Input voltage dependent control for active clamp flyback

1. A power converter, comprising:

a transformer having a primary winding for coupling to an input voltage, a secondary winding providing an output voltage of the power converter, and a sensing circuit for providing a sensing signal through auxiliary winding,

a power switch for coupling to the primary winding of the power converter to control a primary current flow;

a capacitor and a modulation switch coupled in parallel to the power switch, the modulation switch being coupled in series with the capacitor; and

a control circuit, comprising:

a power controller for turning on the power switch to maintain a desired output voltage;

a mode selection circuit for comparing the input voltage with a reference voltage and providing a mode selection signal that indicates a high input voltage mode if the input voltage is higher than the reference voltage and a low input voltage mode if the input voltage is lower than the reference voltage; and

a modulation controller configured to turn on the modulation switch to activate the capacitor according to the mode selection signal, wherein:

in the low input voltage mode, the modulation switch is turned on in a demagnetization-time during which the secondary winding discharges; and

in the high input voltage mode, the modulation switch is turned on in a discontinuous time during which no current flows in the transformer and the sensing signal is characterized by a resonant waveform;

wherein the modulation controller varies a turn-on-time of the modulation switch to add a frequency jitter to a switching frequency of the power converter.

2. The power converter of claim 1 , wherein the modulation controller comprises a delay circuit and a one-shot circuit.

3. The power converter of claim 2 , wherein, in the low input voltage mode, the modulation controller is configured to turn on the modulation switch after a pre-set delay time after the power switch is turned off in every switching cycle of the power converter.

4. The power converter of claim 2 , wherein, in the low input voltage mode, the modulation controller is configured to turn on the modulation switch after a pre-set delay time after the power switch is turned off once in multiple switching cycles of the power converter.

5. The power converter of claim 1 , wherein the control circuit further comprises:

a valley detector for detecting valley points in the resonant waveform in the sensing signal during the discontinuous time; and

a peak detector for detecting peak points in the resonant waveform in the sensing signal during the discontinuous time.

6. The power converter of claim 5 , wherein, in the high input voltage mode, the modulation controller is configured to:

turn on the modulation switch at a first peak point in the resonant waveform after a blanking time; and

turn off the modulation switch after a time period based on a time-varying function to vary a turn-on-time of the modulation switch.

7. The power converter of claim 6 , wherein the power controller is configured to turn on the power switch at a valley point of the resonant waveform after a preset off time of the modulation switch.

8. The power converter of claim 7 , wherein the power controller is configured to turn off the power switch based on a constant on-time, a feedback signal, or a peak primary current sense signal.

9. A control circuit for a power converter, comprising:

a power controller for turning on a power switch to maintain a desired output voltage, wherein the power switch is coupled to a primary winding of the power converter to control a primary current flow, and the output voltage is provided by a secondary winding, and a sensing circuit provides a sensing signal through an auxiliary winding; and

a modulation controller configured to activate a modulation capacitance coupled in parallel to the power switch, wherein:

in a low input voltage mode, the modulation switch is turned on in a demagnetization-time during which the secondary winding discharges; and

in a high input voltage mode, the modulation switch is turned on in a discontinuous time;

wherein the modulation controller is configured to turn on a modulation switch that is coupled in series with a capacitor, and wherein the capacitor and the modulation switch are coupled in parallel with the power switch;

wherein the control circuit further comprises:

a valley detector for providing valley pulse signals at valley points in a resonant waveform in the sensing signal during the discontinuous time; and

a peak detector for providing peak pulse signals at peak points in a resonant waveform in the sensing signal during the discontinuous time.

10. The control circuit of claim 9 , wherein, in the low input voltage mode, the modulation controller is configured to turn on the modulation switch after a pre-set delay time after the power switch is turned off.

11. The control circuit of claim 10 , wherein the modulation controller comprises a delay circuit and a one-shot circuit.

12. The control circuit of claim 9 , wherein, in the high input voltage mode, the modulation controller is configured to:

turn on the modulation switch at a first peak point in the resonant waveform after a blanking time; and

turn off the modulation switch after a time period based on a time-varying function to vary a turn-on-time of the modulation switch.

13. The control circuit of claim 12 , wherein the modulation controller is coupled to the valley detector, the peak detector, and an input voltage magnitude detector to provide a frequency jitter that includes a first portion that varies with the input voltage of the power converter and a second portion that is a time-varying function.

14. A method for controlling a power converter, comprising:

turning on a power switch to maintain a desired output voltage, wherein the power switch is coupled to a primary winding to control a primary current flow and the output voltage is provided by a secondary winding; and

adding a capacitance in parallel to the power switch at a time determined by a magnitude of an input voltage to the power converter, wherein:

in a low input voltage mode, the capacitance is added in a demagnetization-time during which the secondary winding discharges; and

in a high input voltage mode, the capacitance is added in a discontinuous time;

wherein adding the capacitance in parallel to the power switch comprises turning on a modulation switch that is coupled in series with a capacitor, and

wherein the capacitor and the modulation switch are coupled in parallel with the power switch.

15. The method of claim 14 , wherein, in the low input voltage mode, adding the capacitance comprises turning on the modulation switch after a pre-set delay time after the power switch is turned off.

16. The method of claim 14 , wherein, in the high input voltage mode, adding the capacitance comprises:

turning on the modulation switch at a first peak point in a resonant waveform in the discontinuous time after a blanking time; and

turning off the modulation switch after a time period based on a time-varying function to vary a turn-on-time of the modulation switch.