Quasi-resonant valley lockout without feedback reference

1. A method of controlling a power supply, comprising:

providing a power switching transistor in the power supply;

providing a counter;

enabling the counter when the power supply enters discontinuous conduction mode (DCM);

counting a first number of a plurality of resonant voltage valleys occurring at a conduction terminal of the power switching transistor to establish a first threshold;

counting a second number of the resonant voltage valleys occurring at the conduction terminal of the power switching transistor;

enabling conduction of the power switching transistor in response to a feedback signal when the second number of the resonant voltage valleys counted reaches the first threshold; and

counting the first number of the resonant voltage valleys occurring prior to the feedback signal enabling the power switching transistor to establish the first threshold.

2. The method of claim 1 , further including resetting the counter when a timer expires.

3. The method of claim 1 , further including determining the first threshold periodically.

4. The method of claim 1 , wherein counting the second number of the resonant voltage valleys includes

resetting the counter when the power switching transistor becomes non-conductive.

5. A method of controlling a power supply, comprising:

providing a power supply including a transistor;

counting a first number of resonant voltage valleys of the power supply;

enabling conduction of the transistor when the first number of resonant voltage valleys counted reaches a first threshold; and

determining the first threshold when a rate of change of a feedback signal of the power supply exceeds a second threshold.

6. A power supply controller, comprising:

a power switching transistor;

a first counter configured to count a first number of resonant voltage valleys to establish a first threshold;

a second counter configured to count a second number of resonant voltage valleys, wherein the resonant voltage valleys occur at a conduction terminal of the power switching transistor; and

a comparator coupled to an output of the first counter and an output of the second counter to provide an enabling output signal of the power supply controller when the second number of resonant voltage valleys counted reaches the first threshold.

7. The power supply controller of claim 6 , further including a slope detection circuit coupled to a feedback input of the power supply controller and configured to reset the first counter.

8. The power supply controller of claim 6 , further including a timer configured to reset the first counter.

9. The power supply controller of claim 6 , further including a pulse generator coupled to an output of the comparator.

10. A method of controlling a power supply, comprising:

providing a power supply including a transistor;

counting a first number of a plurality of resonant voltage valleys of the power supply;

counting a second number of the resonant voltage valleys of the power supply to determine a first threshold;

determining the first threshold when a rate of change of a feedback signal of the power supply exceeds a second threshold; and

enabling conduction of the transistor when the first number of the resonant voltage valleys counted reaches the first threshold.

11. The method of claim 10 , wherein counting the second number of the resonant voltage valleys includes:

providing a counter; and

enabling the counter when the power supply enters discontinuous conduction mode (DCM).

12. The method of claim 11 , further including resetting the counter when a timer expires.

13. The method of claim 10 , further including determining the first threshold periodically.

14. The method of claim 10 , wherein counting the first number of resonant voltage valleys includes:

providing a counter; and

resetting the counter when the transistor becomes non-conductive.

15. A method of controlling a power supply, comprising:

providing a power supply including a transistor;

providing a feedback signal from an output of the power supply;

counting a first number of resonant voltage valleys of the power supply;

counting a second number of resonant voltage valleys to determine a first threshold;

enabling conduction of the transistor when the first number of resonant voltage valleys counted reaches the first threshold; and

detecting a slope of the feedback signal to reset the first counter.

16. The method of claim 15 , further including providing a low-pass filter for the feedback signal.