Control circuit and method for a flyback power converter to compensate for an entry point of a burst mode

• US 8,406,015 B2
• Filed: 05/12/2009
• Issued: 03/26/2013
• 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
• Primary Examiner(s): Berhane, Adolf
• Assistant Examiner(s): NASH, GARY A
• Application Number:
  US12/453,446
• Publication Number:
  US 20090284994A1
• Time in Patent Office: 1,414 Days
• Field of Search: 323/287, 363 2112- 2118
• US Class Current: 363/21.12
• 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 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;

determining whether to control the flyback converter to enter a burst mode according to the feedback signal and a preset value;

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.

2. The control method of claim 1, 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.

3. The control method of claim 2, wherein the step of determining whether to control the flyback converter to enter a burst mode according to the feedback signal and a preset value comprises generating a mask signal according to the feedback signal and the preset value to mask the clock.

4. The control method of claim 1, wherein the step of generating the compensated feedback signal comprises deducting a sawtooth wave from the feedback signal to generate the compensated feedback signal.

5. 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;

wherein the sawtooth wave is used to stabilize an entry point of a burst mode.

6. The control circuit of claim 5, 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.

7. The control circuit of claim 5, 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.

8. The control circuit of claim 5, further comprising a burst circuit to determine whether to control the flyback converter to enter the burst mode according to the feedback signal and a preset value.

9. The control circuit of claim 5, 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.

10. The control circuit of claim 9, wherein the comparator is a hysteresis comparator.

11. 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;

determining whether to control the flyback converter to enter a burst mode according to the feedback signal and a preset value;

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.

12. The control method of claim 11, 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.

13. The control method of claim 12, wherein the step of determining whether to control the flyback converter to enter a burst mode according to the feedback signal and a preset value comprises generating a mask signal according to the feedback signal and the preset value to mask the clock.

14. The control method of claim 11, 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.