With reference to FIG. 12, the waveforms of the voltage of the first node n1 and the voltage of the second node n2 are shown. At the moment that the switch is turned off, the first diode D1 may be turned on, and the first capacitor may provide a current loop to maintain a current flowing through the primary winding Wp to avoid the voltage spike.

In conclusion, the primary winding Wp to secondary winding Ws leakage inductance energy is recovered by storing the energy in the second capacitor C2, similar to a RCD clamp snubber. The difference is that the clamping winding Wc is a voltage translation of the primary winding Wp referenced to the ground. The energy is recovered from the second capacitor C2 when the switch 12 turns on and the energy is transferred through resonant tank including the first inductor L1 charging the first capacitor C1 up to Vsource. When the switch 12 turns off, the first capacitor C1 slows down switching transition of the switch 12 for a reduction of EMI and switching losses, and returns leakage energy to the power source 13 through the first diode D1.

Further, the third capacitor C3 is a coupling capacitor between the primary winding Wp and the clamping winding We to recover energy from the primary winding Wp to clamp leakage inductance, and the voltage across the third capacitor C3 is typically equal to Vsource.