At turn-off a high voltage spike appears on the MOSFET due to the transformer’s leakage inductance. This voltage spike can exceed the MOSFETs maximum V_DS rating, leading to failure of the device. Therefore, a clamping circuit is required. There are two types of clamping circuits: the RCD clamp and the diode-zener (or TVS) clamp. The TVS clamp provides better light-load efficiency and lower input power at no load than the RCD clamp because the TVS may not activated at lighter output load. The RCD clamp offers additional damping of parasitic ringing and improved EMI. The TVS diode clamp is used in this design example. The series resistor (R_CLAMP), R2 // R8 in the schematic, creates a snubber effect for the TVS diode clamp for improved EMI, but the voltage stress on V_DS is increased.

The total clamping voltage (V_CLAMP) is designed to meet the 90% derating of the primary MOSFET at V_{IN_MAX}. V_CLAMP also needs to be higher than the reflected voltage on primary to limit the clamping loss. Two TVS diodes, D1 and D3, are connected in series to share the high clamping loss at full load. In the schematic, each 160-V clamp diode exhibits about 200 V at peak current, so the equivalent V_CLAMP is around 400V.

The maximum and minimum clamp voltages can be calculated with the following equations.

The voltage rating of the series rectifier diode (D7) needs to be higher than 1.4 kV, which is the summation of 1000 V_{IN_MAX} and 400 V_CLAMP, so a 1.6 kV device is chosen assuming 90% derating. Instead of an ultra-fast type, the slow-recovery P/N junction diode should be considered, so that the reverse recovery could help to damp the high-frequency ringing after clamping and also recycle partial leakage energy to secondary side for increased converter efficiency.