An active clamp flyback converter includes a low-side switch that serves as a power switch and a high-side switch that serves as a clamp switch. The high-side switch is operated in one of two active clamp switching modes, which is selected based on load condition. In a complementary active clamp mode, the switching frequency of the low-side switch is decreased as the load increases. In a modified active clamp mode, the high-side switch is turned on in a same switching cycle first by zero voltage switching (ZVS) and second by quasi-resonant switching (QRS), with a QRS blanking time being increased as the load decreases. A ZVS delay time and dead time between switching are adaptively set based on a duty cycle of the low-side switch. The output voltage of the active clamp flyback converter is sensed from an auxiliary voltage of an auxiliary winding of the transformer.