Fractional valley switching controller

1. A method comprising:

determining a target number of valleys of a resonant waveform at a drain node of a main switch of a power converter, the target number of valleys corresponding to a desired off-time of the main switch and corresponding to a non-integer number of valleys; and

modulating an off-time of the main switch between a plurality of modulated off-times, each of the modulated off-times of the main switch corresponding to an integer number of valleys, the plurality of modulated off-times of the main switch having an average value that corresponds to the desired off-time.

2. The method of claim 1 , wherein:

each modulated off-time of the plurality of modulated off-times corresponds to a respective modulated integer valley number of a plurality of modulated integer valley numbers;

the plurality of modulated integer valley numbers comprises a first sequence of one or more first integer valley numbers in series immediately followed by a second sequence of one or more second integer valley numbers in series;

a length of the first sequence is different than a length of the second sequence; and

the one or more first integer valley numbers are not equal to the one or more second integer valley numbers.

3. The method of claim 1 , further comprising:

measuring the off-time of the main switch;

determining a difference between the desired off-time of the main switch and the measured off-time of the main switch; and

generating the target number of valleys based on the determined difference.

4. The method of claim 1 , further comprising:

measuring the off-time of the main switch;

determining a difference between the measured off-time of the main switch and the desired off-time of the main switch;

producing an off-time adjustment signal based on the determined difference between the measured off-time of the main switch and the desired off-time of the main switch; and

generating the target number of valleys based on the off-time adjustment signal.

5. The method of claim 1 , further comprising:

generating a fractional mode enable signal if a switching frequency of the main switch is greater than a threshold switching frequency; and

generating a fractional mode disable signal if the switching frequency of the main switch is not greater than the threshold switching frequency.

6. The method of claim 5 , further comprising:

generating a plurality of modulated integer valley numbers based on the target number of valleys.

7. The method of claim 6 , further comprising:

generating the plurality of modulated integer valley numbers using a first modulator upon receiving the fractional mode disable signal; and

generating the plurality of modulated integer valley numbers using a second modulator upon receiving the fractional mode enable signal.

8. The method of claim 7 , wherein:

the first modulator generates the plurality of modulated integer valley numbers using a hysteresis controller having two fixed levels.

9. The method of claim 7 , wherein:

the second modulator generates the plurality of modulated integer valley numbers using a hysteresis controller having a varying first level and a fixed second level.

10. The method of claim 7 , wherein:

the second modulator generates the plurality of modulated integer valley numbers using a sigma-delta modulator.

11. A method comprising:

receiving, at a transformer having a primary winding, an input voltage;

providing, by a secondary winding of the transformer, an output voltage to a load;

controlling, by a main switch coupled to the primary winding, a current through the primary winding to provide the output voltage, a resonant waveform at a drain node of the main switch comprising one or more valleys during an off-time of the main switch;

determining a target number of valleys corresponding to a desired off-time of the main switch, the target number of valleys corresponding to a non-integer number of valleys; and

modulating the off-time of the main switch between a plurality of modulated off-times, each of the modulated off-times of the main switch corresponding to a respective integer number of valleys of the resonant waveform, the plurality of modulated off-times of the main switch having an average value that corresponds to the desired off-time.

12. The method of claim 11 , wherein:

each modulated off-time of the plurality of modulated off-times corresponds to a respective modulated integer valley number of a plurality of modulated integer valley numbers;

the plurality of modulated integer valley numbers comprises a first sequence of one or more first integer valley numbers in series immediately followed by a second sequence of one or more second integer valley numbers in series;

a length of the first sequence is different than a length of the second sequence; and

the one or more first integer valley numbers are not equal to the one or more second integer valley numbers.

13. The method of claim 11 , further comprising:

measuring the off-time of the main switch;

determining a difference between the measured off-time of the main switch and the desired off-time;

producing an off-time adjustment signal based on the determined difference; and

generating the target number of valleys based on the off-time adjustment signal.

14. The method of claim 11 , further comprising:

generating a plurality of modulated integer valley numbers based on the target number of valleys, each of the modulated integer valley numbers corresponding to a respective modulated off-time of the plurality of modulated off-times.

15. The method of claim 14 , further comprising:

generating a valley detection signal for each valley of the one or more valleys;

incrementing a valley count upon receiving each of the valley detection signals;

comparing a most recent valley count to a most recent modulated integer valley number of the plurality of modulated integer valley numbers; and

emitting a pulse-width modulation (PWM) reset signal upon determining that the most recent valley count exceeds the most recent modulated integer valley number, the PWM reset signal controlling the off-time of the main switch.

16. The method of claim 15 , further comprising:

generating, using an auxiliary winding of the transformer, an auxiliary voltage corresponding to the one or more valleys of the resonant waveform during the off-time of the main switch; and

generating the valley detection signal for each valley of the one or more valleys based on the auxiliary voltage.

17. The method of claim 14 , further comprising:

generating a fractional mode enable signal if a switching frequency of the main switch is greater than a threshold switching frequency; and

generating a fractional mode disable signal if the switching frequency of the main switch is not greater than the threshold switching frequency.

18. The method of claim 17 , further comprising:

generating, by a first modulator, the plurality of modulated integer valley numbers using the first modulator upon receiving the fractional mode disable signal; and

generating, by a second modulator, the plurality of modulated integer valley numbers upon receiving the fractional mode enable signal.

19. The method of claim 18 , wherein:

the first modulator generates the plurality of modulated integer valley numbers using a hysteresis controller having two fixed levels.

20. The method of claim 18 , wherein:

the second modulator generates the plurality of modulated integer valley numbers using a hysteresis controller having a varying first level and a fixed second level.

21. The method of claim 18 , wherein:

the second modulator generates the plurality of modulated integer valley numbers using a sigma-delta modulator.