Systems and methods for reducing switching loss in power conversion systems

1. A power converter, the power converter comprising:

a primary winding;

a secondary winding coupled to the primary winding;

a first switch including a first switch terminal, a second switch terminal, and a third switch terminal, the first switch being configured to affect a first current associated with the primary winding, the first switch terminal corresponding to a first voltage, the second switch terminal corresponding to a second voltage, the first voltage minus the second voltage being equal to a voltage difference;

a second switch including a fourth switch terminal, a fifth switch terminal, and a sixth switch terminal and configured to affect a second current associated with the secondary winding;

a sampled-voltage generator configured to sample a third voltage before the first switch becomes closed and generate a sampled voltage based at least in part on the third voltage, the third voltage being proportional to the voltage difference if the first switch is not closed;

an error amplifier configured to receive the sampled voltage and a reference voltage and generate an amplified voltage based at least in part on the sampled voltage and the reference voltage;

a threshold voltage generator configured to generate a threshold voltage based on at least information associated with the amplified voltage; and

a drive signal generator configured to receive the threshold voltage and a fourth voltage, generate a drive signal based at least in part on the threshold voltage and the fourth voltage, and output the drive signal to the second switch, the fourth voltage representing the second current;

wherein the drive signal generator is further configured to, in response to the fourth voltage becoming larger than the threshold voltage, generate the drive signal to open the second switch;

wherein the power converter is configured to, in response to the sampled voltage and the reference voltage not being equal, change the threshold voltage with time.

2. The power converter of claim 1 is further configured, in response to the sampled voltage being larger than the reference voltage, increase the threshold voltage with time.

3. The power converter of claim 2 is further configured to cause the sampled voltage to decrease with the increasing threshold voltage.

4. The power converter of claim 1 is further configured, in response to the sampled voltage being smaller than the reference voltage, decrease the threshold voltage with time.

5. The power converter of claim 4 is further configured to cause the sampled voltage to increase with the decreasing threshold voltage.

6. The power converter of claim 1 is further configured to, in response to the sampled voltage and the reference voltage being equal, keep the threshold voltage constant with time.

7. The power converter of claim 6 is further configured to cause the sampled voltage to remain constant in response to the constant threshold voltage.

8. The power converter of claim 1 wherein the drive signal generator includes:

a comparator configured to receive the threshold voltage and the fourth voltage and generate a comparison signal based at least in part on the threshold voltage and the fourth voltage; and

a flip-flop configured to receive the comparison signal and generate the drive signal based at least in part on the comparison signal.

9. The power converter of claim 1 wherein:

the first switch is further configured to affect the first current flowing through the primary winding;

the second switch is further configured to affect the second current flowing through the secondary winding; and

the drive signal generator is further configured to receive the fourth voltage, the fourth voltage being proportional to the second current.

10. The power converter of claim 1 wherein the threshold voltage generator is further configured to:

increase the threshold voltage in response to the decreasing amplified voltage; and

decrease the threshold voltage in response to the increasing amplified voltage.

11. A controller for a power converter, the controller comprising:

a sampled-voltage generator configured to sample a first voltage before a first switch becomes closed and generate a sampled voltage based at least in part on the first voltage, the first voltage being proportional to a voltage difference if the first switch is not closed, the first switch including a first switch terminal, a second switch terminal, and a third switch terminal and configured to affect a first current associated with a primary winding of the power converter, the first switch terminal corresponding to a second voltage, the second switch terminal corresponding to a third voltage, the voltage difference being equal to the second voltage minus the third voltage; and

an error amplifier configured to receive the sampled voltage and a reference voltage, generate an amplified voltage based at least in part on the sampled voltage and the reference voltage, and output the amplified voltage in order for a threshold voltage generator to generate a threshold voltage;

wherein the threshold voltage generator is configured to:

generate the threshold voltage based on at least information associated with the amplified voltage; and

output the threshold voltage to a drive signal generator;

wherein the drive signal generator is configured to:

receive the threshold voltage and a fourth voltage;

generate a drive signal based at least in part on the threshold voltage and the fourth voltage, the fourth voltage representing a second current associated with a secondary winding of the power converter, the secondary winding coupled to the primary winding; and

output the drive signal to a second switch, the second switch including a fourth switch terminal, a fifth switch terminal, and a sixth switch terminal and configured to affect the second current;

wherein the drive signal generator is further configured to, in response to the fourth voltage becoming larger than the threshold voltage, generate the drive signal to open the second switch;

wherein the controller is configured to, in response to the sampled voltage and the reference voltage not being equal, generate the amplified voltage to change the threshold voltage with time.

12. The controller of claim 11 is further configured, in response to the sampled voltage being larger than the reference voltage, generate the amplified voltage to increase the threshold voltage with time.

13. The controller of claim 12 wherein the sampled voltage decreases with the increasing threshold voltage.

14. The controller of claim 11 is further configured, in response to the sampled voltage being smaller than the reference voltage, generate the amplified voltage to decrease the threshold voltage with time.

15. The controller of claim 14 wherein the sampled voltage increases with the decreasing threshold voltage.

16. The controller of claim 11 is further configured to, in response to the sampled voltage and the reference voltage being equal, generate the amplified voltage to keep the threshold voltage constant with time.

17. The controller of claim 16 wherein the sampled voltage remains constant in response to the constant threshold voltage.

