Resonant converters with synchronous rectifier feedback

1. A converter comprising:

an input switch network coupled to a power source, wherein the input switch network comprises a plurality of power switches;

a resonant tank coupled to the plurality of power switches;

a transformer coupled to the resonant tank;

an output stage coupled to the transformer, wherein the output stage comprises a synchronous rectifier formed by a first switch and a second switch;

a comparator coupled to the first switch, wherein the comparator is configured to:

detect a drain-to-source voltage of the first switch;

compare the drain-to-source voltage with a predetermined threshold;

generate a first logic state if the drain-to-source voltage is greater than the predetermined threshold; and

generate a second logic state if the drain-to-source voltage is less than the predetermined threshold, wherein the predetermined threshold is a body diode voltage of the first switch, and wherein the body diode voltage is approximately equal to −0.6 V; and

a control circuit configured to adjust a switching frequency of the power switches and the synchronous rectifier based upon the first logic state and the second logic state.

2. The converter of claim 1 , wherein:

the first logic state is a logic 0; and

the second logic state is a logic 1.

3. The converter of claim 2 , wherein:

in response to the logic 0 sent from the comparator to the control circuit, the control circuit is configured to increase the switching frequency of the power switches and the synchronous rectifier until the drain-to-source voltage is less than the predetermined threshold.

4. The converter of claim 2 , wherein:

in response to the logic 1 sent from the comparator to the control circuit, the control circuit is configured to reduce the switching frequency of the power switches and the synchronous rectifier until the drain-to-source voltage is greater than the predetermined threshold.

5. The converter of claim 1 , further comprising:

a diode coupled between an input of the comparator and a drain of the first switch, wherein:

a cathode of the diode is coupled to the drain of the first switch; and

an anode of the diode is coupled to the input of the comparator.

6. The converter of claim 5 , further comprising:

a bias voltage source coupled to the input of the comparator through a bias resistor.

7. The converter of claim 1 , wherein:

the converter is a half-bridge LLC resonant converter.

8. A method comprising:

providing a resonant converter, wherein the resonant converter comprises:

an input switch network coupled to a power source, wherein the input switch network comprises a plurality of power switches;

a resonant tank coupled to the plurality of power switches;

a transformer coupled to the resonant tank; and

an output stage coupled to the transformer, wherein the output stage comprises a synchronous rectifier formed by a first switch and a second switch;

detecting a drain voltage of the first switch;

comparing the drain voltage with a predetermined voltage threshold, wherein:

the drain voltage is coupled to a negative input of a comparator; and

the predetermined voltage threshold is coupled to a positive input of the comparator;

generating a logic state based upon an output of the comparator;

increasing a switching frequency of the resonant converter until the output of the comparator has a first logic state transition; and

reducing the switching frequency of the resonant converter until the output of the comparator has a second logic state transition.

9. The method of claim 8 , further comprising:

increasing the switching frequency of the resonant converter when the logic state is 0.

10. The method of claim 9 , further comprising:

increasing the switching frequency of the resonant converter until a logic transition occurs from 0 to 1.

11. The method of claim 8 , further comprising:

reducing the switching frequency of the resonant converter when the logic state is 1.

12. The method of claim 11 , further comprising:

reducing the switching frequency of the resonant converter until a logic transition occurs from 1 to 0.

13. The method of claim 8 , wherein:

a drain of the first switch is coupled to a secondary winding of the transformer; and

a source of the first switch is coupled to ground.

14. A method comprising:

providing a resonant converter, wherein the resonant converter comprises:

a power stage comprising:

an input switch network coupled to a power source, wherein the input switch network comprises a plurality of power switches;

a resonant tank coupled to the plurality of power switches;

an isolation device coupled to the resonant tank; and

an output stage coupled to the isolation device through a rectifier, wherein the rectifier comprises a first transistor and a second transistor, and wherein a drain of the first transistor is coupled to the isolation device and a source of the first transistor is coupled to ground; and

a control circuit coupled to the power switches, wherein the control circuit generates a plurality of gate drive signals for the power switches;

detecting a signal across the drain and the source of the first transistor;

comparing the signal with a predetermined threshold to generate a logic state;

in response to a first logic state, increasing a switching frequency until the signal is less than the predetermined threshold; and

in response to a second logic state, decreasing the switching frequency until the signal is greater than the predetermined threshold.

15. The method of claim 14 , further comprising:

generating a logic 0 if the signal is greater than the predetermined threshold; and

generating a logic 1 if the signal is less than the predetermined threshold.

16. The method of claim 15 , further comprising:

in response to the logic 0, increasing the switching frequency until the signal is less than the predetermined threshold; and

in response to the logic 1, decreasing the switching frequency until the signal is greater than the predetermined threshold.

17. The method of claim 14 , wherein:

the plurality of switches comprises a first switch and a second switch connected in series, and wherein a common node of the first switch and the second switch is coupled to the resonant tank.

18. The method of claim 14 , wherein:

the isolation device is a center-tapped transformer.

19. The method of claim 14 , wherein:

the control circuit is coupled to the plurality switches through a plurality of drivers.