Switching Power Supply

Switching power supply apparatus

Switching Power Supply Abstract
A switching power supply apparatus operates with less power consumption as a whole as a result of reduced power loss suffered while the switching operation of the main switching device is being stopped in burst switching control. Burst switching control is achieved by a signal level checker circuit 15 repeatedly turning on and off a switch circuit 17 provided in the line by way of which a switching controller circuit 19 is supplied with operating power. In burst switching control, when the switching operation of a main switching device 5 is being stopped, the supply of operating power to the switching controller circuit 19 is also stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

Switching Power Supply Claims
What is claimed is:

1. A switching power supply apparatus having a serial circuit, including a primary coil of a transformer and a main switching device, connected between a positive and a negative power supply line connected to a direct-current power source, the switching power supply apparatus outputting a direct-current voltage obtained by rectifying with a rectifier a high-frequency voltage induced in a secondary coil of the transformer by the main switching device performing switching operation, wherein the switching power supply apparatus uses as a feedback signal a result of comparison between the direct-current voltage and a predetermined reference voltage, and drives the main switching device by turning on and off, according to a signal level of the feedback signal, supply of operating power to a main switching device driving system that drives the main switching device.

2. A switching power supply apparatus having a serial circuit, including a primary coil of a transformer and a main switching device, connected between a positive and a negative power supply line connected to a direct-current power source, the switching power supply apparatus outputting a direct-current voltage obtained by rectifying with a rectifier a high-frequency voltage induced in a secondary coil of the transformer by the main switching device performing switching operation, wherein the switching power supply apparatus further includes: an output voltage detector that compares the direct-current voltage obtained through rectification with a predetermined reference voltage and that outputs a result of the comparison as a feedback signal; a switching controller that drives and controls the main switching device according to the feedback signal output from the output voltage detector; a signal level checker that monitors a signal level of the feedback signal and that outputs an operation control signal for turning on and off the switching controller according to the monitored signal level; and an operation/nonoperation switcher that is provided in a line by way of which the switching controller is supplied with operating power and that turns on and off the switching controller according to the operation control signal from the signal level checker, the switching power supply apparatus outputting a desired voltage by driving the main switching device with a drive signal from the switching controller that is so turned on and off.

3. A switching power supply apparatus as claimed in claim 2, wherein the feedback signal from the output voltage detector is transmitted to the switching controller through a photodiode of a photocoupler, and the signal level checker monitors the signal level of the feedback signal by comparing a current level flowing through a phototransistor of the photocoupler with a reference current level.

4. A switching power supply apparatus as claimed in claim 3, wherein a current detection resistor is connected in series with the phototransistor of the photocoupler, and the signal level checker turns on and off the switching controller by feeding the switching controller with, as the operation control signal, a signal obtained by comparing a voltage drop across the current detection resistor with a voltage of a current level check reference power source.

5. A switching power supply apparatus as claimed in claim 3, wherein operating power of the signal level checker and the phototransistor of the photocoupler is supplied from subsidiary control power extracted from a node between a plurality of diodes constituting a serial circuit provided in a steady-operation current supply line by way of which a voltage induced in a subsidiary coil of the transformer is supplied after being rectified with the plurality of diodes.

6. A switching power supply apparatus as claimed in claim 2, wherein the operating power of the switching controller is supplied by way of a start-up current supply line by way of which a start-up current is supplied from the positive power supply line through a start-up resistor, or by way of a steady-operation current supply line by way of which a voltage induced in a subsidiary coil of the transformer is supplied after being rectified with a serial circuit composed of a plurality of diodes, and operating power of the signal level checker is supplied from subsidiary control power extracted from a node between the plurality of diodes.

7. A switching power supply apparatus as claimed in claim 2, wherein the switching controller is realized as a pulse-width modulation (PWM) control circuit that outputs, as the drive signal with which to drive the main switching device, a pulse signal that is pulse-width-modulated according to a voltage level of the feedback signal from the output voltage detector.

8. A switching power supply apparatus as claimed in claim 7, wherein used as the PWM control circuit is a PWM control integrated circuit (IC) that is realized as an integrated circuit chip having at least a feedback(FB) terminal to which a voltage related to the feedback signal is input and a capacitor for soft starting(CS) terminal to which a voltage for enabling or disabling an internal circuit is input.

9. A switching power supply apparatus as claimed in claim 2, wherein, when the pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, a start-up corrector is additionally provided to correct start-up of the PWM control IC; a first resistor is connected between a feedback (FB) terminal of the PWM control IC and the negative power supply line; the signal level checker feeds a capacitor for soft starting (CS) terminal controller, which serves as the operation/nonoperation switcher, and the FB terminal with the operation control signal and an inverted feedback signal, respectively, according to a result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects a CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; and the start-up corrector connects and disconnects, through a second resistor, the FB terminal to and from the negative power supply line according to a voltage level of the subsidiary control power.

10. A switching power supply apparatus as claimed in claim 9, wherein the CS terminal controller includes an NPN-type transistor having a collector thereof connected to the CS terminal of the PWM control IC, having an emitter thereof connected to the negative power supply line, and having a base thereof connected to the collector of the other of the transistors included in the signal level checker.

11. A switching power supply apparatus as claimed in claim 9, wherein the start-up corrector includes: a serial circuit composed of a Zener diode and a plurality of resistors connected between a line of the subsidiary control power and the negative power supply line; and an NPN-type transistor having a base thereof connected to a node between the resistors, having a collector thereof connected through the second resistor to the FB terminal of the PWM control IC, and having an emitter thereof connected to the negative supply power line.

12. A switching power supply apparatus as claimed in claim 9, wherein the signal level checker includes, for generation of the reference voltage, voltage division resistors, of which a lower-potential-side resistor is divided into two resistors, with a node therebetween connected through a diode to the CS terminal of the PWM control IC.

13. A switching power supply apparatus as claimed in claim 9, wherein the switching power supply apparatus further includes: a capacitor connected between the CS terminal of the PWM control IC and the negative power supply line; and a diode connected between the capacitor and the CS terminal.

14. A switching power supply apparatus as claimed in claim 2, wherein the signal level checker includes a pair of transistors having emitters thereof connected together to form a comparator, with a base of one of the transistors connected to a node between the current detection resistor and the phototransistor, with a base of the other of the transistors connected to the current level check reference power source, with a collector of the one of the transistors connected to a feedback (FB) terminal of a pulse-width modulation (PWM) control integrated circuit (IC), and with a collector of the other of the transistors connected to a capacitor for soft starting (CS) terminal controller.

15. A switching power supply apparatus as claimed in claim 2, wherein, when a pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, the switching power supply apparatus further includes: a current adjuster connected between a feedback (FB) terminal of the PWM control IC and the negative power supply line to adjust a current output from the FB terminal according to the signal level of the feedback signal; and a capacitor for soft starting (CS) terminal controller that serves as the operation/nonoperation switcher by connecting and disconnecting a CS terminal of the PWM control IC to and from the negative power supply line according to an output signal of the signal level checker.

16. A switching power supply apparatus as claimed in claim 15, wherein the current adjuster includes an NPN-type transistor having a collector thereof connected to the FB terminal of the PWM control IC, having an emitter thereof connected through a resistor to the negative power supply line, and having a base thereof connected to a line of the feedback signal.

17. A switching power supply apparatus as claimed in claim 15, wherein the current adjuster includes an NPN-type transistor having a collector thereof connected to the FB terminal of the PWM control IC, having an emitter thereof connected through a resistor to the negative power supply line, and having a base thereof connected to a line of the feedback signal, and in series with the resistor connected between the base of the NPN-type transistor and the negative power supply line is connected an NPN-type transistor having a collector and a base thereof connected together.

18. A switching power supply apparatus as claimed in claim 2, wherein, when a pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, a start-up corrector is additionally provided to correct start-up of the PWM control IC; a start-up switcher is additionally provided to turn on and off supply of operating power to the signal level checker; a first resistor is connected between a feedback (FB) terminal of the PWM control IC and the negative power supply line; the signal level checker feeds a capacitor for soft starting (CS) terminal controller, which serves as the operation/nonoperation switcher, and the FB terminal with the operation control signal and an inverted feedback signal, respectively, according to a result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects a CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; the start-up corrector detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up corrector connects, through a second resistor, the FB terminal of the PWM control IC to the negative power supply line and, if not, the start-up corrector cuts off the second resistor; and the start-up switcher detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up switcher turns on supply of the operating power to the signal level checker and, if not, the start-up switcher turns off supply of the operating power to the signal level checker.

19. A switching power supply apparatus as claimed in claim 18, wherein the start-up switcher includes an NPN-type transistor having a collector thereof connected to a node between a current detection resistor connected to a line of the feed back signal and an internal reference voltage line of the signal level checker, having a base thereof connected to the phototransistor, and having an emitter thereof connected to the negative power supply line.

20. A switching power supply apparatus as claimed in claim 18, wherein the start-up corrector includes an NPN-type transistor having a collector thereof connected through the second resistor to the FB terminal of the PWM control IC, having a base thereof connected through a resistor to the phototransistor, and having an emitter thereof connected to the negative power supply line.