18. A controller for a power converter, the controller comprising:

a threshold voltage generator configured to generate a threshold voltage based on at least information associated with an amplified voltage generated by an error amplifier; and

a drive signal generator configured to receive the threshold voltage and a first voltage, generate a drive signal based at least in part on the threshold voltage and the first voltage, and output the drive signal to a first switch, the first voltage representing a first current, the first switch including a first switch terminal, a second switch terminal, and a third switch terminal and configured to affect the first current associated with a secondary winding coupled to a primary winding of the power converter;

wherein the error amplifier is configured to:

receive a sampled voltage from a sampled-voltage generator and a reference voltage; and

generate the amplified voltage based at least in part on the sampled voltage and the reference voltage;

wherein the sampled-voltage generator is configured to:

sample a second voltage before a second switch becomes closed; and

generate the sampled voltage based at least in part on the second voltage, the second voltage being proportional to a voltage difference if the second switch is not closed;

wherein the second switch including a fourth switch terminal, a fifth switch terminal, and a sixth switch terminal, the second switch being configured to affect a second current associated with the primary winding, the fourth switch terminal corresponding to a third voltage, the fifth switch terminal corresponding to a fourth voltage, the third voltage minus the fourth voltage being equal to the voltage difference;

wherein the drive signal generator is further configured to, in response to the first voltage becoming larger than the threshold voltage, generate the drive signal to open the first switch;

wherein the threshold voltage generator is further configured to, in response to the sampled voltage and the reference voltage not being equal, change the threshold voltage with time.

19. The controller of claim 18 is further configured, in response to the sampled voltage being larger than the reference voltage, increase the threshold voltage with time.

20. The controller of claim 19 wherein the sampled voltage decreases with the increasing threshold voltage.

21. The controller of claim 18 is further configured, in response to the sampled voltage being smaller than the reference voltage, decrease the threshold voltage with time.

22. The controller of claim 21 wherein the sampled voltage increases with the decreasing threshold voltage.

23. The controller of claim 18 is further configured to, in response to the sampled voltage and the reference voltage being equal, keep the threshold voltage constant with time.

24. The controller of claim 23 wherein the sampled voltage remains constant in response to the constant threshold voltage.

25. The controller of claim 18 wherein the drive signal generator includes:

a comparator configured to receive the threshold voltage and the first voltage and generate a comparison signal based at least in part on the threshold voltage and the first voltage; and

a flip-flop configured to receive the comparison signal and generate the drive signal based at least in part on the comparison signal.

26. The power converter of claim 18 wherein:

the first switch is further configured to affect the first current flowing through the secondary winding;

the second switch is further configured to affect the second current flowing through the primary winding; and

the drive signal generator is further configured to receive the first voltage, the first voltage being proportional to the first current.

27. The controller of claim 18 wherein the threshold voltage generator is further configured to:

increase the threshold voltage in response to the decreasing amplified voltage; and

decrease the threshold voltage in response to the increasing amplified voltage.

28. A method for a power converter, the method comprising:

sampling a first voltage before a first switch becomes closed, the first switch including a first switch terminal, a second switch terminal, and a third switch terminal, the first switch being configured to affect a first current associated with a primary winding of the power converter;

generating a sampled voltage based at least in part on the first voltage, the first voltage being proportional to a voltage difference if the first switch is not closed, the first switch terminal corresponding to a second voltage, the second switch terminal corresponding to a third voltage, the second voltage minus the third voltage being equal to the voltage difference;

receiving the sampled voltage and a reference voltage;

generating an amplified voltage based at least in part on the sampled voltage and the reference voltage;

generating a threshold voltage based on at least information associated with the amplified voltage;

receiving the threshold voltage and a fourth voltage;

generating a drive signal based at least in part on the threshold voltage and the fourth voltage; and

outputting the drive signal to a second switch, the fourth voltage representing a second current, the second switch including a fourth switch terminal, a fifth switch terminal, and a sixth switch terminal and configured to affect the second current associated with a secondary winding coupled to the primary winding;

wherein the generating a drive signal based at least in part on the threshold voltage and the fourth voltage includes, in response to the fourth voltage becoming larger than the threshold voltage, generating the drive signal to open the second switch;

wherein the generating a threshold voltage based on at least information associated with the amplified voltage includes, in response to the sampled voltage and the reference voltage not being equal, changing the threshold voltage with time.

29. The method of claim 28 wherein the generating a threshold voltage based on at least information associated with the amplified voltage includes, in response to the sampled voltage being larger than the reference voltage, increasing the threshold voltage with time.

30. The method of claim 29 , and further comprising causing the sampled voltage to decrease with the increasing threshold voltage.

31. The method of claim 28 wherein the generating a threshold voltage based on at least information associated with the amplified voltage includes, in response to the sampled voltage being smaller than the reference voltage, decreasing the threshold voltage with time.

32. The method of claim 31 , and further comprising causing the sampled voltage to increase with the decreasing threshold voltage.

33. The method of claim 28 wherein the generating a threshold voltage based on at least information associated with the amplified voltage includes, in response to the sampled voltage and the reference voltage being equal, keeping the threshold voltage constant with time.

34. The method of claim 33 , and further comprising causing the sampled voltage to remain constant in response to the constant threshold voltage.

35. The method of claim 28 wherein the generating a threshold voltage based on at least information associated with the amplified voltage includes:

receiving the threshold voltage;

generating a comparison signal based at least in part on the threshold voltage and the fourth voltage;

receiving the comparison signal; and

generating the drive signal based at least in part on the comparison signal.

36. The method of claim 28 wherein:

the first current flows through the primary winding;

the second current flows through the secondary winding; and

the fourth voltage is proportional to the second current.

37. The method of claim 28 wherein the generating a threshold voltage based on at least information associated with the amplified voltage includes:

increasing the threshold voltage in response to the decreasing amplified voltage; and

decreasing the threshold voltage in response to the increasing amplified voltage.