Power supply with near valley switching

1. An electrical circuit comprising:

a controller that is configured to detect a zero-crossing of a drain-source resonance voltage of a metal oxide field effect transistor (MOSFET) that is coupled to a primary winding of a transformer relative to an input voltage, to generate a signal indicating occurrence of a near valley window time period in response to detecting the zero-crossing of the drain-source resonance voltage of the MOSFET, and to turn on the MOSFET before or after a minimum value of the drain-source resonance voltage of the MOSFET within a range of time during the near valley window time period when a blanking period during which the MOSFET is prohibited from being turned on ends within the near valley window time period,

wherein the controller comprises:

a clamping circuit that is configured to clamp a sensed drain-source voltage of the MOSFET to generate a clamped voltage;

a replica circuit that generates a replica of the clamped voltage within the controller; and

a zero-crossing detection circuit that detects the zero-crossing of the drain-source resonance voltage of the MOSFET from the replica of the clamped voltage.

2. The electrical circuit of claim 1 , wherein the controller is configured to turn on the MOSFET at a next near valley window time period or at an end of a waiting period when the blanking period ends outside the near valley window time period.

3. The electrical circuit of claim 1 , wherein the capacitance comprises lumped parasitic capacitance at a drain of the MOSFET.

4. The electrical circuit of claim 1 , wherein the controller further comprises:

a one-shot circuit that generates the signal indicating the occurrence of the near valley window time period in response to detecting the zero-crossing of the drain-source resonance voltage of the MOSFET.

5. A method comprising:

sensing a drain-source resonance voltage of a primary switch at resonance, the primary switch being at resonance when an inductance of a primary winding of a transformer to which the primary switch is coupled resonates with a capacitance when the primary switch is turned off;

allowing the primary switch to be turned on within a range of time during a near valley window time period that occurs during a negative half cycle of the drain-source resonance voltage of the primary switch relative to an input voltage; and

turning on the primary switch before or after a minimum value of the drain-source resonance voltage of the primary switch during the near valley window time period,

wherein sensing the drain-source resonance voltage of the primary switch at resonance comprises:

detecting an auxiliary voltage at an auxiliary winding of the transformer;

clamping the auxiliary voltage to generate a clamped voltage; and

detecting a zero-crossing of the drain-source resonance voltage of the primary switch from the clamped voltage.

6. The method of claim 5 , wherein the primary switch is turned on during the near valley window time period when a blanking period during which the primary switch is prevented from being turned on ends during the near valley window time period.

7. The method of claim 5 , further comprising:

turning on the primary switch outside the near valley time period when a blanking period during which the primary switch is prevented from being turned on does not end during the near valley window time period and a waiting period expires.

8. The method of claim 5 , further comprising:

initiating the near valley window time period in response to the detection of the zero-crossing of the drain-source resonance voltage of the primary switch.

9. The method of claim 5 , wherein the primary switch comprises a metal oxide semiconductor field effect transistor (MOSFET) and the capacitance is at the drain of the MOSFET.

10. A switched mode power supply comprising:

a transformer having a primary winding and a secondary winding;

a metal oxide semiconductor (MOS) transistor that is coupled to the primary winding; and

a controller that is configured to control a switching operation of the primary switch to couple and decouple the primary winding to an input voltage, to detect a drain-source resonance voltage of the MOS transistor at resonance, to start a near valley window time period in response to zero-crossing of the drain-source resonance voltage, and to allow the MOS transistor to be turned on before or after a minimum value of the drain-source resonance voltage of the MOS transistor within a range of time during the near valley window time period,

wherein the controller is an integrated circuit (IC) and comprises:

a clamping circuit that is configured to clamp a sensed drain-source resonance voltage of the MOSFET to generate a clamped voltage;

a replica circuit that generates a replica of the clamped voltage inside the controller; and

a zero-crossing detection circuit that detects the zero-crossing of the drain-source resonance voltage of the MOS transistor from the replica of the clamped voltage.

11. The power supply of claim 10 , wherein the MOS transistor is a MOSFET.

12. The power supply of claim 10 , wherein the controller is configured to turn on the MOS transistor when a blanking period during which the MOS transistor is prevented from being turned on ends during the near valley window time period.

13. The power supply of claim 12 , wherein the controller is configured to turn on the MOS transistor in a next near valley window time period when the blanking period ends outside the near valley window time period.

14. The power supply of claim 12 , wherein the controller is configured to turn on the MOS transistor at an end of a waiting period when the blanking period ends outside the near valley window time period.