21. A switching power supply apparatus as claimed in claim 2, wherein, when a pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, a start-up corrector is additionally provided to correct start-up of the PWM control IC; a first resistor is connected between a feedback (FB) terminal of the PWM control IC and the negative power supply line; the signal level checker feeds a capacitor for soft starting (CS) terminal controller, which serves as the operation/nonoperation switcher, and the FB terminal with the operation control signal and an inverted feedback signal, respectively, according to a result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects a CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; and the start-up corrector detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up corrector connects, through a diode and the second resistor, the FB terminal of the PWM control IC to the negative power source line and turns on supply of operating power to the signal level checker and, if not, the start-up corrector cuts off the diode and the second resistor and turns off supply of the operating power to the signal level checker.

22. A switching power supply apparatus as claimed in claim 21, wherein the start-up corrector includes an NPN-type transistor having a collector thereof connected through the diode and the second resistor to the FB terminal of the PWM control IC, having a base thereof connected through a resistor to the phototransistor, and having an emitter thereof connected to the negative power supply line.

23. A switching power supply apparatus as claimed in claim 21, wherein the signal level checker includes, for generation of the reference voltage, voltage division resistors, of which a lower-potential-side resistor is divided into two resistors, with a node therebetween connected through a diode to the CS terminal controller, and the CS terminal controller is connected through another diode to the CS terminal of the PWM control IC.

24. A switching power supply apparatus as claimed in claim 2, wherein, when a pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, a start-up switcher is additionally provided to turn on and off supply of operating power to the signal level checker; a current adjuster is additionally provided that is connected between a feedback (FB) terminal of the PWM control IC and the negative power supply line to adjust a current output from the FB terminal according to the signal level of the feedback signal; the signal level checker feeds a capacitor for soft starting (CS) terminal controller, which serves as the operation/nonoperation switcher, with the operation control signal according to a result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects a CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; and the start-up switcher detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up switcher turns on supply of operating power to the signal level checker and, if not, the start-up switcher turns off supply of operating power to the signal level checker.

25. A switching power supply apparatus having a serial circuit, including a primary coil of a transformer and a main switching device, connected between a positive and a negative power supply line connected to a direct-current power source, the switching power supply apparatus outputting a desired direct-current voltage by controlling the main switching device according to a feedback signal obtained as a result of comparison between a direct-current voltage obtained through rectification of a high-frequency voltage induced in a secondary coil of the transformer by the main switching device performing switching operation and a previously set reference voltage, wherein a signal level of the feedback signal is compared with a signal level of a previously generated oscillation signal; according to a result of the comparison, an on-state duty of a drive signal to be fed to the main switching device is determined and switching between burst switching control and continuous switching control is performed; and while switching operation of the main switching device is being stopped in burst switching control, supply of operating power for driving the main switching device is stopped.

26. A switching power supply apparatus as claimed in claim 25, wherein burst switching control is achieved by turning on and off supply of operating power to a switching controller that drives the main switching device.

27. A switching power supply apparatus as claimed in claim 25, wherein, when a pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, a capacitor is connected between a feedback (FB) terminal of the PWM control IC and an internal power terminal connected to an internal power supply line.

28. A switching power supply apparatus as claimed in claim 25, wherein, when a pulse-width modulation (PWM) control integrated circuit (IC) is used as the switching controller, a serial circuit composed of a capacitor and a resistor is connected between a feedback (FB) terminal of the PWM control IC and an internal power terminal connected to an internal power supply line.

Switching Power Supply Description
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching power supply apparatus used as a direct-current power source in an electronic appliance.

2. Description of the Prior Art

A conventionally known example of such a switching power supply apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. H10-304658. The switching power supply apparatus disclosed in this publication is provided with a main switch that turns on and off a direct current applied to the primary coil of a transformer, a secondary-side rectifying/smoothing circuit that rectifies and smoothes the on/off signal induced in the secondary coil of the transformer so as to supply it as a main output signal, a subsidiary power source that rectifies and smoothes the on/off signal induced in the bias coil of the transformer so as to supply it as a subsidiary supply voltage, an error amplifier that generates an error voltage signal that represents the difference between the subsidiary supply voltage output from the subsidiary power source and a reference voltage, and a comparator that feeds the main switch with an on/off control signal in such a way as to reduce the error voltage signal output from the error amplifier. This switching power supply apparatus is further provided with a light-load switching controller portion that temporarily stops the on/off operation of the main switch when the main output voltage becomes higher than an upper limit voltage and that restarts the on/off operation of the main switch when the main output voltage becomes lower than a lower limit voltage.

In this conventional switching power supply apparatus, control is so performed that the on/off operation of the main switch is temporarily stopped when the main output voltage output from the secondary-side rectifying/smoothing circuit becomes higher than the upper limit voltage, and that the on/off operation of the main switch is restarted when the main output voltage becomes lower than the lower limit voltage.

Here, however, while the on/off operation of the main switch is temporarily stopped when the main output voltage becomes higher than the upper limit voltage, operating power is kept supplied to the individual circuits and control devices provided in the control circuit that drives and controls the main switch. This causes wasteful power loss. Specifically, in a configuration where the on/off operation of the main switch is temporarily stopped when the main output voltage output from the secondary-side rectifying/smoothing circuit becomes higher than the upper limit voltage and the on/off operation of the main switch is restarted when the main output voltage becomes lower than the lower limit voltage, i.e., in so-called burst switching control, the operating power is kept supplied to all the circuits and control devices provided in the control circuit even while the switching operation is being stopped. This causes wasteful consumption of the supply current, resulting in wasteful power loss.

Also in the conventional switching power supply apparatuses disclosed in Japanese Patent Applications Laid-Open Nos. 2001-346378 and 2002-58238, as in the switching power supply apparatus described above, even during the period in which the switching operation of the main switch is being stopped in burst switching control, the operating power is kept supplied to all the circuits and control devices provided in the switching signal controlling circuit. This causes wasteful consumption of the supply current, resulting in wasteful power loss.

Incidentally, in the conventional switching power supply apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-86745, to reduce power consumption in a stand-by state, the switching operation of the main switching device is stopped in that state. Thus, the aim of this invention is not to reduce the power loss suffered while the switching operation of the main switching device is being stopped in burst switching control.

SUMMARY OF THE INVENTION

To facilitate a complete understanding of the invention, a glossary of terms and acronyms is provided on the last page of the specification before the claims.

An object of the present invention is to provide a switching power supply apparatus that operates with less power consumption as a whole as a result of reduced power loss suffered while the switching operation of the main switching device is being stopped in burst switching control.

To achieve the above object, according to one aspect of the present invention, a switching power supply apparatus has a serial circuit, including the primary coil of a transformer and a main switching device, connected between a positive and a negative power supply line connected to a direct-current power source. The switching power supply apparatus outputs a direct-current voltage obtained by rectifying with a rectifier a high-frequency voltage induced in the secondary coil of the transformer by the main switching device performing switching operation. Here, the switching power supply apparatus uses as a feedback signal the result of comparison between the direct-current voltage and a predetermined reference voltage, and drives the main switching device by turning on and off, according to the signal level of the feedback signal, supply of operating power to a main switching device driving system that drives the main switching device.

In this switching power supply apparatus according to the invention, for example, in heavy-load operation, the output voltage decreases. To correct this, a lower-level feedback signal is generated. This causes the operating power to the main switching device driving system to be kept supplied thereto, and thus the main switching device continues switching operation. On the other hand, in light-load operation, when the output voltage becomes higher than a predetermined value, a higher-level feedback signal is generated. This causes the supply of the operating power to the switching device driving system to be stopped, and thus the main switching device stops switching operation. As a result, the output voltage returns to the predetermined value.

That is, with this switching power supply apparatus according to the invention, while the switching operation of the main switching device is being stopped in burst switching control, the supply of the operating power to the main switching device driving system is also stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

According to another aspect of the present invention, a switching power supply apparatus has a serial circuit, including the primary coil of a transformer and a main switching device, connected between a positive and a negative power supply line connected to a direct-current power source. The switching power supply apparatus outputs a direct-current voltage obtained by rectifying with a rectifier a high-frequency voltage induced in the secondary coil of the transformer by the main switching device performing switching operation. Here, the switching power supply apparatus further includes: an output voltage detector that compares the direct-current voltage obtained through rectification with a predetermined reference voltage and that outputs the result of the comparison as a feedback signal; a switching controller that drives and controls the main switching device according to the feedback signal output from the output voltage detector; a signal level checker that monitors the signal level of the feedback signal and that outputs an operation control signal for turning on and off the switching controller according to the monitored signal level; and an operation/nonoperation switcher that is provided in the line by way of which the switching controller is supplied with operating power and that turns on and off the switching controller according to the operation control signal from the signal level checker.

In this switching power supply apparatus according to the invention, in light-load operation, when the output voltage tends to increase, i.e., when the output voltage is higher than a predetermined value, and thus the signal level of the feedback signal output from the output voltage detector is, for example, high, the signal level checker feeds the operation/nonoperation switcher with an operation control signal that requests nonoperation, and thus the operation/nonoperation switcher stops the supply of the operating power to the switching controller.

As a result, the main switching device stops switching operation, and thus the output voltage starts to decrease gradually. When the signal level of the feedback signal from the output voltage detector becomes, for example, low, the signal level checker feeds the operation/nonoperation switcher with an operation control signal that requests operation, and thus the operation/nonoperation switcher starts the supply of the operating power to the switching controller.

As a result, the main switching device restarts switching operation, and thus the output voltage starts to increase gradually. When the signal level of the feedback signal becomes high again, the signal level checker feeds the operation/nonoperation switcher with an operation control signal that requests nonoperation, and thus the operation/nonoperation switcher stops the supply of the operating power to the switching controller. As a result, the main switching device stops switching operation, and thus the output voltage starts to decrease gradually. As this sequence of operations is repeated, the output voltage is kept at the predetermined value.

