Inrush current control during starting of resonant converters

1. A converter with soft start comprising:

a transformer;

first and second switches connected to the transformer to supply power to the transformer;

first and second capacitors connected in series, the first capacitor is connected to the first switch and the second capacitor is connected to the second switch;

a controller connected to the first and second switches and arranged to, during startup of the converter, switch the first switch with a variable duty cycle and switch the second switch with either a fixed duty cycle or a variable duty cycle with pulses larger than pulses of the variable duty cycle of the first switch; and

a bleed device arranged to set initial conditions of the converter before startup of the converter by discharging the second capacitor before startup; wherein

the converter is an LLC converter; and

the first and the second capacitors are split resonant capacitors.

2. The converter of claim 1 , wherein the bleed device is connected in parallel with the second switch.

3. The converter of claim 2 , wherein:

the first switch is a high-side switch; and

the second switch is a low-side switch.

4. The converter of claim 2 , wherein:

the first switch is a low-side switch; and

the second switch is a high-side switch.

5. The converter of claim 1 , wherein the bleed device is connected in parallel with the second capacitor.

6. The converter of claim 5 , wherein:

the first switch is a high-side switch;

the second switch is a low-side switch;

the first capacitor is a high-side capacitor; and

the second capacitor is a low-side capacitor.

7. The converter of claim 5 , wherein:

the first switch is a low-side switch;

the second switch is a high-side switch;

the first capacitor is a low-side capacitor; and

the second capacitor is a high-side capacitor.

8. The converter of claim 1 , wherein the bleed device discharges the second capacitor before startup to zero or nearly zero.

9. The converter of claim 1 , further comprising a resonant inductor.

10. The converter of claim 9 , wherein the resonant inductor is a discrete inductor or a leakage inductance of the transformer.

11. The converter of claim 1 , wherein the bleed device includes a resistor.

12. The converter of claim 1 , wherein the bleed device includes a switch.

13. The converter of claim 1 , wherein switching of the first switch is center-aligned, left-aligned, or right-aligned.

14. A soft-start method for a converter comprising the steps of:

providing a converter that includes:

a transformer;

first and second switches connected to the transformer to supply power to the transformer; and

first and second capacitors connected in series, the first capacitor is connected to the first switch and the second capacitor is connected to the second switch;

before startup of the converter, using a bleed device to set initial conditions of the converter by discharging the second capacitor before startup; and

during startup of the converter, switching the first switch with a variable duty cycle and switching the second switch with either a fixed duty cycle or a variable duty cycle with pulses larger than pulses of the variable duty cycle of the first switch; wherein

the converter is an LLC converter; and

the first and the second capacitors are split resonant capacitors.

15. The soft-start method of claim 14 , wherein the bleed device is connected in parallel with the second switch.

16. The soft-start method of claim 14 , wherein discharging of the second capacitor before startup discharges the second capacitor to zero or nearly zero.

17. A converter with soft start comprising: a transformer including primary and secondary windings; a primary circuit connected to the primary windings and including:

first and second switches connected to the transformer to supply power to the transformer; and

first and second capacitors connected in series, the first capacitor is connected to the first switch and the second capacitor is connected to the second switch;

a controller connected to the first and second switches and arranged to, during startup of the converter, switch the first switch with a variable duty cycle and switch the second switch with either a fixed duty cycle or a variable duty cycle with pulses larger than pulses of the variable duty cycle of the first switch; and

a bleed device arranged to set initial conditions of the converter before startup of the converter by discharging the second capacitor before startup; and a secondary circuit connected to the secondary windings and not including an inductor; wherein the converter is an LLC converter; and the first and second capacitors are split resonant capacitors.