Switching power converter with reduced voltage switching of the primary switch

1. A method of operation a switching power converter, comprising:

activating, during a first switching period, a primary switch coupled to a primary winding of a transformer of a flyback topology, the activating induces a positive current in the primary winding;

deactivating the primary switch and activating a secondary switch coupled to a secondary winding of the transformer, the activating the secondary switch during the first switching period and for a duration selected based on a signal indicative of negative current from a prior switching period, and the activating the secondary switch results in a positive current through the secondary winding followed by a negative current through the secondary winding;

discharging capacitance of the primary switch by a negative current in the primary winding responsive to the negative current in the secondary winding; and then

activating the primary switch during a second switching period subsequent to the first switching period,

wherein the first switching period begins after the prior switching period ends.

2. The method of claim 1 wherein the duration is shorter than a prior duration in the prior switching period if the signal indicative of negative current indicates an excess negative current.

3. The method of claim 2 wherein the duration of the first switching period being shorter than the prior duration results in increasing a switching frequency.

4. The method of claim 2 wherein the duration is shorter than the prior duration in the prior switching period by at least one selected from a group consisting of: an amount proportional to the excess negative current; and a predetermined amount.

5. The method of claim 1 wherein the duration is longer than a prior duration in the prior switching period if the signal indicative of negative current indicates an insufficient negative current.

6. The method of claim 1 further comprising, during the activation of the secondary switch, flowing positive current in the primary winding only through a diode to a clamp capacitor, the positive current in the primary winding during the activation of the secondary switch attributable to leakage inductance of the transformer.

7. The method of claim 1 wherein the signal indicative of negative current is at least one selected from a group consisting of: a voltage of a switch node between the primary winding and the primary switch; and a voltage of a monitor node between the secondary winding and the secondary switch.

8. A controller for a switching power converter, the controller comprising:

a primary-drive terminal, a secondary-drive terminal, a sense terminal, and a feedback terminal;

a primary controller coupled to the primary-drive terminal and the feedback terminal, the primary controller configured to assert the primary-drive terminal in a charge mode of a first switching period;

a secondary controller coupled to the secondary-drive terminal, the secondary controller configured to assert the secondary-drive terminal during a discharge mode in the first switching period;

a modulation controller configured to:

sense a signal indicative of negative current flow by way of the sense terminal during the first switching period; and

adjust a duration of assertion of the secondary-drive terminal during a second switching period subsequent to the first switching period, the adjustment responsive to the signal indicative of negative current flow during the first switching period,

wherein the second switching period begins after the first switching period ends.

9. The controller of claim 8 wherein when the modulation controller adjusts the duration of assertion, the modulation controller is further configured to make the duration of assertion shorter than a duration of assertion during the first switching period if the signal indicative of negative current flow indicates an excess negative current.

10. The controller of claim 9 wherein the duration of assertion during the second switching period being shorter than the duration of assertion during the first switching period results in increasing a switching frequency.

11. The controller of claim 8 wherein when the modulation controller adjusts the duration of assertion, the modulation controller is further configured to make the duration of assertion longer than a duration of assertion during the first switching period if the signal indicative of negative current flow indicates an insufficient negative current.

12. The controller of claim 11 wherein the duration of assertion during the second switching period being longer than the duration of assertion during the first switching period results in decreasing a switching frequency.

13. The controller of claim 8 wherein the controller does not control a clamp switch of a clamp circuit of the switching power converter.

14. A switching power converter comprising:

a primary circuit comprising:

a primary winding of a transformer, the primary winding having a first terminal coupled to an input node, and a second terminal defining a switch node;

a primary field effect transistor (primary FET) coupled between the switch node and a ground or common of the primary circuit;

a clamp diode in series with a clamp capacitor, the clamp diode and clamp capacitor coupled between the input node and the switch node;

a secondary circuit comprising:

a secondary winding of the transformer arranged for flyback operation; and

a secondary FET coupled between a terminal of the secondary winding and ground or common of the secondary circuit;

a controller defining a main drive output coupled to a gate of the primary FET, a second drive output coupled to a gate of the secondary FET, and a sense input coupled to a signal indicative of voltage at the switch node;

the controller is configured to:

assert a gate of the primary FET during a charge mode of a first switching period; and then

assert a gate of the secondary FET during a discharge mode of the first switching period;

sense a signal indicative of negative current flow by way of the sense input during the first switching period; and

adjust a duration of assertion of the gate of the secondary FET during a discharge mode of a second switching period subsequent to the first switching period, the adjustment responsive to the signal indicative of negative current flow during the first switching period,

wherein the second switching period begins after the first switching period ends.

15. The switching power converter of claim 14 wherein when the controller adjusts the duration of assertion, the controller is further configured to make the duration of assertion shorter than a duration of assertion during the first switching period if the signal indicative of negative current flow indicates an excess negative current.

16. The switching power converter of claim 15 wherein the duration of assertion during the second switching period being shorter than the duration of assertion during the first switching period results in increasing a switching frequency.

17. The switching power converter of claim 14 wherein when the controller adjusts the duration of assertion, the controller is further configured to make the duration of assertion longer than a duration of assertion during the first switching period if the signal indicative of negative current flow indicates an insufficient negative current.

18. The switching power converter of claim 17 wherein the duration of assertion during the second switching period being longer than the duration of assertion during the first switching period results in decreasing a switching frequency.

19. The switching power converter of claim 17 wherein the controller makes the duration of assertion during the second switching period longer by at least one selected from a group consisting of: an amount proportional to the insufficient negative current; and a predetermined amount.

20. The switching power converter of claim 14 wherein the controller does not control a clamp switch of a clamp circuit of the switching power converter.