In this switching power supply apparatus, when the output voltage tends to decrease, i.e., when the output voltage is lower than a predetermined value, and thus the signal level of the feedback signal output from the output voltage detector is, for example, low, the signal level checker feeds the operation/nonoperation switcher with an operation control signal that requests operation, and thus the operation/nonoperation switcher continues supplying the operating power to the switching controller so that switching operation is performed continuously.

With this switching power supply apparatus according to the invention, burst switching control is achieved as a result of the signal level checker repeatedly turning on and off the operation/nonoperation switcher provided in the line by way of which the switching controller is supplied with operating power. Moreover, while the switching operation of the main switching device is being stopped in burst switching control, the supply of the operating power to the switching controller is also stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

Preferably, the feedback signal from the output voltage detector is transmitted to the switching controller through the photodiode of a photocoupler, and the signal level checker monitors the signal level of the feedback signal by comparing the current level flowing through the phototransistor of the photocoupler with a reference current level.

With this configuration, burst switching operation is controlled according to the result of comparison between the current value through the phototransistor and the reference current value. The signal level of the feedback signal (i.e., the current value through the phototransistor) represents the load current value of the switching power supply apparatus. Thus, it is possible to correctly set the load current value at which switching between continuous switching operation and burst switching operation is performed.

In burst switching operation, the output voltage fluctuates. However, since the signal level of the feedback signal represents the output voltage value, it is possible to correctly set the upper and lower limits of the output voltage.

Preferably, a current detection resistor is connected in series with the phototransistor of the photocoupler, and the signal level checker turns on and off the switching controller by feeding the switching controller with, as the operation control signal, a signal obtained by comparing the voltage drop across the current detection resistor with the voltage of a current level check reference power source.

With this configuration, it is possible to make the signal level checker detect the signal level of the feedback signal according to the voltage drop across the current detection resistor, compare the signal level with the voltage of the current level check reference power source, and turn on and off the supply of the operating power to the switching controller according to the result of the comparison.

Preferably, the operating power of the switching controller is supplied by way of the start-up current supply line by way of which a start-up current is supplied from the positive power supply line through a start-up resistor, or by way of the steady-operation current supply line by way of which a voltage induced in the subsidiary coil of the transformer is supplied after being rectified with a serial circuit composed of a plurality of diodes, and the operating power of the signal level checker is supplied from subsidiary control power extracted from a node between the plurality of diodes.

With this configuration, when the switching power supply apparatus starts to start up, it is possible to prevent, by the action of the diodes, the current that is supposed to flow to the start-up current supply line from flowing to the steady-operation current supply line. This helps reduce the time required for start-up, and also helps reduce the resistance of the start-up resistor and thereby reduce power consumption.

In other words, as compared with a switching power supply apparatus that is not provided with the function of controlling burst switching in such a way as to stop the supply of the operating power to the switching controller that performs burst switching operation when the switching power supply apparatus starts to start up, the switching power supply apparatus of the present invention starts up in as short a time while reducing the unnecessary power consumption by the start-up resistor.

Preferably, the operating power of the signal level checker and the phototransistor of the photocoupler is supplied from subsidiary control power extracted from a node between a plurality of diodes constituting a serial circuit provided in the steady-operation current supply line by way of which a voltage induced in the subsidiary coil of the transformer is supplied after being rectified with the plurality of diodes.

With this configuration, when the switching power supply apparatus starts to start up, the diodes prevent the start-up current from flowing to the subsidiary control power. This helps shorten the start-up time. Moreover, in the steady operation, the direct-current voltage obtained by rectifying the voltage induced in the subsidiary coil of the transformer is fed as the operating power to the signal level checker and the phototransistor of the photocoupler. This ensures stable operation.

Preferably, the switching controller is realized as a PWM control circuit that outputs, as the drive signal with which to drive the main switching device, a pulse signal that is pulse-width-modulated according to the voltage level of the feedback signal from the output voltage detector.

With this configuration, the main switching device is driven with a drive signal accurately commensurate with the voltage level of the feedback signal. This helps enhance the stability of the output voltage of the switching power supply apparatus.

Preferably, used as the PWM control circuit is a PWM control IC (for example, an IC with the product number FA5511 manufactured by Fuji Electric Co., Ltd.) that is realized as an integrated circuit chip having at least an FB terminal to which a voltage related to the feedback signal is input and a CS terminal to which a voltage for enabling or disabling an internal circuit is input.

With this configuration, it is possible to reduce the space occupied by the circuit that drives the main switching device, and to enhance the stability of the output voltage, leading to miniaturization of the apparatus.

Preferably, when a PWM control IC is used as the switching controller, a start-up corrector is additionally provided to correct the start-up of the PWM control IC; a first resistor is connected between the FB terminal of the PWM control IC and the negative power supply line; the signal level checker feeds a CS terminal controller, which serves as the operation/nonoperation switcher, and the FB terminal with the operation control signal and an inverted feedback signal, respectively, according to the result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects the CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; and the start-up corrector connects and disconnects, through a second resistor, the FB terminal to and from the negative power supply line according to the voltage level of the subsidiary control power.

With this configuration, at the start-up of the switching power supply apparatus, when the voltage of the subsidiary control power increases, immediately before a current starts to flow through the phototransistor, the start-up corrector connects the second resistor in parallel with the first resistor and thereby reduces the resistance between the FB terminal and the negative power supply line. This causes the potential at the FB terminal to decrease. In this way, when the switching power supply apparatus starts to start up, the voltage at the FB terminal is kept at the optimum level to permit reliable rising of the output voltage. In addition, in the steady state, the switching power supply apparatus is permitted to output a reliably stabilized voltage.

Preferably, the signal level checker includes a pair of transistors having the emitters thereof connected together to form a comparator, with the base of one of the transistors connected to the node between the current detection resistor and the phototransistor, with the base of the other of the transistors connected to the current level check reference power source, with the collector of the one of the transistors connected to the FB terminal of the PWM control IC, and with the collector of the other of the transistors connected to the CS terminal controller.

With this configuration, it is possible to easily realize the comparator for comparing the signal level of the feedback signal with the current level of the current level check reference power.

Preferably, the CS terminal controller includes an NPN-type transistor having the collector thereof connected to the CS terminal of the PWM control IC, having the emitter thereof connected to the negative power supply line, and having the base thereof connected to the collector of the other of the transistors included in the signal level checker.

With this configuration, where the CS terminal controller is provided with the NPN transistor connected in the manner described above, it is possible, with a simple configuration, to enable and disable the PWM control IC.

Preferably, the start-up corrector includes: a serial circuit composed of a Zener diode and a plurality of resistors connected between the line of the subsidiary control power and the negative power supply line; and an NPN-type transistor having the base thereof connected to a node between the resistors, having the collector thereof connected through the second resistor to the FB terminal of the PWM control IC, and having the emitter thereof connected to the negative supply power line.

With this configuration, where the start-up corrector is provided with the serial circuit and the NPN-type transistor described above, it is possible, with a simple configuration, to make the switching power supply apparatus output a reliably stabilized voltage in the steady operation.

Preferably, the signal level checker includes, for generation of the reference voltage, voltage division resistors, of which a lower-potential-side resistor is divided into two resistors, with the node therebetween connected through a diode to the CS terminal of the PWM control IC.

With this configuration, by varying the resistances of the individual division resistors for generating the reference voltage, it is possible to freely and accurately set the fluctuation width and burst switching period of the output voltage in burst switching operation. In particular, by making the fluctuation width of the output voltage as wide as applications permit, it is possible to reduce unnecessary power consumption in burst switching operation.

Preferably, the switching power supply apparatus further includes: a capacitor connected between the CS terminal of the PWM control IC and the negative power supply line; and a diode connected between the capacitor and the CS terminal.

With this configuration, in burst switching operation, it is possible to quicken, by the action of the diode, the fluctuation of the voltage level at the CS terminal and thereby quicken the speed of switching between a state in which switching operation is performed and a state in which switching operation is stopped. Moreover, in burst switching operation, it is possible to reduce the fluctuation width of the output voltage and increase the accuracy of the upper and lower limits of the output voltage. Moreover, when the load current abruptly increases during burst switching operation, it is possible to shorten the time required to shift to continuous switching operation and thereby prevent a decrease in the output voltage.

Preferably, when a PWM control IC is used as the switching controller, the switching power supply apparatus further includes: a current adjuster connected between the FB terminal of the PWM control IC and the negative power supply line to adjust the current output from the FB terminal according to the signal level of the feedback signal; and a CS terminal controller that serves as the operation/nonoperation switcher by connecting and disconnecting the CS terminal of the PWM control IC to and from the negative power supply line according to an output signal of the signal level checker.

With this configuration, at the start-up of the switching power supply apparatus, the current adjuster adjusts the voltage at the FB terminal of the PWM control IC to a high value. Thus, the PWM control IC makes the main switching device perform switching operation with a great on-state duty, and thereby reduces the start-up time. Moreover, the CS terminal controller connects and disconnects the CS terminal of the PWM control IC to and from the negative power supply line according to the output signal of the signal level checker, and thereby turns on and off the PWM control IC.

With this configuration, the current adjuster has a simpler configuration than the start-up corrector, but nevertheless achieves the same effect. That is, it is possible, with a simpler configuration, to reduce the start-up time of the switching power supply apparatus.

