Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

In the topology Switching power supply (smps) with a transformer, switch off the voltage and current overlap is the main part of power losses when the switch switched off. At the meantime, because of the stray inductance and stray capacitance in the circuit, when the power switch is off, the circuit will also appear over-voltage and generate vibration. It will damage the switch if the peak voltage is too high.  Meanwhile, the existence of oscillation will increase output ripple. To reduce the turn-off loss and voltage spikes, you need parallel the snubber circuit on both ends of the switch to improve the circuit performance.

    The main functions of the buffer circuit: one is to reduce turn-on or turn-off losses; the second is to reduce the voltage or current spikes; third is to reduce dV / dt or dI / dt. The turn off losses is very small because the current decline rate is very quick. The MOSFET is still using the turn-off buffer circuit, but its role is not to reduce the turn-off losses, it  reduce the transformer leakage inductance voltage spike. In this paper, the discuss point is turn-off snubber circuit for the MOSFET

RC snubber circuit design

    RC snubber circuit design must be familiar with the main circuit topology used in the situation. Figure l shows the composition of the RC snubber forward converter. Figure, when Q is off, the collector voltage starts to rise to 2Vdc, and capacitor C limits the collector voltage rise rate, while reducing the rise in voltage and decrease current overlap, thus reducing the loss Q switch. And before the next time the switch turns off, C must have full voltage 2Vdc is done and the discharge path for the C, Q, R. 
 
    Assuming no buffered switch circuit, in Figure 1, the reset winding turns and primary turn of the forward converter are the same. Thus, when Q off instantly, the energy stored in the magnetizing inductance and leakage inductance release, the polarities of primary windings voltage is reverse, the collector voltage of forward converter switch quickly rose to 2Vdc. Meanwhile, the excitation current flows through diode D to reset winding, and finally reduced to zero, then the voltage of Q drops to Vdc. Figure 2 shows the switch collector current and voltage waveforms. Can be seen, the switch without snubber circuit, when the Q switch off, the leakage inductance voltage spikes is large and the produced  turn-off loss is large too. Severely, it may burn the switch severe, therefore, must give the switch tube buffer circuits in the switching power supply (smps).