Reduced voltage switching of a main switch in flyback power converters

1. A method of operating a power converter, comprising:

driving a primary current with a first polarity in a primary winding of a transformer, and a secondary winding of the transformer arranged for flyback operation;

creating, by the primary current, a charge voltage on an auxiliary winding of the transformer;

charging, by the charge voltage, a capacitance coupled to the auxiliary winding;

ceasing the driving of the primary current and discharging an energy in a field associated with the secondary winding to create a secondary current in the secondary winding; and after the secondary current reaches zero and prior to again driving the primary current

reducing voltage across a main switch coupled to the primary winding, the reducing by coupling the capacitance and an inductor in series with the auxiliary winding.

2. The method of claim 1 wherein reducing voltage across the main switch further comprises resonating a capacitance in a primary circuit with the inductor.

3. A method of operating a power converter, comprising:

driving a primary current with a first polarity in a primary winding of a transformer, and a secondary winding of the transformer arranged for flyback operation;

creating, by the primary current, a charge voltage on an auxiliary winding of the transformer;

charging, by the charge voltage, a capacitance coupled to the auxiliary winding;

ceasing the driving of the primary current and discharging an energy in a field associated with the secondary winding to provide an output voltage of the power converter; and

reducing voltage across a main switch coupled to the primary winding, the reducing by coupling the capacitance to the auxiliary winding and creating a resonance between a drain-to-source capacitance of the main switch being a field effect transistor and an inductor coupled in series with the auxiliary winding.

4. The method of claim 3 wherein the capacitance further comprises a valley fill circuit.

5. The method of claim 3 wherein charging the capacitance further comprises charging a series combination of a first capacitor and a second capacitor.

6. The method of claim 3 further comprising supplying operational power to a primary-side controller by the auxiliary winding.

7. A method of operating a power converter, comprising:

driving a primary current with a first polarity in a primary winding of a transformer, and a secondary winding of the transformer arranged for flyback operation;

creating, by the primary current, a charge voltage on an auxiliary winding of the transformer;

charging, by the charge voltage, a series combination of a first capacitor and a second capacitor coupled to the auxiliary winding;

ceasing the driving of the primary current and discharging an energy in a field associated with the secondary winding to provide an output voltage of the power converter; and

including a primary in the primary winding by coupling the capacitance to the auxiliary winding and discharging, through the auxiliary winding, a parallel combination of the first capacitor and the second capacitor.

8. The method of claim 7 wherein charging the capacitance further comprises charging a valley fill circuit.

9. The method of claim 7 wherein reducing voltage across the main switch further comprises reducing voltage across the main switch to a range of voltages between 100 milli-Volts (mV) and −100 mV, inclusive.

10. A power converter comprising:

a primary circuit comprising:

a primary winding of a transformer, the primary winding defining a first lead and a second lead, the first lead coupled to an input voltage;

a main switch defining a control input, a first connection, a second connection, the first connection coupled to the second lead of the primary winding, and the second connection coupled to ground on the primary circuit;

a secondary circuit comprising a secondary winding arranged for flyblack operation, and the secondary circuit defining a voltage output of the power converter;

an auxiliary circuit comprising:

an auxiliary winding of the transformer;

a resonance switch defining a control input, the resonance switch coupled in series with the auxiliary winding;

an inductor coupled in series with the auxiliary winding;

a first capacitor coupled in series with the auxiliary winding and the inductor;

a primary controller comprising:

a main gate terminal coupled to the control input of the main switch;

an auxiliary driver circuit coupled to the control input of the resonance switch;

wherein the primary controller is configured to:

assert the control input of the main switch to drive a primary current with a first polarity through the primary winding;

de-assert the control input of the main switch to discharge an energy in a field associated with the secondary winding; and

reduce voltage across the main switch by assertion of the control input of the resonance switch, assertion of the control input of the resonance switch causes current to flow from the first capacitor through the auxiliary winding and thereby induce a primary current with a second polarity opposite the first polarity.

11. The power converter of claim 10 wherein when the primary controller reduces voltage across the main switch, the primary controller is configured to reduce voltage across the main switch to a range of voltages between 100 milli-Volts (mV) and −100 mV, inclusive.

12. The power converter of claim 10 wherein the auxiliary circuit further comprises:

the first capacitor defining a first lead coupled to the auxiliary winding, and a second lead;

a first diode defining and anode coupled to the second lead of the first capacitor;

a second capacitor defining a first lead coupled the cathode of the first diode, and a second lead coupled to ground within the secondary circuit;

wherein, during time periods when the primary current with the first polarity flows through the primary winding, the first and second capacitors charge in series.

13. The power converter of claim 12 wherein the auxiliary circuit further comprises:

a second diode defining an anode coupled to ground within the secondary circuit, and a cathode coupled to the second lead of the first capacitor;

a third diode defining an anode coupled to the first lead of the second capacitor, and a cathode coupled to the first lead of the first capacitor;

wherein, during time periods when the control input of the resonance switch is asserted, flowing current from the first and second capacitors in parallel.

14. A power converter comprising:

a primary circuit comprising:

a primary winding of a transformer, the primary winding defining a first lead and a second lead, the first lead coupled to an input voltage;

a main field effect transistors (main FET) defining a gate, a drain, and a source, the drain coupled to the second lead of the primary winding, and the source coupled to ground on the primary circuit;

a secondary winding arranged for flyblack operation and defining a voltage output of the power converter;

an auxiliary circuit comprising:

an auxiliary winding of the transformer;

a resonance FET defining a gate, the resonance FET coupled in series with the auxiliary winding;

an inductor coupled in series with the auxiliary winding;

a valley fill circuit coupled to the inductor;

a primary controller comprising:

a gate terminal coupled to the gate of the main FET;

an auxiliary driver circuit coupled to the gate of the resonance FET;

wherein the primary controller is configured to:

assert the gate of the main FET to drive a primary current with a first polarity through the primary winding;

de-assert the gate of the main FET to discharge an energy in a field associated with the secondary winding;

reduce voltage across the main switch by assertion of the fate of the resonance FET, assertion of the gate of the resonance FET causes current to flow from the valley fill circuit through the auxiliary winding and thereby induce a primary current with a second polarity opposite the first polarity.

15. The power converter of claim 14 wherein when the primary controller reduces voltage across the main switch, the primary controller is configured to reduce voltage across the main switch to a range of voltages between 100 milli-Volts (mV) and −100 mV, inclusive.

16. The power converter of claim 14 wherein the valley fill circuit further comprises:

a first capacitor defining a first lead coupled to the auxiliary winding, and a second lead;

a first diode defining and anode coupled to the second lead of the first capacitor;

a second capacitor defining a first lead coupled the cathode of the first diode, and a second lead coupled to ground within the secondary circuit;

wherein, during time periods when the primary current with the first polarity flows through the primary winding, the first and second capacitors charge in series.

17. The power converter of claim 16 wherein the valley fill circuit further comprises:

a second diode defining an anode coupled to ground within the secondary circuit, and a cathode coupled to the second lead of the first capacitor;

a third diode defining an anode coupled to the first lead of the second capacitor, and a cathode coupled to the first lead of the first capacitor;

wherein, during time periods when the control input of the resonance switch is asserted, flowing current from the first and second capacitors in parallel.

18. The power converter of claim 14 wherein the auxiliary winding produces a supply voltage coupled to the primary controller.