According to another aspect of the present invention, the switching power supply apparatus has a serial circuit, including the primary coil of a transformer and a main switching device, connected between a positive and a negative power supply line connected to a direct-current power source. The switching power supply apparatus outputs a desired direct-current voltage by controlling the main switching device according to a feedback signal obtained as a result of comparison between a direct-current voltage obtained through rectification of a high-frequency voltage induced in the secondary coil of the transformer by the main switching device performing switching operation and a previously set reference voltage. Here, the signal level of the feedback signal is compared with the signal level of a previously generated oscillation signal. According to the result of the comparison, the on-state duty of the drive signal to be fed to the main switching device is determined and switching between burst switching control and continuous switching control is performed. Moreover, while the switching operation of the main switching device is being stopped in burst switching control, supply of the operating power for driving the main switching device is stopped.

In this switching power supply apparatus according to the invention, the on-state duty of the drive signal to be fed to the main switching device is determined according to the result of comparison between the signal level of the previously generated oscillation signal and the signal level of the feedback signal. This makes it possible to accurately control the switching of the main switching device. Moreover, switching between burst switching and continuous switching is also performed according to the result of the comparison. This makes it possible to accurately perform the switching. Moreover, while the switching operation of the main switching device is being stopped, the supply of the operating power for driving the main switching device is stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

Preferably, burst switching control is achieved by turning on and off the supply of operating power to the switching controller that drives the main switching device. This helps reduce the power loss suffered while the switching operation is being stopped.

Preferably, when a PWM control IC is used as the switching controller, a capacitor is connected between the FB terminal of the PWM control IC and an internal power terminal connected to an internal power supply line.

With this configuration, in a case where, for phase compensation of the output voltage stabilizing control system, a serial circuit composed of a capacitor and a resistor is connected between the FB terminal of the PWM control IC and the negative power supply line, even when the load current abruptly increases during burst switching operation, it is possible to quicken the control of the burst switching operation control system as much as possible and thereby prevent a decrease in the output voltage of the switching power supply apparatus. In addition, it is possible to reduce unnecessary power consumption in the burst switching operation.

Preferably, when a PWM control IC is used as the switching controller, a serial circuit composed of a capacitor and a resistor is connected between the FB terminal of the PWM control IC and an internal power terminal connected to an internal power supply line.

With this configuration, in a case where, for phase compensation of the output voltage stabilizing control system, a serial circuit composed of a capacitor and a resistor is connected between the FB terminal of the PWM control IC and the negative power supply line, even when the load current abruptly increases during burst switching operation, it is possible to quicken the control of the burst switching operation control system as much as possible and thereby prevent a decrease in the output voltage of the switching power supply apparatus. In addition, it is possible to achieve phase compensation in the output voltage stabilizing control system with almost no effects on the burst switching operation characteristics.

Preferably, the current adjuster includes an NPN-type transistor having the collector thereof connected to the FB terminal of the PWM control IC, having the emitter thereof connected through a resistor to the negative power supply line, and having the base thereof connected to the line of the feedback signal, and in series with the resistor connected between the base of the NPN-type transistor and the negative power supply line is connected an NPN-type transistor having the collector and base thereof connected together.

With this configuration, even when characteristics change as temperature varies, the current adjuster can suppress the variation of the predetermined current value (of the load current) at which switching between burst switching operation and continuous switching operation is performed. This helps stabilize the output voltage.

Preferably, when a PWM control IC is used as the switching controller, a start-up corrector is additionally provided to correct the start-up of the PWM control IC; a start-up switcher is additionally provided to turn on and off the supply of operating power to the signal level checker; a first resistor is connected between the FB terminal of the PWM control IC and the negative power supply line; the signal level checker feeds a CS terminal controller, which serves as the operation/nonoperation switcher, and the FB terminal with the operation control signal and an inverted feedback signal, respectively, according to the result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects the CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; the start-up corrector detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up corrector connects, through a second resistor, the FB terminal of the PWM control IC to the negative power supply line and, if not, the start-up corrector cuts off the second resistor; and the start-up switcher detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up switcher turns on the supply of the operating power to the signal level checker and, if not, the start-up switcher turns off the supply of the operating power to the signal level checker.

With this configuration, at the start of start-up, power immediately starts to be supplied to the PWM control IC, and the switching power supply apparatus starts switching operation. This switching operation causes the output voltage of the switching power supply apparatus to increase until a feedback signal is generated, when the feedback signal is detected by the start-up corrector and the start-up switcher. As a result, the start-up corrector connects the second resistor in addition to and in parallel with the first resistor, and the start-up switcher starts to supply an operating current to the signal level checker. Supplied with the operating current, the signal level checker starts to operate, and, during the period in which the signal level of the feedback signal is lower than the voltage level of the current level check reference power, the CS terminal controller keeps the CS terminal of the PWM control IC disconnected from the negative power supply line so that operating power is kept supplied to the PWM control IC. Thus, switching operation is continued to permit the output voltage of the switching power supply apparatus to increase to a predetermined value.

Thereafter, when the load of the switching power supply apparatus is light, and the signal level of the feedback signal is found to be higher than the voltage level of the current level check reference power, the CS terminal controller connects the CS terminal of the PWM control IC to the negative power supply line to turn off the supply of operating power to the PWM control IC and thereby stop the switching operation of the switching power supply apparatus. As the output voltage decreases, the signal level of the feedback signal decreases until it becomes lower than the current level check reference, when the signal level checker turns the operation control signal low. This causes the CS terminal controller to disconnect the CS terminal of the PWM control IC from the negative power supply line so that operating power is supplied to the PWM control IC. This sequence of operations is repeated to achieve burst oscillation operation.

On the other hand, when the load of the switching power supply apparatus is heavy, and the signal level of the feedback signal does not reach the voltage level of the current level check reference power, the signal level checker turns the operation control signal low. This causes the CS terminal controller to disconnect the CS terminal of the PWM control IC from the negative power supply line so that continuous switching operation is continued.

In particular, the start-up corrector is so configured as to cut off the second resistor at start-up to increase the resistance between the FB terminal of the PWM control IC and the negative power supply line and thereby make the potential at the FB terminal higher. This ensures reliable start-up operation. On the other hand, in the steady operation, the start-up corrector connects the second resistor in parallel with the first resistor to make the potential at the FB terminal of the PWM control IC lower. This permits the PWM control IC to reliably control the switching power supply apparatus to output a stabilized voltage.

Preferably, the start-up switcher includes an NPN-type transistor having the collector thereof connected to the node between a current detection resistor connected to the line of the feed back signal and the internal reference voltage line of the signal level checker, having the base thereof connected to the phototransistor, and having the emitter thereof connected to the negative power supply line.

With this configuration, it is possible to realize the start-up switcher with a simple circuit.

Preferably, the start-up corrector includes an NPN-type transistor having the collector thereof connected through the second resistor to the FB terminal of the PWM control IC, having the base thereof connected through a resistor to the phototransistor, and having the emitter thereof connected to the negative power supply line.

With this configuration, it is possible to realize the start-up corrector with a simple circuit.

Preferably, when a PWM control IC is used as the switching controller, a start-up corrector is additionally provided to correct the start-up of the PWM control IC; a first resistor is connected between the FB terminal of the PWM control IC and the negative power supply line; the signal level checker feeds a CS terminal controller, which serves as the operation/nonoperation switcher, and the FB terminal with the operation control signal and an inverted feedback signal, respectively, according to the result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects the CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; and the start-up corrector detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up corrector connects, through a diode and the second resistor, the FB terminal of the PWM control IC to the negative power source line and turns on the supply of operating power to the signal level checker and, if not, the start-up corrector cuts off the diode and the second resistor and turns off the supply of the operating power to the signal level checker.

With this configuration, at the start of start-up, power immediately starts to be supplied to the PWM control IC, and the switching power supply apparatus starts switching operation. This switching operation causes the output voltage of the switching power supply apparatus to increase until a feedback signal is generated, when the feedback signal is detected by the start-up corrector. As a result, the start-up corrector connects the second resistor in addition to and in parallel with the first resistor, and starts to supply an operating current to the signal level checker. Supplied with the operating current, the signal level checker starts to operate, and, during the period in which the signal level of the feedback signal is lower than the voltage level of the current level check reference power, the CS terminal controller keeps the CS terminal of the PWM control IC disconnected from the negative power supply line so that operating power is kept supplied to the PWM control IC. Thus, switching operation is continued to permit the output voltage of the switching power supply apparatus to increase to a predetermined value.

The diode prevents a current from flowing through the signal level checker in a predetermined timing period, and thereby prevents the signal level checker from operating unnecessarily, contributing to higher operation accuracy.

Preferably, the start-up corrector includes an NPN-type transistor having the collector thereof connected through the diode and the second resistor to the FB terminal of the PWM control IC, having the base thereof connected through a resistor to the phototransistor, and having the emitter thereof connected to the negative power supply line.

With this configuration, it is possible to realize the start-up corrector with a simple circuit.

Moreover, the other diode prevents a high-level voltage from being applied to the CS terminal of the PWM control IC when the switching power supply apparatus starts to start up. This is because, when a high-level voltage is applied to the CS terminal, the output of the PWM control IC is turned off.

