Synchronous rectifier gate drive shutdown circuit

1. A power converter, comprising:

a transformer having a primary winding, a first secondary winding, and a second secondary winding;

a bridge input circuit comprising at least two primary power switches for coupling input power to the primary winding;

a self-driven synchronous rectifier circuit connected to the second secondary winding, wherein the self-driven synchronous rectifier circuit includes a first self-driven synchronous rectifier for rectifying a voltage across the first secondary winding, and wherein a control terminal of the first synchronous rectifier is controlled by a voltage across the second secondary winding;

a gate drive circuit comprising a first diode connected to the control terminal of the first synchronous rectifier for introducing a positive dc level shift thereto when the at least two primary switches are off; and

a gate drive shutdown circuit connected to the first diode of the gate drive circuit, the gate drive shutdown circuit including a switch for removing the dc level shift from the control terminal of the first synchronous rectifier when operation of the bridge input circuit is terminated.

2. The power converter of claim 1 , wherein the bridge input circuit is a half bridge input circuit.

3. The power converter of claim 1 , wherein the bridge input circuit is a full bridge input circuit.

4. The power converter of claim 1 , wherein the self-driven synchronous rectifier circuit includes a self-driven current doubler synchronous rectifier circuit.

5. The power conveder of claim 1 , wherein the switch of the gate drive shutdown circuit includes a control terminal responsive to a signal received from the self-driven synchronous rectifier circuit.

6. The power converter of claim 1 , wherein the switch of the gate drive shutdown circuit includes a control terminal responsive to a signal received from the bridge input circuit.

7. The power converter of claim 6 , wherein the signal received from the bridge input circuit is a PWM control signal.

8. The power converter of claim 6 , wherein the gate drive shutdown circuit further includes an isolator for isolating the control terminal of the switch from the bridge input circuit.

9. The power converter of claim 8 , wherein the isolator includes an opto-coupler.

10. The power converter of claim 8 , wherein the isolator includes a transformer.

11. A power converter, comprising:

a transformer having a primary winding and a secondary winding:

a bridge input circuit connected to the primary winding;

a first control-driven synchronous rectifier connected to the secondary winding for rectifying a voltage across the secondary winding during a first phase;

a second control-driven synchronous rectifier connected to the secondary winding for rectifying the voltage across the secondary winding during a second phase;

an output circuit including a first output inductor connected to the first control-driven synchronous rectifier and a second output inductor connected to the second control-driven synchronous rectifier;

a synchronous rectifier control circuit for controlling the first and second control-driven synchronous rectifiers wherein the synchronous rectifier control circuit includes a driver having a first output terminal connected to a control terminal of the first control-diven synchronous rectifier and a second output terminal connected to a control terminal of the second control-driven synchronous rectifier; and

a gate drive shutdown circuit connected to the first and second control-driven synchronous rectifiers for turning on the second synchronous rectifier when primary switches of the bridge input circuit are off and there is negative current in the first and second output inductors.

12. The power converter of claim 11 , wherein the gate drive shutdown circuit includes:

a first resistor connected to the first synchronous rectifier; and

a second resistor connected to the second synchronous rectifier.

13. The power converter of claim 1 , wherein the switch of the gate drive shutdown circuit comprises a conduction channel between a first terminal and a second terminal, wherein the first terminal is connected to the anode of the first diode of the gate drive circuit and the second terminal is connected to ground such that when the switch is turned on, the first diode introduces the positive dc level shift to the control terminal of the first synchronous rectifier.

14. The power converter of claim 13 , wherein the switch of the gate drive shutdown circuit includes a control terminal responsive to a signal received from the self-driven synchronous rectifier circuit.

15. The power converter of claim 1 , wherein the switch of the gate drive shutdown circuit includes a control terminal responsive to a signal received from the bridge input circuit.

16. The power converter of claim 11 , wherein the bridge input circuit is a half bridge input circuit.

17. The power converter of claim 11 , wherein the bridge input circuit is a full bridge input circuit.

18. The power converter of claim 12 , wherein the synchronous rectifier control circuit further comprises a control transformer coupled to the driver.

19. The power converter of claim 18 , wherein the control transformer comprises:

a primary winding responsive to a PWM signal that is used to control the primary switches of the bridge input circuit; and

a secondary winding coupled to the driver.

20. The power converter of claim 19 , wherein the control transformer has a different core than the transformer.