Apparatus and efficiency point tracking method for high efficiency resonant converters

1. A converter comprising:

an input stage coupled to a power source, wherein the input stage 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;

an efficiency point tracking indicator coupled to the converter, wherein:

the efficiency point tracking indicator is a current flowing through a common mode capacitor coupled to the transformer, wherein the common mode capacitor is directly connected to a midpoint of a secondary side of the transformer;

a detector coupled to the efficiency point tracking indicator; and

a control circuit configured to receive an efficiency point tracking signal from the detector and adjust a switching frequency of the power switches based upon the efficiency point tracking signal.

2. The converter of claim 1 , wherein:

the common mode capacitor is coupled to ground through a resistor, wherein the detector is configured to detect the current flowing through the common mode capacitor through detecting a voltage across the resistor.

3. The converter of claim 1 , wherein the detector comprises:

a dc blocking capacitor;

a rectifier coupled to the dc blocking capacitor, wherein the rectifier is configured to convert an ac signal into a dc signal;

a load resistor; and

a voltage divider coupled to the rectifier.

4. The converter of claim 3 , wherein the rectifier comprises:

a first capacitor and a second capacitor connected in series; and

a first diode and a second diode connected in series, wherein the load resistor is coupled between a common node of the first capacitor and the second capacitor, and a common node of the first diode and the second diode.

5. The converter of claim 3 , wherein the rectifier comprises a diode and a capacitor, and wherein:

the diode is coupled between an input node and an output node of the rectifier;

the capacitor is coupled between the output node of the rectifier and ground; and

the load resistor is coupled between the input node of the rectifier and ground.

6. The converter of claim 1 , wherein:

the control circuit is configured to adjust the switching frequency of the power switches until a magnitude of the efficiency point tracking signal from the detector is less than a predetermined threshold.

7. A method comprising:

providing a resonant converter, wherein the resonant converter comprises:

an input stage coupled to a power source, wherein the input stage 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;

detecting a signal indicating a soft switching process of the resonant converter, wherein the signal is proportional to a current flowing through a common mode capacitor, wherein the common mode capacitor is directly connected to a midpoint of a secondary side of the transformer; and

adjusting, by a control circuit, a switching frequency of the resonant converter until a magnitude of the signal is less than a predetermined threshold.

8. The method of claim 7 , further comprising:

in response to a load increase, applying a higher gate drive voltage to the plurality of power switches; and

in response to a load drop, applying a lower gate drive voltage to the plurality of power switches.

9. The method of claim 7 , wherein:

the common mode capacitor is coupled to a midpoint of a secondary winding of the transformer.

10. A method comprising:

providing a first resonant converter, wherein the first resonant converter comprises:

a power stage comprising:

an input stage coupled to a power source, wherein the input stage 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; 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 indicating a soft switching process of the first resonant converter, wherein the signal is proportional to a current flowing through a common mode capacitor, wherein the common mode capacitor is directly connected to a midpoint of a secondary side of the isolation device;

detecting a load current of the first resonant converter; and

configuring a switching frequency of the first resonant converter such that:

adjusting the switching frequency until a magnitude of the signal is less than a predetermined threshold when the load current is greater than a predetermined load level; and

increasing the switching frequency until an output voltage of the first resonant converter is within a predetermined regulation range when the load current is less than the predetermined load level.

11. The method of claim 10 , further comprising:

providing a second resonant converter, wherein:

the first resonant converter and the second resonant converter are connected in parallel; and

the second resonant converter is of a same structure as the first resonant converter.

12. The method of claim 11 , further comprising:

adjusting the switching frequency of the first resonant converter until the switching frequency of the first resonant converter is approximately equal to a resonant frequency of the first resonant converter; and

adjusting the switching frequency of the second resonant converter until the switching frequency of the second resonant converter is approximately equal to a resonant frequency of the second resonant converter.

13. The method of claim 10 , further comprising:

increasing a gate bias voltage of the first resonant converter when the load current is greater than the predetermined load level; and

reducing the gate bias voltage of the first resonant converter when the load current is less than the predetermined load level, wherein a gate driver of the first resonant converter is a lossless gate driver.

14. The method of claim 10 , further comprising:

increasing a duty cycle of the first resonant converter when the load current is greater than the predetermined load level; and

reducing the duty cycle of the first resonant converter when the load current is less than the predetermined load level, wherein a gate driver of the first resonant converter is a PWM gate driver.

15. The method of claim 10 , wherein:

the plurality of gate drive signals are sent to the power switches through a plurality of gate drivers, wherein the gate drivers are lossless gate drivers.

16. The method of claim 10 , further comprising:

in response to an input voltage change, adjusting the switching frequency until the output voltage of the first resonant converter is within a second predetermined regulation range.