Preferably, when a PWM control IC is used as the switching controller, a start-up switcher is additionally provided to turn on and off the supply of operating power to the signal level checker; a current adjuster is additionally provided that is connected between the FB terminal of the PWM control IC and the negative power supply line to adjust the current output from the FB terminal according to the signal level of the feedback signal; the signal level checker feeds a CS terminal controller, which serves as the operation/nonoperation switcher, with the operation control signal according to the result of checking of the signal level of the feedback signal; the CS terminal controller connects and disconnects the CS terminal of the PWM control IC to and from the negative power supply line according to the operation control signal; and the start-up switcher detects whether or not the feedback signal is present so that, if the feedback signal is present, the start-up switcher turns on the supply of operating power to the signal level checker and, if not, the start-up switcher turns off the supply of operating power to the signal level checker.

With this configuration, at the start-up of the switching power supply apparatus, the current adjuster adjusts the current at the FB terminal of the PWM control IC. Thus, the PWM control IC makes the main switching device perform switching operation with a great on-state duty, and thereby reduces the start-up time. Moreover, on detecting the feedback signal, the start-up switcher starts to supply operating power to the signal level checker. Moreover, the CS terminal controller connects and disconnects the CS terminal of the PWM control IC to and from the negative power supply line according to the output signal of the signal level checker, and thereby turns on and off the PWM control IC. In this way, it is possible to realize burst switching operation with high power use efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:

FIG. 1 is a circuit diagram of the switching power supply apparatus of a first embodiment of the invention;

FIG. 2 is a circuit diagram of the switching power supply apparatus of a second embodiment of the invention;

FIG. 3 is a circuit diagram of the switching power supply apparatus of a third embodiment of the invention;

FIG. 4 is a circuit diagram of the switching power supply apparatus of a fourth embodiment of the invention;

FIG. 5 is a circuit diagram of a typical circuit configuration of a switching power supply apparatus employing FA5511, for reference purposes;

FIG. 6 is a circuit diagram showing an outline of the circuit configuration of FA5511;

FIG. 7 is a signal waveform diagram illustrating the start-up operation of the switching power supply apparatus shown in FIG. 5;

FIG. 8 is a signal waveform diagram illustrating the start-up operation of the switching power supply apparatus shown in FIG. 4;

FIG. 9 is a circuit diagram of the switching power supply apparatus of a fifth embodiment of the invention;

FIG. 10 is a circuit diagram of the switching power supply apparatus of a sixth embodiment of the invention;

FIG. 11 is a circuit diagram of the switching power supply apparatus of a seventh embodiment of the invention;

FIG. 12 is a circuit diagram of the switching power supply apparatus of an eighth embodiment of the invention;

FIG. 13 is a circuit diagram of the switching power supply apparatus of a ninth embodiment of the invention;

FIG. 14 is a circuit diagram of the switching power supply apparatus of a tenth embodiment of the invention;

FIG. 15 is a circuit diagram of the switching power supply apparatus of an eleventh embodiment of the invention;

FIG. 16 is a circuit diagram of the switching power supply apparatus of a twelfth embodiment of the invention;

FIG. 17 is a signal waveform diagram illustrating the start-up operation of the switching power supply apparatuses shown in FIGS. 16 and 19;

FIG. 18 is a circuit diagram of the switching power supply apparatus of a thirteenth embodiment of the invention;

FIG. 19 is a circuit diagram of the switching power supply apparatus of a fourteenth embodiment of the invention;

FIG. 20 is a circuit diagram of the switching power supply apparatus of a fifteenth embodiment of the invention;

FIG. 21 is a circuit diagram of the switching power supply apparatus of a sixteenth embodiment of the invention; and

FIG. 22 is a circuit diagram of the switching power supply apparatus of a seventeenth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First Embodiment

FIG. 1 is a circuit diagram of the switching power supply apparatus of a first embodiment of the invention.

In the switching power supply apparatus shown in FIG. 1, a transformer 3 has its primary coil 4 connected, at its one end, to a positive power supply line 1 and, at its other end, through a main switching device 5 to a negative power supply line 2. The main switching device 5 is realized with, for example, an FET (field-effect transistor). The transformer 3 has its secondary coil 6 connected, at its one end, through a diode 7 to an output line 25 and, at its other end, to an output line 26. Between the output lines 25 and 26, there are connected a capacitor 45 and an output voltage detector circuit 9. The output terminal of the output voltage detector circuit 9 is connected by way of a line 9a to the input terminal of a signal level checker circuit 15 and to the input terminal of a switching controller circuit 19 to feed each of them with a feedback signal.

An operating power source 16 has its negative end connected to the negative power supply line 2, and has its positive end connected by way of a line 16a to the power terminal of the signal level checker circuit 15 and to the input terminal of a switch circuit 17. The signal level checker circuit 15 feeds an operation control signal by way of a line 15a to the control terminal of the switch circuit 17. The output terminal of the switch circuit 17 is connected by way of a line 17a to the power terminal of the switching controller circuit 19. The output terminal of the switching controller circuit 19 is connected to the control terminal of the main switching device 5.

Next, the operation of the switching power supply apparatus of the first embodiment will be described. When a voltage from a nonillustrated direct-current power source is applied between the positive and negative power supply lines 1 and 2, the main switching device 5, under the control of the switching controller circuit 19, performs switching operation, and thereby causes a high-frequency current to flow through the primary coil 4 of the transformer 3. This induces a high-frequency voltage in the secondary coil 6 of the transformer 3. This high-frequency voltage is rectified by the diode 7 and then smoothed by the capacitor 45, and is thereby converted into a direct-current voltage. This direct-current voltage is applied between the output lines 25 and 26 so as to be output as the output voltage of the switching power supply apparatus.

The output voltage detector circuit 9 compares the output voltage between the output lines 25 and 26 with a predetermined reference voltage, and feeds the result of the comparison in the form of a feedback signal by way of the line 9a to the signal level checker circuit 15 and the switching controller circuit 19. The switching controller circuit 19 operates from the power supplied thereto from the operating power source 16 through the switch circuit 17, and, by controlling the timing with which the main switching device 5 is turned on and off according to the feedback signal, performs control in such a way that a desired direct-current voltage is output between the output lines 25 and 26.

When the load connected between the output lines 25 and 26 (the output terminals of the switching power supply apparatus) consumes a small amount of electric power (i.e., in light-load operation), the output voltage between the output lines 25 and 26 (the output voltage of the switching power supply apparatus) tends to be higher. To correct this, the output voltage detector circuit 9 outputs to the line 9a a feedback signal having, for example, a higher level.

On the other hand, when the load connected between the output lines 25 and 26 consumes a large amount of electric power (i.e., in heavy-load operation), the output voltage between the output lines 25 and 26 tends to be lower. To correct this, the output voltage detector circuit 9 outputs to the line 9a a feedback signal having, for example, a lower level.

When the output voltage between the output lines 25 and 26 (the output voltage of the switching power supply apparatus) is higher than the reference voltage, and the feedback signal output by way of the line 9a has a higher level, the signal level checker circuit 15 feeds an operation control signal by way of the line 15a to the switch circuit 17 so as to turn the switch circuit 17 off.

With the switch circuit 17 turned off, the switching controller circuit 19 ceases to be supplied with the voltage from the operating power source 16, and thus stops operating. As a result, the main switching device 5 stops operating, and thus permits the output voltage between the output lines 25 and 26 (the output voltage of the switching power supply apparatus) to decrease gradually.

As the output voltage decreases, the level of the feedback signal from the output voltage detector circuit 9 becomes, for example, lower. Then, the signal level checker circuit 15 feeds an operation control signal by way of the line 15a to the switch circuit 17 so as to turn the switch circuit 17 on. This causes the voltage from the operating power source 16 to be supplied to the switching controller circuit 19, and thus the switching controller circuit 19 restarts operating, and makes the main switching device 5 perform switching operation.

Consequently, the output voltage between the output lines 25 and 26 (the output voltage of the switching power supply apparatus) increases, and meanwhile the output voltage detector circuit 9 feeds a higher-level feedback signal by way of the line 9a to the signal level checker circuit 15. Then, the signal level checker circuit 15 turns the switch circuit 17 off to stop the operation of the switching controller circuit 19 and thereby stop the switching operation by the main switching device 5.

When the load connected between the output lines 25 and 26 is a heavy load that consumes a considerably large amount of electric power, the output voltage tends to be considerably low. In this case, the signal level checker circuit 15 keeps the switch circuit 17 continuously on. Thus, the switching controller circuit 19 makes the main switching device 5 perform switching operation continuously and thereby stabilizes the output voltage.

As described above, bust switching operation is achieved by repeatedly stopping switching operation when the output voltage of the switching power supply apparatus increases and restarting switching operation when the output voltage decreases. This stabilizes the output voltage.

In burst switching operation, the operating power of the signal level checker circuit 15 is supplied thereto without passing through the switch circuit 17, and therefore the signal level checker circuit 15 keeps operating even when switching operation is being stopped. However, the power consumption of the signal level checker circuit 15 is far lower than that of the switching controller circuit 19, and accordingly the switching power supply apparatus operates with less power consumption, contributing to energy saving.

Second Embodiment

FIG. 2 is a circuit diagram of the switching power supply apparatus of a second embodiment of the invention.

In the switching power supply apparatus shown in FIG. 2, a transformer 3 has its primary coil 4 connected, at its one end, to a positive power supply line 1 and, at its other end, through a main switching device 5 to a negative power supply line 2. The transformer 3 has its secondary coil 6 connected, at its one end, through a diode 7 to an output line 25 and, at its other end, to an output line 26. Between the output lines 25 and 26, there are connected a capacitor 45 and an output voltage detector circuit 9.

