Multi-mode operation and control of a resonant converter

1. A method of controlling a switched-mode power supply, the method comprising:

operating the switched-mode power supply in a normal operating mode comprising regulating a feedback signal to a first signal level; and

when the feedback signal crosses a first threshold, operating the switched-mode power supply in a second operating mode comprising

periodically asserting a high-side switch signal and a low-side switch signal having a dead-time between each assertion of the high-side switch signal and each assertion of the low-side switch signal, and

adjusting the dead-time to regulate the feedback signal to a second signal level different from the first signal level.

2. The method of claim 1 , further comprising generating the feedback signal based on a load requirement.

3. The method of claim 1 , further comprising, changing an operating mode of the switched-mode power supply from the second operating mode to the normal operating mode when the feedback signal crosses a second threshold.

4. The method of claim 3 , wherein the second threshold is between the first threshold and the first signal level.

5. The method of claim 1 , further comprising de-asserting the high-side switch signal and the low-side switch signal when the feedback signal crosses a third threshold.

6. The method of claim 1 , wherein:

in the normal operating mode, an assertion time during which the high-side switch signal or the low-side switch signal is asserted is greater than the dead-time between assertions of the high-side switch signal and the low-side switch signal; and

in the second operating mode, the dead-time between assertions of the high-side switch signal and the low-side switch signal is greater than the assertion time during which the high-side switch signal or the low-side switch signal as asserted.

7. The method of claim 6 , wherein the assertion time during which the high-side switch signal or the low-side switch signal is asserted is about 1.5 times greater than the dead-time between assertions of the high-side switch signal and the low-side switch signal.

8. The method of claim 1 , wherein an on-time of the high-side switch signal or of the low-side switch signal is constant in the second operating mode.

9. The method of claim 1 , wherein:

the switched-mode power supply is a resonant power supply; and

operating the switched-mode power supply in the normal operating mode comprises adjusting a frequency of the high-side switch signal or of the low-side switch signal to regulate the feedback signal to the first signal level.

10. A switch controller comprising:

a first threshold circuit coupled to a feedback signal and configured to generate a mode select signal based on the feedback signal;

a second threshold circuit coupled to the feedback signal and configured to generate a low load signal based on the feedback signal;

a dead-time calculation circuit coupled to the feedback signal and configured to generate a dead-time signal based on the feedback signal; and

a switching signal generator circuit coupled to the first threshold circuit, the second threshold circuit, and the dead-time calculation circuit, the switching signal generator circuit configured to

alternate between a normal operating mode and a burst operating mode based on the mode select signal,

when in the burst operating mode, adjust a dead-time duration between assertions of a high-side switch signal and a low-side switch signal based on the dead-time signal, and

when in the burst operating mode, disable assertions of the high-side switch signal and the low-side switch signal based on the low load signal.

11. The switch controller of claim 10 , wherein an output voltage is regulated at a first level in the normal operating mode and at a second level in the burst operating mode, the first level different from the second level.

12. The switch controller of claim 10 , wherein

the first threshold circuit comprises

a first comparator configured to compare the feedback signal to a first threshold and generate a burst-on signal based on comparing the feedback signal to the first threshold, and

a second comparator configured to compare the feedback signal to a second threshold and generate a leaving burst mode signal based on comparing the feedback signal to the second threshold;

the mode select signal comprises the burst-on signal and the leaving burst mode signal; and

the switching signal generator circuit is configured to enter the burst operating mode when the burst-on signal is asserted and exit the burst operating mode when the leaving burst mode signal is asserted.

13. The switch controller of claim 12 , wherein the second threshold circuit comprises a third comparator configured to compare the feedback signal to a third threshold and generate assertions of the low load signal based on comparing the feedback signal to the third threshold.

14. The switch controller of claim 13 , wherein the dead-time calculation circuit comprises a fourth comparator configured to compare the feedback signal to a fourth threshold and generate the dead-time signal based on comparing the feedback signal to the fourth threshold.

15. The switch controller of claim 10 , wherein the switching signal generator circuit comprises:

a mode select circuit coupled to the first threshold circuit and the second threshold circuit;

an on-time circuit coupled to the dead-time calculation circuit, wherein, when in the burst operating mode, the on-time circuit is configured to assert a switch enable signal for an on-time duration based on the dead-time signal; and

a switch drive logic circuit coupled to the on-time circuit, wherein, when in the burst operating mode, the switch drive logic circuit is configured to alternatingly assert the high-side switch signal and the low-side switch signal when the switch enable signal is asserted.

16. A method of operating a switch controller, the method comprising:

generating a mode select signal at a first threshold circuit based on a feedback signal;

generating a low load signal at a second threshold circuit based on the feedback signal;

generating a dead-time signal at a dead-time calculation circuit based on the feedback signal;

alternating between a normal operating mode and a burst operating mode based on the mode select signal;

when in the burst operating mode, adjusting a dead-time duration between assertions of a high-side switch signal and a low-side switch signal based on the dead-time signal; and

when in the burst operating mode, disabling assertions of the high-side switch signal and the low-side switch signal based on the low load signal.

17. The method of claim 16 , further comprising:

regulating an output voltage at a first level in the normal operating mode; and

regulating the output voltage at a second level in the burst operating mode, the first level different from the second level.

18. The method of claim 16 , wherein

the mode select signal comprises a burst-on signal and a leaving burst mode signal, and generating the mode select signal comprises generating the burst-on signal based on comparing the feedback signal to a first threshold and generating the leaving burst mode signal based on comparing the feedback signal to a second threshold; and

alternating between the normal operating mode and the burst operating mode based on the mode select signal comprises entering the burst operating mode when the burst-on signal is asserted and exiting the burst operating mode when the leaving burst mode signal is asserted.

19. The method of claim 18 , wherein generating the low load signal comprises comparing the feedback signal to a third threshold and generating assertions of the low load signal based on comparing the feedback signal to the third threshold.

20. The method of claim 19 , wherein generating the dead-time signal comprises comparing the feedback signal to a fourth threshold and generating the dead-time signal based on comparing the feedback signal to the fourth threshold.