Application Note AN-4147 Desig

This arTIcle presents some design guidelines for the RCDsnubber of flyback converters. When the MOSFET turns off,a high-voltage spike occurs on the drain pin because of a resonance

between the leakage inductor (Llk) of the main transformerand the output capacitor (COSS) of the MOSFET. Theexcessive voltage on the drain pin may lead to an avalanchebreakdown and eventually damage the MOSFET. Therefore,it is necessary to add an addiTIonal circuit to clamp the voltage.