The output voltage detector circuit 9 is composed of two serial circuits connected between the output lines 25 and 26, more specifically one composed of a photocoupler 20, a resistor 21, and a shunt regulator 22 and another composed of output voltage division resistors 23 and 24. The photocoupler 20 is composed of a photodiode 20a and a phototransistor 20b. The control terminal of the shunt regulator 22 is connected to the node between the output voltage division resistors 23 and 24. The shunt regulator 22 compares the voltage at the node between the output voltage division resistors 23 and 24 with a reference voltage that has previously been prepared internally, and permits a current commensurate with the result of the comparison to flow through the photodiode 20a.

An operating power source 16 has its negative end connected to the negative power supply line 2, and has its positive end connected to a steady-state operating current supply line 16a. A signal level checker circuit 15 is composed of a Zener diode 191, a resistor 201, a comparator 18, and a current detection resistor 28.

The Zener diode 191 has its cathode connected to the steady-state operating current supply line 16a, and has its anode connected to one end of the resistor 201 and to the non-inverting input terminal of the comparator 18. The other end of the resistor 201 is connected to the negative power supply line 2. The current detection resistor 28 has its one end connected to the steady-state operating current supply line 16a, and has its other end connected to the inverting input terminal of the comparator 18 and to the collector of the phototransistor 20b of the photocoupler 20.

The comparator 18 has its positive power terminal connected to the steady-state operating current supply line 16a, and has its negative power terminal connected to the negative power supply line 2. The output terminal of the comparator 18 is connected by way of a line 15a to the control terminal of a switch circuit 17. The emitter of the phototransistor 20b is connected by way of a line 19a to the control terminal of the switching controller circuit 19. The output terminal of the switching controller circuit 19 is connected to the control terminal of the main switching device 5.

Next, the operation of the switching power supply apparatus of the second embodiment will be described. When a direct-current voltage from the operating power source 16 is applied between the positive and negative power supply lines 1 and 2, the main switching device 5, under the control of the switching controller circuit 19, performs switching operation, and thereby causes a high-frequency current to flow through the primary coil 4 of the transformer 3. This induces a high-frequency voltage in the secondary coil 6 of the transformer 3. This high-frequency voltage is rectified by the diode 7 and then smoothed by the capacitor 45, and is thereby converted into a direct-current voltage. This direct-current voltage is applied between the output lines 25 and 26 so as to be output as the output voltage of the switching power supply apparatus.

The output voltage detector circuit 9 compares the output voltage between the output lines 25 and 26 with a predetermined reference voltage, and feeds the result of the comparison in the form of a feedback signal, on one hand, via the node between the collector of the phototransistor 20b and the current detection resistor 28 to the signal level checker circuit 15 and, on the other hand, by way of the line 19a to the switching controller circuit 19.

More specifically, in the output voltage detector circuit 9, the shunt regulator 22 compares the voltage at the node between the output voltage division resistors 23 and 24 with a reference voltage that has previously been prepared internally, and makes a current commensurate with the result of the comparison flow through the photodiode 20a. The phototransistor 20b supplies a current commensurate with the current flowing through the photodiode 20a from the operating power source 16 through current detection resistor 28 to the switching controller circuit 19. Thus, according to the current supplied, the switching controller circuit 19 controls the switching operation of the main switching device 5, and thereby controls the output voltage of the switching power supply apparatus (the voltage between the output lines 25 and 26) so as to make it equal to a predetermined value.

In the signal level checker circuit 15, the comparator 18 compares the voltage drop across the current detection resistor 28 with the voltage of the current level check reference power generated by the Zener diode 191 and the resistor 201, and feeds a signal commensurate with the result of the comparison by way of the line 15a to the switch circuit 17. The Zener diode 191 may be replaced with a resistor.

When the load connected between the output lines 25 and 26 (the output terminals of the switching power supply apparatus) consumes a small amount of electric power (i.e., in light-load operation), the output voltage between the output lines 25 and 26 (the output voltage of the switching power supply apparatus) tends to be higher. To correct this, the output voltage detector circuit 9 increases the current flowing through the phototransistor 20b.

The comparator 18, as the result of comparing the current value through the phototransistor 20b with the reference current level set by the current level check reference power, outputs a high-level operation control signal, and feeds this high-level operation control signal by way of the line 15a to the control terminal of the switch circuit 17, which is thereby turned off. This stops the supply of the supply voltage to the switching controller circuit 19, and thus the switching controller circuit 19 stops operating. Consequently, the main switching device 5 stops operating, and thus the output voltage of the switching power supply apparatus decreases gradually.

As the output voltage decreases, the current value through the phototransistor 20b decreases. Then, the comparator 18, as the result of comparing the current value through the phototransistor 20b with the reference current level set by the current level check reference power, outputs a low-level operation control signal, and feeds this low-level operation control signal by way of the line 15a to the control terminal of the switch circuit 17, which is thereby turned on. This starts the supply of the supply voltage to the switching controller circuit 19, and thus the switching controller circuit 19 starts to operate. Consequently, the main switching device 5 starts to operate, and thus the output voltage of the switching power supply apparatus increases gradually.

As the output voltage increases, the current value through the phototransistor 20b increases. Then, the comparator 18, as the result of comparing the current value through the phototransistor 20b with the reference current level set by the current level check reference power, outputs a high-level operation control signal, and feeds this high-level operation control signal by way of the line 15a to the control terminal of the switch circuit 17, which is thereby turned off. This stops the supply of the supply voltage to the switching controller circuit 19, and thus the switching controller circuit 19 stops operating. Consequently, the main switching device 5 stops operating, and thus the output voltage of the switching power supply apparatus decreases gradually. Thereafter, this sequence of control is repeated, and burst oscillation is thereby maintained. In this way, the output voltage of the switching power supply apparatus is kept approximately constant.

Incidentally, among the operations described above, those belonging to the first part of the sequence described above, namely the turning off of the switch circuit 17, the stopping of the switching operation of the main switching device 5, the decrease in the output voltage, the decrease in the current through the phototransistor 20b, and the output of the low-level signal from the comparator 18, are not performed simultaneously, but, because of delays produced by various portions of the circuit, performing all these operations requires a certain operation time, and, during this operation time, the switching power supply apparatus stops switching operation.

Likewise, the operations belonging to the second part of the sequence described above, namely the turning on of the switch circuit 17, the starting of the switching operation of the main switching device 5, the increase in the output voltage, the increase in the current through the phototransistor 20b, and the output of the high-level signal from the comparator 18, are not performed simultaneously, but, because of delays produced by various portions of the circuit, performing all these operations requires a certain operation time, and, during this operation time, the switching power supply apparatus continues switching operation.

The theory that the operation times described above help maintain the periods during which the switching power supply apparatus keeps performing and stops performing switching operation applies not only in this embodiment but also in the first embodiment.

A small degree of hysteresis may be introduced in the control by slightly lowering the voltage level of the current level check reference power fed to the non-inverting input terminal of the comparator 18 at the same time that the switch circuit 17 is turned off and, likewise, slightly raising the voltage level of the current level check reference power fed to the non-inverting input terminal of the comparator 18 at the same time that the switch circuit 17 is turned on. This helps make longer the periods during which the switching power supply apparatus keeps performing and stops performing switching operation.

On the other hand, when the load connected between the output lines 25 and 26 consumes a large amount of electric power (i.e., in heavy-load operation), the output voltage between the output lines 25 and 26 tends to be lower. This causes the current flowing through the phototransistor 20b to decrease, and thus causes the voltage drop across the current detection resistor 28 to become lower than the voltage across the Zener diode 191. Accordingly, the comparator 18 outputs a low-level operation control signal, and thus the switch circuit 17 is kept continuously on, permitting the switching power supply apparatus to perform continuous switching operation.

Here, it should be noted that burst switching is achieved according to the result of comparison between the current value through the phototransistor 20b and the reference current value (the voltage across the Zener diode 191 as converted into a current value). The signal level of the feedback signal from the output voltage detector circuit 9 (i.e., the current value through the phototransistor 20b) represents the load current value of the switching power supply apparatus. Thus, it is possible to correctly set the load current value at which switching between continuous switching operation and burst switching operation is performed.

In burst switching operation, the output voltage fluctuates as described earlier. However, since the signal level of the feedback signal, i.e., the current value through the phototransistor 20b, also represents the output voltage value of the switching power supply apparatus as has already been described and will also be described later, it is possible to correctly set the upper and lower limits of the output voltage.

The signal level of the feedback signal may be detected on the line 19a leading to the control terminal of the switching controller circuit 19. However, as will be described later, this configuration cannot cope with a case where a current flows out of the switching controller circuit 19 via its control terminal. That is, as the supply of operating power to the switching controller circuit 19 is turned on and off, the current value flowing out of it via its control terminal varies, and thus the voltage value at the control terminal no longer correctly represents the output voltage and the load current as described earlier.

In this way, burst switching operation is achieved by repeatedly stopping switching operation when the output voltage of the switching power supply apparatus increases and restarting stitching operation when the output voltage decreases. This helps stabilize the output voltage.

Third Embodiment

FIG. 3 is a circuit diagram of the switching power supply apparatus of a third embodiment of the invention. FIG. 3 is a circuit diagram showing the detailed circuit configuration of the operating power source 16 shown in FIGS. 1 and 2. In FIG. 3, such circuit components that find their counterparts in FIGS. 1 and 2 are identified with the same reference numerals, and their explanations will not be repeated.

