An active clamp switching power converter circuit includes a sensing circuit that generates a sensed auxiliary winding voltage of an auxiliary winding around the core having a same polarity as the secondary winding. A controller controls switching of a power switch coupled to the primary winding to control current through the primary winding and controls switching of an active clamp switch based on the sensed voltage to control leakage energy when the power switch is turned off. The controller regulates timing of the switching to achieve a zero voltage switching condition prior to turning on the power switch for power efficient operation.