An active clamp switching power converter circuit includes a primary-side sensing circuit that generates a sensed voltage based on an auxiliary winding voltage of an auxiliary winding around the core having a same polarity as the primary winding. Based on the sensed voltage, 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 to control leakage current when the power switch is turned off. The controller regulates timing of the switching to turn on the power switch based on timing of a zero voltage switching condition for power efficient operation.