Control circuit and method for a flyback converter

• US 20090284994A1
• Filed: 05/12/2009
• Published: 11/19/2009
• Est. Priority Date: 05/14/2008
• Status: Active Grant

• Current Assignee: Richtek Technology Corporation (MediaTek, Inc.)
• Original Assignee: Richtek Technology Corporation (MediaTek, Inc.)
• Inventors: Lin, Tzu-Chen, Huang, Pei-Lun
• Granted Patent: US 8,406,015 B2
• Time in Patent Office: Days
• US Class Current: 363/21.130
• CPC Class Codes: H02M 1/0032 Control circuits allowing low power mode operation, e.g. in standby mode
• CPC Class Codes: H02M 3/335 using semiconductor devices only
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A control circuit and method are provided for a flyback converter converting an input voltage to an output voltage, to compensate for an entry point of a burst mode of the flyback converter, so that the entry point is not affected by the input voltage, and audible noise resulted from a higher input voltage is reduced without impacting the light load efficiency of the flyback converter.

Claims

1. A control circuit for a flyback converter including a transformer connected with a power switch switched by a control signal for the transformer to convert an input voltage into an output voltage, the control circuit comprising:

a compensator compensating a feedback signal with a sawtooth wave to generate a compensated feedback signal, the feedback signal being a function of the output voltage; and

a pulse width modulation circuit connected to the compensator to generate the control signal according to the compensated feedback signal and a current sense signal, the current sense signal being a function of a current flowing through the power switch.

2. The control circuit of claim 1, wherein the pulse width modulation circuit comprises:

a comparator connected to the compensator to compare the compensated feedback signal with the current sense signal to generate a comparison signal; and

a flip-flop connected to the comparator to generate the control signal according to the comparison signal and a clock.

3. The control circuit of claim 1, wherein the compensator comprises an adder having a positive input to receive the feedback signal, a negative input to receive the sawtooth wave, and an output to provide the compensated feedback signal.

4. The control circuit of claim 1, further comprising a burst circuit to determine, according to the feedback signal and a preset value, whether or not the flyback converter to enter a burst mode.

5. The control circuit of claim 4, wherein the burst circuit comprises a comparator to generate a mask signal according to the feedback signal and the preset value to reduce the number of times the power switch is to be switched.

6. The control circuit of claim 5, wherein the comparator is a hysteresis comparator.

7. The control circuit of claim 4, wherein the sawtooth wave is used to stabilize an entry point of the burst mode.

8. A control method for a flyback converter including a transformer connected with a power switch switched by a control signal for the transformer to convert an input voltage into an output voltage, the control method comprising:

detecting the output voltage to generate a feedback signal;

detecting a current flowing through the power switch to generate a current sense signal;

according to the feedback signal and a preset value, determining whether or not the flyback converter to enter a burst mode;

compensating the feedback signal to generate a compensated feedback signal; and

generating the control signal according to the compensated feedback signal and the current sense signal;

wherein the feedback signal is compensated to prevent an entry point of the burst mode from being affected by the input voltage.

9. The control method of claim 8, wherein the step of generating the control signal comprises:

comparing the compensated feedback signal with the current sense signal to generate a comparison signal; and

generating the control signal according to the comparison signal and a clock.

10. The control method of claim 9, wherein the step of determining whether or not the flyback converter to enter a burst mode comprises generating a mask signal according to the feedback signal and the preset value to mask the clock.

11. The control method of claim 8, wherein the step of generating the compensated feedback signal comprises deducting a sawtooth wave from the feedback signal to generate the compensated feedback signal.

12. A control circuit for a flyback converter including a transformer connected with a power switch switched by a control signal for the transformer to convert an input voltage into an output voltage, the control circuit comprising:

a compensator compensating a current sense signal with a sawtooth wave to generate a compensated current sense signal, the current sense signal being a function of a current flowing through the power switch; and

a pulse width modulation circuit connected to the compensator to generate the control signal according to a feedback signal and the compensated current sense signal, the feedback signal being a function of the output voltage.

13. The control circuit of claim 12, wherein the pulse width modulation circuit comprises:

a comparator connected to the compensator to compare the feedback signal with the compensated current sense signal to generate a comparison signal; and

a flip-flop connected to the comparator to generate the control signal according to the comparison signal and a clock.

14. The control circuit of claim 12, wherein the compensator comprises an adder having a first positive input to receive the current sense signal, a second positive input to receive the sawtooth wave, and an output to provide the compensated current sense signal.

15. The control circuit of claim 12, further comprising a burst circuit to determine, according to the feedback signal and a preset value, whether or not the flyback converter to enter a burst mode.

16. The control circuit of claim 15, wherein the burst circuit comprises a comparator to generate a mask signal according to the feedback signal and the preset value to reduce the number of times the power switch is to be switched.

17. The control circuit of claim 16, wherein the comparator is a hysteresis comparator.

18. The control circuit of claim 15, wherein the sawtooth wave is used to stabilize an entry point of the burst mode.

19. A control method for a flyback converter including a transformer connected with a power switch switched by a control signal for the transformer to convert an input voltage into an output voltage, the control method comprising:

detecting the output voltage to generate a feedback signal;

detecting a current flowing through the power switch to generate a current sense signal;

according to the feedback signal and a preset value, determining whether or not the flyback converter to enter a burst mode;

compensating the current sense signal to generate a compensated current sense signal; and

generating the control signal according to the feedback signal and the compensated current sense signal;

wherein the current sense signal is compensated to prevent an entry point of the burst mode from being affected by the input voltage.

20. The control method of claim 19, wherein the step of generating the control signal comprises:

comparing the feedback signal with the compensated current sense signal to generate a comparison signal; and

generating the control signal according to the comparison signal and a clock.

21. The control method of claim 20, wherein the step of determining whether or not the flyback converter to enter a burst mode comprises generating a mask signal according to the feedback signal and the preset value to mask the clock.

22. The control method of claim 19, wherein the step of generating the compensated current sense signal comprises adding a sawtooth wave to the current sense signal to generate the compensated current sense signal.