In FIG. 3, the operating power of the switching controller circuit 19 is supplied thereto by way of a start-up current supply line 29a by way of which a start-up current is supplied from the positive power supply line 1 through a start-up resistor 29, or by way of a steady-state operating currant supply line 16a by way of which a voltage induced in a subsidiary coil 32 of the transformer 3 is supplied through a serial circuit composed of a plurality of diodes 30 and 31. The operating power of the signal level checker circuit 15 and the phototransistor 20b of the photocoupler 20 is supplied thereto from subsidiary control power extracted from the node between the diodes 30 and 31.

The circuit corresponding to the operating power source 16 described earlier is composed of the subsidiary coil 32 of the transformer 3, the diode 31, a capacitor 33, the diode 30, the start-up resistor 29, and a capacitor 46. In this switching power supply apparatus, at the start of start-up, when a direct-current voltage from a nonillustrated direct-current power source is applied between the positive and negative power supply lines 1 and 2, a charge current flows through the capacitor 46 by way of the start-up resistor 29, and, as will be described later, since the switch circuit 17 is on, when the charge voltage of the capacitor 46 reaches a predetermined voltage level, the switching controller circuit 19 starts to operate and starts to supply the main switching device 5 with a drive signal.

Thus, the switching power supply apparatus starts switching operation, and a high-frequency voltage is induced in the subsidiary coil 32 of the transformer 3. This high-frequency voltage is rectified and smoothed by the diode 31 and the capacitor 33, and is thereby converted into a direct-current voltage. The phototransistor 20b and the comparator 18 operate from the operating power supplied thereto from the capacitor 33, and operate in such a way as to keep the output voltage of the switching power supply apparatus at a predetermined value and achieve burst switching control when the load is light as described earlier.

During the start-up operation of the switching power supply apparatus, the diode 30 prevents a current from flowing from the positive power supply line 1 through the start-up resistor 29 to the capacitor 33, and thus helps shorten the time required for the charge voltage of the capacitor 46 to reach the predetermined voltage level. On completion of the start-up of the switching power supply apparatus, the capacitor 46 is charged mainly by the current fed thereto from the capacitor 33 through the diode 30, and supplies operating power to the switching controller circuit 19 through the switch circuit 17.

When the switching power supply apparatus starts to start up, the charge voltage of the capacitor 33 is zero, and therefore the comparator 18 is not operating. However, since the output terminal of this comparator 18 is pulled down by a resistor 62, the switch circuit 17 is in an on state.

Likewise, when the switching power supply apparatus starts to start up, the charge voltage of the capacitor 33 is zero, and therefore no current flows through the phototransistor 20b. Thus, the switching controller circuit 19 controls the main switching device 5 on the assumption that the output voltage of the switching power supply apparatus is lower than the predetermined value. Thereafter, as the output voltage of the switching power supply apparatus increases, the charge voltage of the capacitor 33 increases until a current flows through the phototransistor 20b, when the switching power supply apparatus starts to operate in a predetermined steady state.

As described above, during the period after the switching power supply apparatus starts to start up until it starts to operate in the steady state, the switching controller circuit 19 and the switch circuit 17 operates by using as operating power the charge voltage of the capacitor 46. Accordingly, to prevent the charge voltage of the capacitor 46 from becoming lower than the permitted minimum operating voltage during that period, the capacitor 46 needs to be given a sufficiently high capacitance.

By increasing the resistance of the start-up resistor 29, it is possible to reduce the power loss through the start-up resistor 29. However, making the resistance too high results in lengthening the time required to charge the capacitor 46 when the switching power supply apparatus starts up, slowing down its start-up.

In this embodiment, when the switching power supply apparatus starts up, the diode 30 prevents the charge accumulated in the capacitor 46 from flowing out of it to the phototransistor 20b and the comparator 18. This helps reduce the time required for start-up. Moreover, by increasing the resistance of the start-up resistor 29, it is possible to reduce power consumption.

In the switching power supply apparatus of this embodiment, the signal level checker circuit 15 achieves burst switching control by repeatedly turning on and off the switch circuit 17 provided in the line by way of which the switching controller circuit 19 is supplied with operating power. Moreover, in burst switching control, while the switching operation of the main switching device 5 is being stopped, the supply of operating power to the switching controller circuit 19 is also stopped. This helps reduce the power loss suffered while the switching operation is being stopped, and thus helps reduce the power consumption of the apparatus as a whole.

Fourth Embodiment

FIG. 4 is a circuit diagram of the switching power supply apparatus of a fourth embodiment of the invention. In FIG. 4, such circuit components that find their counterparts in FIGS. 1 to 3 are identified with the same reference numerals, and their explanations will not be repeated. The switching power supply apparatus of this embodiment incorporates a PWM (pulse-width modulation) control IC, for example one with the product number FA5511 manufactured by Fuji Electric Co., Ltd. In FIG. 4, FA5511 is shown as an IC 38.

FIG. 6 shows an outline of the configuration of FA5511. In FIG. 6, when operating power is supplied to a Vcc terminal T6, this operating power is supplied to an output buffer 101, an operation control circuit 102, and a 5 V voltage regulator 103. When the voltage supplied via the Vcc terminal T6 becomes higher than a predetermined operation starting voltage, the 5 V voltage regulator 103 is brought into an output-enabled state, and thus supplies stabilized 5 V power, on one hand, by way of an internal supply line 104 to a PWM logic circuit 105 and an OSC (oscillation circuit ) 106 and, on the other hand, by way of the internal supply line 104 and then through a diode 107 and a resistor 108 to an FB terminal T2.

An internal power terminal T7 is connected to the internal supply line 104, and to this internal power terminal T7 is externally connected a capacitor 40 for eliminating noise from the internal supply line 104. This capacitor 40 prevents noise from being superimposed on the power supplied by way of the internal supply line 104, and thereby prevents erroneous control.

The oscillation frequency of the OSC 106 is set by the resistance of a resistor 36 that is externally connected via a terminal T1. The oscillation signal generated by the OSC 106 is fed to the PWM logic circuit 105. The FB terminal T2 is pulled up to the internal supply line 104 through a serial circuit composed of a diode 107 and a resistor 108, and thus a voltage divided by the serial circuit and a circuit element externally connected to the FB terminal T2 is supplied to the PWM logic circuit 105.

The PWM logic circuit 105 performs, in the manner that will be described later, logic calculation on the voltage level at the FB terminal T2, the voltage level at a CS terminal T8, which will be described later, and the oscillation signal fed from the OSC 106, and feeds the output buffer 101 with a drive signal for driving the main switching device 5 (see FIG. 4). The output buffer 101 current-amplifies the drive signal, and then feeds it as the drive signal to the main switching device 5, which is externally connected via an output terminal T5.

Via a terminal T3, a current detection signal from the main switching device 5 is fed in. When the current flowing through the main switching device 5 exceeds a predetermined level, the PWM logic circuit 105 shuts off the drive signal for the main switching device 5 (reduces it to a low level) to protect the main switching device 5. A terminal T4 serves as a common ground terminal of the internal circuit of FA5511, and is connected to the negative power supply line 2 (see FIG. 4) of the switching power supply apparatus.

To a CS terminal T8 is externally connected a capacitor 41. At the same time that the operation control circuit 102 outputs an output enable signal to the 5 V voltage regulator 103 as described earlier, the operation control circuit 102 feeds the capacitor 41 with a weak current, with which the capacitor 41 is charged gradually. When the switching power supply apparatus is operating in the steady state, the operation control circuit 102 controls the charge voltage of the capacitor 41 in such a way that it does not exceed a predetermined voltage level.

When the potential at the CS terminal T8 is forcibly turned low with an external circuit, the operation control circuit 102 disables the 5 V voltage regulator 103 and thereby stops the supply of power to the internal supply line 104, and simultaneously the 5 V voltage regulator 103 outputs a disable signal to the output buffer 101. Thus, when the potential at the CS terminal T8 is forcibly turned low with an external circuit, the power consumption by FA5511 is greatly reduced.

The switching power supply apparatus of this embodiment exploits the above-described function of FA5311. Specifically, when the output voltage of the switching power supply apparatus is high, and thus the signal level of the feedback signal is high, the signal level checker circuit 15 (see FIG. 4) forcibly turns the potential at the CS terminal T8 low with an external circuit, and thereby stops the operation of the output buffer 101, PWM logic circuit 105, and OSC 106. This causes the switching power supply apparatus to stop operating, and as a result the signal level of the feedback signal decreases. Then, the signal level checker circuit 15 ceases to forcibly turn the potential at the CS terminal T8 low, and thereby restarts the switching power supply apparatus. In this way, in light-load operation of the switching power supply apparatus, burst switching operation is achieved.

FIG. 5 shows, for reference proposes, the circuit configuration of a switching power supply apparatus having a typical circuit configuration in a case where it adopts FA5511. In FIG. 5, such circuit components that find their counterparts in FIG. 4 are identified with the same reference numerals, and their explanations will not be repeated. FIG. 7 shows the waveforms of the signals observed at relevant points in the switching power supply apparatus during the period after it starts to start up until it starts to operate in the steady state. In FIG. 7, at (a) is shown the voltage 701 across the capacitor 46 shown in FIG. 5; at (b) are shown the voltage 702 at the FB terminal T2 of the IC 38, i.e., FA5511, shown in FIG. 5, the oscillation signal 703 that the OSC 106 (see FIG. 6) feeds to the PWM logic circuit 105 (see FIG. 6), and the voltage 704 at the CS terminal T8; at (c) is shown the output signal 705 output via the output terminal T5.

