Input voltage dependent control for active clamp flyback

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.