Startup method and system for resonant converters

1. A method comprising:

providing a resonant converter comprising:

a switching network comprising a plurality of switches;

a resonant tank comprising:

a series resonant inductor coupled to the switching network and a primary side of a transformer; and

a series resonant capacitor coupled to the switching network and the primary side of the transformer;

a synchronous rectifier coupled to a secondary side of the transformer; and

a driver coupled to the switching network and the synchronous rectifier, wherein the driver is of an adjustable bias voltage;

configuring the switching network to operate a switching frequency higher than a resonant frequency of the resonant tank when the resonant converter is in a startup process; and

ramping up the adjustable bias voltage during the startup process, wherein during the ramping up the adjustable bias voltage, the adjustable bias voltage increases from zero to a steady bias voltage through a plurality of ramp-up stages.

2. The method of claim 1 , wherein:

the switching frequency is about one and a half times the resonant frequency.

3. The method of claim 1 ,

wherein the adjustable bias voltage increases from zero to a steady bias voltage with a slew rate less than or equal to 1000 mV/us.

4. The method of claim 1 ,

wherein:

in a first ramp-up stage, the adjustable bias voltage increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage increases from the Miller-plateau voltage of the switches to the steady bias voltage.

5. The method of claim 4 , wherein:

a first bias voltage slew rate in the first ramp-up stage is approximately equal to a third bias voltage slew rate in the third ramp-up stage; and

the first bias voltage slew rate is greater than a second bias voltage slew rate in the second ramp-up stage.

6. The method of claim 1 ,

wherein:

in a first ramp-up stage, the adjustable bias voltage increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage increases from the Miller-plateau voltage of the switches to the steady bias voltage through a non-linear voltage ramp-up process.

7. The method of claim 6 , wherein:

the non-linear voltage ramp-up process comprises an exponential function.

8. The method of claim 1 ,

wherein in a first ramp-up stage, the adjustable bias voltage non-linearly increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage slowly increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage non-linearly increases from the Miller-plateau voltage of the switches to the steady bias voltage.

9. The method of claim 8 , wherein:

the first ramp-up stage comprises a plurality of first sub-segments; and

the third ramp-up stage comprises a plurality of third sub-segments.

10. A system comprising:

an input power source;

a switching network comprising:

a first pair of switches coupled between the input power source; and

a second pair of switches coupled between the input power source;

a resonant tank connected between the switching network and a primary side of a transformer, wherein the resonant tank comprises:

a series resonant inductor coupled to the switching network and the transformer; and

a series resonant capacitor coupled to the switching network and the transformer;

a rectifier coupled to a secondary side of the transformer; and

a driver coupled to the rectifier and the switching network, wherein:

the driver is of an adjustable bias voltage; and

the driver is configured to generate gate drive signals for the switching network and the rectifier, and wherein the gate drive signals are of a switching frequency higher than a resonant frequency of the resonant tank during a startup process;

wherein the adjustable bias voltage is configured to increase from zero to a steady bias voltage through a multiple ramp-up stages.

11. The system of claim 10 , wherein:

the switching network, the resonant tank, the transformer and the rectifier form an LLC resonant converter.

12. The system of claim 11 , wherein:

the adjustable bias voltage is configured to increase during a startup process of the LLC resonant converter.

13. The system of claim 12 , wherein:

in a first ramp-up stage, the adjustable bias voltage linearly increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage slowly increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage linearly increases from the Miller-plateau voltage of the switches to the steady bias voltage.

14. The system of claim 12 , wherein:

in a first ramp-up stage, the adjustable bias voltage linearly increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage slowly increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage non-linearly increases from the Miller-plateau voltage of the switches to the steady bias voltage.

15. The system of claim 10 , wherein:

the transformer is a center-tapped transformer; and

the rectifier is a synchronous rectifier.

16. A method comprising:

providing a resonant converter comprising:

a switching network comprising a plurality of switches;

a resonant tank coupled between the switching network and a transformer, wherein the resonant tank comprises:

a series resonant inductor coupled to the switching network and the transformer; and

a series resonant capacitor coupled to the switching network and the transformer; and

a driver having an adjustable bias voltage; and

in response to a startup process of the resonant converter, configuring the switching network to operate a switching frequency higher than a resonant frequency of the resonant tank;

wherein the method further comprises increasing the adjustable bias voltage from zero to a steady bias voltage through a plurality of ramp-up stages.

17. The method of claim 16 , wherein:

in a first ramp-up stage, the adjustable bias voltage linearly increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage slowly increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage linearly increases from the Miller-plateau voltage of the switches to the steady bias voltage.

18. The method of claim 16 , wherein:

in a first ramp-up stage, the adjustable bias voltage linearly increases from zero to a turn-on threshold voltage of the switches;

in a second ramp-up stage, the adjustable bias voltage slowly increases from the turn-on threshold voltage to a Miller-plateau voltage of the switches; and

in a third ramp-up stage, the adjustable bias voltage non-linearly increases from the Miller-plateau voltage of the switches to the steady bias voltage.

19. The method of claim 16 , further comprising:

coupling the switching network to a dc input power source;

coupling a primary side of the resonant converter and a secondary side of the resonant converter through the transformer;

coupling a rectifier to the secondary side of the transformer; and

coupling an output filter to the rectifier.