Now, with reference to FIGS. 5 and 7, the operation of this switching power supply apparatus will be described. First, when, at a time point t0, a direct-current voltage is applied between the positive and negative power supply lines 1 and 2, the voltage 701 across the capacitor 46 increases gradually owing to a charge current supplied thereto through the start-up resistor 29. When, at a time point t1, the voltage reaches the predetermined operation starting voltage of FA5511, the voltage on the internal supply line 104 inside the IC 38 rises as described earlier, and thus the OSC 106, PWM logic circuit 105, and output buffer 101 start to operate.

Thus, the OSC 106 feeds the PWM logic circuit 105 with an oscillation signal 703 having constant upper and lower limits and a constant period, and, as a result of the capacitor 41 being charged with the weak current fed thereto from the operation control circuit 102, the voltage 704 at the CS terminal T8 increases gradually. At the time point t1, the voltage between the output lines 25 and 26 is still zero, and therefore no current flows through the shunt regulator 22 and the phototransistor 20b. Thus, the voltage 702 at the FB terminal T2 of the IC 38 is high.

When whichever of the voltage 704 at the CS terminal T8 and the voltage 702 at the FB terminal T2 is lower is higher than the voltage of the oscillation signal 703 output from the OSC 106, the PWM logic circuit 105 outputs an output signal (a pulse signal) 705 of which the level is higher than the voltage at the output terminal T5 of the output buffer 101. Thus, during the period from the time point t1 to a time point t2, during which the level of the voltage 704 at the CS terminal T8 is lower than the level of the oscillation signal 703 output from the OSC 106, the output signal 705 remains low. At the time point t2, when the level of the voltage 704 at the CS terminal T8 momentarily exceeds the level of the oscillation signal 703 of the OSC 106, the output signal 705 becomes high and then remains high for the corresponding period, turning the main switching device 5 on.

Thereafter, as the voltage 704 increases, the period during which the output signal 705 remains high becomes increasingly long, and correspondingly the power supplied from the secondary coil 6 of the transformer 3 through the diode 7 to between the output lines 25 and 26 increases. Thus, the voltage between the output lines 25 and 26 increases until, at a time point t3, a current starts to flow through the shunt regulator 22 and the phototransistor 20b, when the voltage 702 at the FB terminal T2 starts to decrease.

Next, when, at a time point t5, the voltage 702 at the FB terminal T2 becomes lower than the voltage 704 at the CS terminal T8, the period during which the output signal 705 at the output terminal T5 is high is determined by the result of comparison between the level of the oscillation signal 703 of the OSC 106 and the voltage 702 at the FB terminal T2. Since the level of the voltage 702 represents the feedback signal output from the output voltage detector circuit 9, the switching power supply apparatus now starts to operate in the steady state in which it outputs a predetermined voltage.

On the other hand, the charge voltage 701 of the capacitor 46 tends to slightly decrease during the period from the time point t1 to the time point t3, because during that period more current flows to the Vcc terminal T6 than is supplied from the start-up resistor 29. However, this decrease is so controlled as not to go below the minimum operating Vcc voltage of the IC 38, i.e., FA5511, by giving the capacitor 46 a sufficiently high capacitance.

As described earlier, the output voltage of the switching power supply apparatus increases, and correspondingly the charge voltage 701 of the capacitor 46 starts to increase at a time point t4 and reaches the steady-state stable voltage at a time point t6.

It is to be understood that the circuit configuration of the switching power supply apparatus explained with reference to FIG. 5 is a mere example of a typical circuit configuration in a case where FA5511 is adopted, and thus does not incorporate the function of achieving burst switching in light-load operation which will be described later in connection with this particular embodiment.

Next, the operation of the switching power supply apparatus of this embodiment shown in FIG. 4 will be described with reference to a signal waveform diagram shown in FIG. 8. In FIG. 8, at (a) is shown the voltage 801 across the capacitor 46 shown in FIG. 4; at (b) are shown the voltage 804 at the FB terminal T2 of the IC 38, i.e., FA5511, shown in FIG. 4, the oscillation signal 803 that the OSC 106 (see FIG. 6) feeds to the PWM logic circuit 105 (see FIG. 6), and the voltage 805 at the CS terminal T8 of the IC 38; at (c) is shown the output signal 806 output via the output terminal T5 of the IC 38.

First, when, at a time point T0, a direct-current voltage is applied between the positive and negative power supply lines 1 and 2, the voltage 801 across the capacitor 46 increases gradually owing to a charge current supplied thereto through the start-up resistor 29. When, at a time point T1, the voltage reaches the predetermined operation starting voltage of FA5511, the voltage on the internal supply line 104 inside the IC 38 rises as described earlier, and thus the OSC 106, PWM logic circuit 105, and the output buffer 101 start to operate.

Thus, the OSC 106 feeds the PWM logic circuit 105 with an oscillation signal 803 having constant upper and lower limits and a constant period, and, as a result of the capacitor 41 being charged with the weak current fed thereto from the operation control circuit 102, the voltage 805 at the CS terminal T8 increases gradually. At the time point T1, the charge voltage of the capacitor 33 is zero, the output current of the signal level checker circuit 15 is zero, and the switch of a start-up corrector circuit 35 is, as will be described later, off. Accordingly, the voltage 804 at the FB terminal T2 of the IC 38 is a division voltage that results from voltage division by the diode 107 provided inside the IC 38, the resistor 108, and a resistor 39a (see FIG. 4).

This division voltage has its value set to be slightly higher than the lower-limit voltage level of the oscillation signal 803.

When whichever of the voltage 805 at the CS terminal T8 and the voltage 804 at the FB terminal T2 is lower is higher than the voltage level of the oscillation signal 803 output from the OSC 106, the PWM logic circuit 105 outputs an output signal 806 via the output terminal T5 of the output buffer 101.

Thus, during the period from the time point T1 to a time point T2, during which the level of the voltage 805 at the CS terminal T8 is lower than the level of the oscillation signal 703 output from the OSC 106, the output signal 806 remains low. At the time point T2, when the level of the voltage 805 at the CS terminal T8 momentarily exceeds the level of the oscillation signal 803 of the OSC 106, the output signal 705 becomes high and then remains high for the corresponding period, turning the main switching device 5 on.

This causes the voltage between the output lines 25 and 26 to slightly increase, and thus causes the charge voltage of the capacitor 33 to increase in such a way as to correspond to the increase in the voltage between the output lines 25 and 26. Consequently, a current starts to be supplied from the capacitor 33 through the signal level checker circuit 15 to the FB terminal T2 of the IC 38, and thus the voltage 804 at the FB terminal T2 starts to increase.

When the current value through the phototransistor 20b is lower than a predetermined value set within the signal level checker circuit 15, the signal level checker circuit 15 supplies a current to the FB terminal T2 of the IC 38; by contrast, when the current value through the phototransistor 20b is higher than the predetermined value set within the signal level checker circuit 15, the signal level checker circuit 15 feeds a current to a CS terminal controller circuit 37, but supplies no current to either of the FB terminal T2 and the CS terminal T8.

While the signal level checker circuit 15 is supplying a current to the FB terminal T2 of the IC 38, when the current value through the phototransistor 20b increases, the signal level checker circuit 15 decreases the supply current (inverted feedback signal); by contrast, when the current value through the phototransistor 20b decreases, the signal level checker circuit 15 increases the supply current (inverted feedback signal).

The supply current also depends on the operating power of the signal level checker circuit 15; that is, it also depends on the charge voltage of the capacitor 33. Thus, as described earlier, after the switching power supply apparatus starts to start up, as the voltage between the output lines 25 and 26 increases, and thus as the charge voltage of the capacitor 33 increases, the supply current increases.

Thereafter, when the supply current increases until the steady-operation state is reached in which the voltage between the output lines 25 and 26 are stabilized, the voltage between the output lines 25 and 26 and the charge voltage of the capacitor 33 are stabilized at constant values determined by the predetermined output voltage of the switching power supply apparatus and the winding ratio between the secondary coil 6 and the subsidiary coil 32 of the transformer 3. Thus, now, the supply current depends solely on the current value through the phototransistor 20b as described above.

Next, after the time point T2, as the voltage 805 at the CS terminal T8 increases, the period during which the output signal 806 output via the output terminal T5 remains high becomes increasingly long, thus the voltage between the output lines 25 and 26 increases, thus the charge voltage of the capacitor 33 increases, and thus the current supplied from the signal level checker circuit 15 increases. As a result of this course of events, the voltage 804 at the FB terminal T2 increases gradually.

After a time point T3, when the voltage 805 at the CS terminal T8 becomes higher than the voltage 804 at the FB terminal T2, as described earlier, the PWM logic circuit 105 compares the voltage 804 at the FB terminal T2 with the oscillation signal 803 of the OSC 106, and, according to the result of the comparison, outputs the output signal 806 through the output buffer 101 via the output terminal T5 so as to feed the output signal 806 as the drive signal to the main switching device 5.

As described above, the charge voltage of the capacitor 33 depends on the voltage between the output lines 25 and 26 and the winding ratio between the secondary coil 6 and subsidiary coil 32 of the transformer 3. Thus, after the time point T2, as the voltage between the output lines 25 and 26 increases, the charge voltage of the capacitor 33 increases describing a curve 802 shown at (a) in FIG. 8. When, at a time point T4, the voltage of the capacitor 33 becomes higher than a predetermined