Pre-charge techniques for a multi-level flying capacitor converter

1. A circuit, comprising:

a first transistor having a first control input;

a second transistor having a second control input;

a capacitor coupled between the first and second transistors; and

a controller coupled to the first and second control inputs, the controller configured to:

during a first mode, with a first time-varying duty cycle, toggle on and off the first transistor while toggling off and on the second transistor, such that the second transistor is on when the first transistor is off; and

during a second mode following the first mode, with a second time-varying duty cycle, toggle on and off the first transistor while toggling off and on the second transistor, such that the second transistor is on when the first transistor is off.

2. The circuit of claim 1 , wherein the first mode is a pre-charge mode during which the controller causes a voltage on the capacitor to increase.

3. The circuit of claim 1 , wherein the first time-varying duty cycle repeatedly changes between a first duty cycle value and a second duty value, and the second time-varying duty cycle sinusoidally changes.

4. The circuit of claim 1 , wherein:

the first time-varying duty cycle sinusoidally changes from a first lower duty cycle value to a first higher duty cycle value;

the second time-varying duty cycle sinusoidally changes from a second lower duty cycle value to a second higher duty cycle value; and

the first lower duty cycle value is larger than the second lower duty cycle value, and the first higher duty cycle value is smaller than the second higher duty cycle value.

5. The circuit of claim 1 , wherein:

the first time-varying duty cycle sinusoidally changes with a first amplitude that increases; and

the second time-varying duty cycle sinusoidally changes with a second amplitude that is constant.

6. The circuit of claim 1 , wherein the circuit is a flying capacitor converter.

7. The circuit of claim 6 , wherein the circuit is a three-phase, flying capacitor converter.

8. The circuit of claim 1 , further comprising:

a first gate driver coupled to the first transistor;

a second gate driver coupled to the second transistor; and

a bootstrap circuit coupled to the first and second gate drivers.

9. A circuit, comprising:

a controller configured to:

during a first mode, with a first time-varying duty cycle, toggle a first control signal to turn on and off a first transistor while toggling a second control signal to turn off and on a second transistor such that the second transistor is on when the first transistor is off; and

during a second mode following the first mode, with a second time-varying duty cycle, toggle the first control signal to turn on and off the first transistor while toggling the second control signal to turn off and on the second transistor, such that the second transistor is on when the first transistor is off.

10. The circuit of claim 9 , wherein the first mode is a pre-charge mode during which the controller causes a voltage on a capacitor to increase.

11. The circuit of claim 10 , wherein the controller is configured to: detect whether the voltage on the capacitor has reached a threshold level; and, responsive to detecting that the voltage has reached the threshold level; discontinue the pre-charge mode and initiate the second mode.

12. The circuit of claim 9 , wherein the controller is configured to: repeatedly change the first time-varying duty cycle between a first duty cycle value and a second duty value; and to implement the second time-varying duty cycle as a sinusoidally-varying duty cycle.

13. The circuit of claim 12 , wherein the first duty cycle value is greater than 0.5, and the second duty cycle value is a difference between 1.0 and the first duty cycle value.

14. The circuit of claim 9 , wherein the controller is configured to:

sinusoidally vary the first time-varying duty cycle between a first lower duty cycle value and a first higher duty cycle value; and

sinusoidally vary the second time-varying duty cycle between a second lower duty cycle value and a second higher duty cycle value;

in which the first lower duty cycle value is larger than the second lower duty cycle value, and the first higher duty cycle value is smaller than the second higher duty cycle value.

15. The circuit of claim 9 , wherein the controller is configured to:

sinusoidally vary the first time-varying duty cycle to have a first amplitude that increases; and

sinusoidally vary the second time-varying duty cycle to have a second amplitude that is constant.

16. A method, comprising:

during a pre-charge mode of a flying capacitor converter, with a variable duty cycle, toggling on and off a first transistor while toggling off and on a second transistor, such that the second transistor is on when the first transistor is off; and

during the pre-charge mode, varying the duty cycle.

17. The method of claim 16 , wherein varying the duty cycle during the pre-charge mode includes switching the duty cycle between a first duty cycle value and a second duty cycle value.

18. The method of claim 17 , wherein the first duty cycle value is greater than 0.5, and the second duty cycle value is a difference between 1.0 and the first duty cycle value.

19. The method of claim 16 , wherein varying the duty cycle during the pre-charge mode includes sinusoidally varying the duty cycle to have an increasing amplitude.

20. The method of claim 16 , wherein varying the duty cycle during the pre-charge mode includes sinusoidally varying the duty cycle to have a first amplitude, and the method further comprises:

detecting whether a voltage on a capacitor has reached a threshold level; and

responsive to detecting that the voltage has reached the threshold level, discontinuing the pre-charge mode and initiating an operational mode in which the duty cycle is sinusoidally varied to have a second amplitude that is larger than the first amplitude.