Systems and methods of synchronous rectification in active clamp flyback power converters

1. A method of operating a power converter having a transformer arranged for flyback operation, the method comprising:

sensing a first slope of voltage on a secondary winding of the transformer, the first slope indicative of the power converter entering a charge mode of the transformer;

modifying, responsive the sensing, an operational state of a secondary rectifier (SR), driver coupled to a secondary rectifier;

making the secondary rectifier conductive by the SR driver during a discharge mode of the transformer;

sensing a second slope of voltage on the secondary of the transformer, the second slope indicative of ending of the discharge mode of the power converter; and then

returning, responsive to sensing the second slope, the SR driver to an original operational state.

2. The method of claim 1 :

wherein modifying the operational state of the SR driver further comprises increasing a minimum conduction time of the secondary rectifier during discharge modes;

wherein returning the SR driver to the original operational state further comprises decreasing the minimum conduction time of the secondary rectifier.

3. The method of claim 2 wherein increasing the minimum conduction time further comprises increasing based a resistance of resistor external to and coupled to the SR driver.

4. The method of claim 2 wherein returning to the original operational state further comprises returning the minimum conduction time to a predetermined value.

5. The method of claim 2 wherein sensing the first slope further comprises sensing that the first slope is positive and above a first predetermined value.

6. The method of claim 1 :

wherein modifying the operational state further comprises arming the SR driver;

wherein making the secondary rectifier conductive during the discharge mode further comprises making the secondary rectifier conductive only if SR driver is armed;

wherein returning to the original operational state further comprises disarming the SR driver.

7. The method of claim 6 wherein arming the SR driver further comprises arming the SR driver when the first slope is positive and above a first predetermined value.

8. The method of claim 6 wherein making the secondary rectifier conductive during the discharge mode further comprises:

monitoring, during periods of time when the SR driver is armed, a voltage on the secondary winding of the transformer; and

making the secondary rectifier conductive when the voltage on the secondary winding indicates a body diode of the secondary rectifier is conducting.

9. A driver for a secondary-side synchronous rectifier, the driver comprising:

a drain-sense terminal and a gate terminal;

a controller configured to:

sense a first slope of a voltage on the drain-sense terminal, the first slope indicative of a power converter entering a charge mode of a transformer arranged for flyback operation;

change, responsive to entering the charge mode, an operational state of the driver from an original state to a modified state;

assert the gate terminal during a discharge mode of the transformer;

sense a second slope of the voltage at the drain-sense terminal, the second slope indicative of the power converter entering the discharge mode; and

return, responsive to sensing the second slope, the operational state to the original state.

10. The driver of claim 9 :

wherein when the controller changes the operational state to the modified state, the controller is configured to increase a minimum conduction time during discharge modes;

wherein when the controller returns the operational state to the original state, the controller is configured to decrease the minimum conduction time.

11. The driver of claim 10 further comprising:

an on-time terminal; and

wherein when the controller increases the minimum conduction time, the controller is further configured to increase based a resistance sensed through the on-time terminal.

12. The driver of claim 10 wherein when the controller returns the operational state to the original state, the controller is configured to return the minimum conduction time to a predetermined value.

13. The driver of claim 10 wherein when the controller senses the first slope, the controller is configured to sense that the first slope being positive and above a first predetermined value.

14. The driver of claim 9 :

wherein when the controller changes the operational state to the modified state, the controller is further configured to arm the driver;

wherein when the controller asserts the gate terminal during the discharge mode of the transformer, the controller is further configured to assert the gate terminal only if the driver is armed; and

wherein when the controller returns the operational state to the original state, the controller is further configured to disarm the driver.

15. The driver of claim 14 wherein the controller is further configure to, during periods when the when the driver is disarmed, refrain from arming the driver when a rate of change of voltage on the drain-sense terminal indicates a clamp FET on a primary side is made conductive.

16. The driver of claim 14 wherein when the controller asserts the gate terminal, the controller is further configured to:

monitor, during periods when the driver is armed, a voltage on the drain-sense terminal; and

assert the gate terminal when the voltage on the drain-sense terminal indicates a body diode of a synchronous rectifier field effect transistor is conducting.

17. A power converter comprising:

a primary side comprising:

a primary winding of a transformer, the primary winding has a first lead coupled to an input voltage, and a second lead that defines a switch node;

a primary field effect transistor (FET) coupled between the switch node and ground on the primary side;

a clamp FET coupled between the switch node and a clamp capacitor;

a secondary side comprising:

a secondary winding of the transformer arranged for flyback operation;

a secondary rectifier FET (SR FET) defining a drain, a source, and a gate, the drain coupled to the secondary winding;

a driver for the SR FET, the driver defining a gate terminal coupled to the gate, and a drain-sense terminal coupled to the drain;

wherein the driver is configured to:

sense the power converter has entered a charge mode of the transformer; and then

change an operational state of the driver from an original state to a modified state;

make the SR FET conductive during a discharge mode of the transformer;

sense the discharge mode has ended; and then

return the operational state of the driver to the original state.

18. The power converter of claim 17 :

wherein when the driver changes the operational state to the modified state, the driver is configured to increase a minimum conduction time of the SR FET during discharge modes;

wherein when the driver returns the operational state to the original state, the driver is configured to decrease the minimum conduction time of the SR FET.

19. The power converter of claim 18 wherein when the driver returns the operational state to the original state, the driver is configured to return the minimum conduction time to a predetermined value.

20. The power converter of claim 17 :

wherein when the driver changes the operational state to the modified state, the driver is further configured to assert an armed signal within the driver;

wherein when the driver makes the SR FET conductive during the discharge mode, the driver is further configured to make the SR FET conductive only if the armed signal is asserted; and

wherein when the driver returns the operational state to the original state, the driver is further configured to de-assert the armed signal.