Active clamp flyback converters and control methods thereof

1. A control method in use of a power converter using an active-clamp flyback topology, the power converter comprising an active-clamp circuit connected in parallel with a primary winding of a transformer, the active-clamp circuit comprising a high-side switch and a capacitor connected in series, the power converter further comprising a low-side switch connecting the primary winding to a first power line, the control method comprising:

switching the low-side switch to generate consecutive switching cycles including a modified flyback cycle and a normal flyback cycle;

making each of the consecutive switching cycles having a cycle time not less than a blanking time generated in response to a load of the power converter;

constantly turning the high-side switch OFF during the normal flyback cycle; and

turning the high-side switch ON after the blanking time during the modified flyback cycle to perform zero-voltage switching for the low-side switch.

2. The control method as claimed in claim 1 , wherein a terminal voltage at the low-side switch oscillates after a demagnetization time to generate at least one peak and one valley, the control method comprising:

detecting whether the peak appears; and

turning the high-side switch ON about when the peak appears.

3. The control method as claimed in claim 1 , wherein a terminal voltage at the low-side switch oscillates after a demagnetization time to generate at least one peak and one valley, the control method comprising:

detecting whether the valley appears; and

turning the high-side switch ON about when the valley appears.

4. The control method as claimed in claim 1 , wherein only one of the consecutive switching cycles is a modified flyback cycle.

5. The control method as claimed in claim 1 , comprising:

switching the low-side switch to repeatedly generate the consecutive switching cycles.

6. The control method as claimed in claim 5 , wherein the consecutive switching cycles are N in total and N is a predetermined constant.

7. The control method as claimed in claim 5 , comprising:

providing a counter counting a number of the consecutive switching cycles.

8. The control method as claimed in claim 1 , wherein the consecutive switching cycles are N in total and the transformer comprises an auxiliary winding, the control method comprising:

adaptively changing N to keep a voltage at an end of the primary winding substantially staying within an acceptable range.

9. The control method as claimed in claim 1 , comprising:

during the modified flyback cycle, turning the high-side switch ON only after the blanking time.

10. A power converter using an active-clamp flyback topology, comprising:

a transformer with a primary winding;

a low-side switch for connecting the primary winding to a first power line;

a high-side switch connected in series with a capacitor to form an active-clamp circuit connected in parallel with the primary winding; and

a control circuit, configured to control the high-side switch and the low-side switch in response to a compensation signal and a current-sense signal;

wherein the control circuit selectively operates the power converter in one of operation modes including a flyback mode;

when operated in the flyback mode, the control circuit switches the low-side switch to generate consecutive switching cycles including a normal flyback cycle and a modified flyback cycle;

during a normal flyback cycle, the control circuit constantly turns the high-side switch OFF;

during a modified flyback cycle, the control circuit turns ON the high-side switch after a blanking time to perform zero-voltage switching for the low-side switch; and

the control circuit determines the blanking time in response to the compensation signal.

11. The power converter as claimed in claim 10 , wherein when operated in the flyback mode, the control circuit makes the current-sense signal have a signal peak independent from the compensation signal.

12. The power converter as claimed in claim 10 , wherein the consecutive switching cycles are N in total and N is a predetermined constant.

13. The power converter as claimed in claim 10 , wherein the consecutive switching cycles are N in total and N is determined in response to a voltage at an end of the primary winding.

14. The power converter as claimed in claim 10 , wherein a terminal voltage at the low-side switch oscillates after a demagnetization time to generate at least one peak and one valley, the control circuit detects whether the peak appears, and the control circuit turns the high-side switch ON about when the peak appears.

15. The power converter as claimed in claim 10 , wherein a terminal voltage at the low-side switch oscillates after a demagnetization time to generate at least one peak and one valley, the control circuit detects whether the valley appears, and the control circuit turns the high-side switch ON about when the valley appears.

16. The power converter as claimed in claim 10 , wherein the control circuit turns ON the high-side switch right after an end of the blanking time.