With reference to FIG. 11, when the switch 12 is turned off, the voltage of the first node n1 starts to rise. As the voltage of the first node n1 starts to rise, the energy stored in the first capacitor C1 is returned to the power source 13 through the first diode D1. Namely, the first capacitor C1 discharges to output a current Ic1 to return the energy stored in the first capacitor C1 to the power source 13. Further, the clamping current Iclamp may flow to the second capacitor C2 to avoid voltage spikes.

In other words, when the voltage of the first node n1 starts to rise, the energy stored in the first capacitor C1 is returned to the power source 13 through the first diode D1. The reduced rising of the voltage of the first node n1 allows a current flowing through the switch 12 to return to zero while the voltage of the first node n1 has little or no voltage applied, reducing switching losses and improving efficiency.

Besides, when the switch 12 is turned off, the primary current Ip flowing through the primary winding Wp and magnetic flux produced by the primary current Ip drop. A voltage induced in the secondary winging Ws is forward-biasing the output diode Dout, allowing an output current Iout to flow from the secondary winding Ws of the transformer. The energy from the transformer recharges the output capacitor Cout and supplies the load 14.

Further, since the clamping winding Wc is magnetically coupled with the primary winding Wp, the clamping winding Wc may create a voltage offset equal to

$V_{\mathrm{out}}\times \frac{N_{}}{}$