Why Leakage Inductance Destroys Flyback MOSFETs
In a flyback transformer, not all magnetic flux generated by the primary winding couples into the secondary winding. The uncoupled portion manifests as Primary Leakage Inductance ((L_{ ext{lk}})). When the primary MOSFET switches off, the main magnetizing current transfers to the secondary, but the energy trapped in the leakage inductance has nowhere to go. Without a clamp snubber, it generates an instantaneous inductive spike ((V = L_{ ext{lk}} imes rac{di}{dt})) that destroys the MOSFET in microseconds.
The RCD Clamp Architecture and Power Formula
An RCD snubber consists of a fast-recovery Diode, a Storage Capacitor ((C_{ ext{clamp}})), and a Bleeder Resistor ((R_{ ext{clamp}})) across the primary winding:
Clamp Resistor: R_clamp = V_clamp² / P_snub
Peak MOSFET Drain Voltage: V_ds,max = V_in,max + V_clamp
The Crucial Vclamp vs. Vor Rule
The clamp voltage (V_{ ext{clamp}}) must always be set higher than the reflected secondary voltage ((V_{ ext{or}}))—typically (1.5 imes V_{ ext{or}}) to (2.0 imes V_{ ext{or}}). If (V_{ ext{clamp}}) is set too close to or below (V_{ ext{or}}), the clamp diode turns on during the normal secondary flyback conduction cycle, discharging the transformer's main energy into the snubber resistor, incinerating the resistor and destroying efficiency.