Power converter with adaptive active clamp

A method of operation for an active-clamp flyback converter, comprising:

• measuring a peak charge time period during which an active-clamp capacitor charges to a peak charge in response to a switching off of a power switch transistor;
• during an off-time of the power switch transistor after the active-clamp capacitor is charged to the peak charge, switching on an active-clamp switch transistor at an active-clamp on-time that equals a difference between a subsequent power switch on-time of the power switch transistor and the peak charge time period;
• switching off the active-clamp switch transistor at an active-clamp off-time that equals the active-clamp on-time plus the peak charge time period;
• and
• switching on the power switch transistor at the subsequent power switch on-time.

An active-clamp controller for an active-clamp flyback converter, comprising:

• a comparator configured to compare a voltage across an active-clamp resistor to a threshold voltage;
• a counter configured to provide a count representing a peak charge time of an active-clamp capacitor, the counter being further configured to stop a counting of the count in response to an output signal from the comparator indicating that the voltage across the active-clamp resistor equals the threshold voltage;
• and
• an active-clamp switch transistor gate driver configured to switch on an active-clamp switch transistor at an active-clamp on-time that equals an on-time of a power switch transistor minus the peak charge time.