Converter and driving method thereof

1. A power converter comprising:

a transformer comprising a primary side that includes a primary winding and a secondary side that includes a secondary winding;

a main switch coupled to a first end of the primary winding;

a clamp capacitor and a clamp switch coupled between the first end of the primary winding and a second end of the primary winding;

a first output unit coupled to the secondary winding and including a first diode;

a second output unit coupled to an auxiliary winding; and

a switching controller configured to vary a switching frequency of the clamp switch with an output load of the converter and to control the main switch in response to detection of zero-crossing of a current flowing to the first diode,

wherein the switching controller indirectly receives information of the current flowing to the first diode of the first output unit through the second output unit.

2. The converter of claim 1 , wherein the switching controller is further configured to turn on the main switch in response to the detection of zero-crossing of the current and to complementarily switch the main switch and the clamp switch.

3. The converter of claim 1 , wherein the second output unit includes a second diode and the information of the current corresponds to an anode voltage of the second diode coupled to the auxiliary winding.

4. The converter of claim 3 , wherein the switching controller includes:

a comparator comparing the anode voltage of the second diode and a reference voltage; and

a latch generating a first switching control signal in response to an output of the comparator to control the main switch.

5. The converter of claim 4 , wherein the switching controller further includes an inverting device inverting the first switching control signal to generate a second switching control signal and providing the second switching signal to the clamp switch.

6. The converter of claim 1 , wherein the auxiliary winding shares a common ground with the primary winding.

7. The converter of claim 1 , wherein the second output unit includes a ground that is the same as a primary side ground.

8. A circuit for controlling a power converter, the circuit comprising:

a first output unit coupled to a secondary winding and including a first diode;

a second output unit coupled to an auxiliary winding and including a node, the node generating a signal indicative of a current flowing through the first diode; and

a switching controller configured to vary a switching frequency of a main switch with an output load of the power converter and to turn on the main switch in response to the signal indicating that the current flowing through the first diode is decreased to zero.

9. The circuit of claim 8 , further comprising:

a clamp switch coupled to the main switch; and

a clamp capacitor having a first end coupled to the clamp switch and a second end coupled to a primary winding of the power converter,

wherein the switching controller is further configured to complementarily switch the main switch and the clamp switch.

10. The circuit of claim 9 , wherein the second output unit further includes a second diode and the signal is an anode voltage of the second diode, and

wherein the switching controller includes:

a first switching controller including a comparator comparing the anode voltage of the second diode and a reference voltage; and

a latch generating a first switching control signal in response to an output of the comparator to control the main switch.

11. The circuit of claim 10 , wherein the switching controller further includes an inverting device inverting the first switching control signal to generate a second switching control signal and providing the second switching signal to the clamp switch.

12. The circuit of claim 8 , wherein the power converter comprises a transformer comprising a primary side that includes a primary winding and a secondary side that includes the secondary winding.

13. The circuit of claim 8 ,

wherein the first switching controller indirectly receives information of the current flowing to the first diode of the first output unit through the second output unit.

14. The circuit of claim 8 , wherein the main switch is coupled between a first terminal of a primary winding of the power converter and a ground, and

wherein the power converter comprises a clamp switch coupled between the first terminal of the primary winding and a first end of a clamp capacitor, the clamp capacitor having a second end coupled to a second terminal of the primary winding.

15. The circuit of claim 8 ,

wherein the switching controller includes:

a first switching controller controlling a turn-on time of the main switch in response to the signal indicative of the current flowing through the first diode; and

a second switching controller controlling a turn-off time of the main switch in response to a signal indicative of an output voltage of the first output unit.

16. A method of controlling a power converter, wherein the power converter includes a first output unit coupled to a secondary winding and including a first diode and a second output unit coupled to an auxiliary winding and including a node, the method comprising:

generating a signal at the node, the signal indicative of a current flowing through the first diode;

varying a switching frequency of a main switch with an output load of the power converter; and

turning on the main switch in response to the signal indicating that the current flowing through the first diode is decreased to zero.

17. The method of claim 16 , wherein the power converter further includes a clamp switch that is coupled to the main switch and a clamp capacitor, the clamp capacitor having a first end coupled to the clamp switch and a second end coupled to a primary winding of the power converter, the method further comprising complementarily switching the main switch and the clamp switch.

18. The method of claim 17 , wherein the signal is an anode voltage of a second diode included in the second output unit, the method further comprising:

comparing the anode voltage of the second diode and a reference voltage; and

generating a first switching control signal in response to a comparison result to control the main switch.

19. The method of claim 18 , further comprising:

inverting the first switching control signal to generate a second switching control signal; and

providing the second switching signal to the clamp switch.

20. The method of claim 16 , wherein varying the switching frequency of the main switch includes increasing the switching frequency as the output load of the power converter is decreased.