How to design a snubber for a flyback converter

A snubber circuit is essential for Flyback converter, to prevent the transistor from burning up. The overshoot on the transistor voltage, is due to the leakage inductance, $L_k$ , of the transformer.

Variables

$P_s$ - max power dissipated by the snubber resistor
$R_s$ - snubber resistor
$C_s$ - snubber capacitor
$T_s$ - switching period
$f_s$ - switching frequency
$V_g$ - Input voltage to the converter
$V_t$ - max acceptable transistor voltage
$I$ - average input current
$L_m$ - magnetizing inductance of the transformer
$L_k$ - magnetizing inductance of the transformer
$V_{t-peak}$ - peak transistor voltage, spec from datasheet

Transistor snubber design

Leakage inductance

It is not easy to calculate the leakage inductance of a transformer, but it can be measured after the transformer is built, or if a prebuilt transformer is used, it can be obtained from a datasheet. It can be assumed that the leakage inductance is 3% of the magnetizing inductance, $V_m$ .

$L_k \approx 0.03 * L_m$

Snubber resistor

To calculate the snubber resistance, $R_s$ , an acceptable max transistor voltage, $V_t$ . You want to select a $V_t$ that has a wide margin from the peak transistor voltage rating specified in it's datasheet. It still must be greater than the transistors blocking voltage, $V_g + V/n$
$V_{t-peak} > V_t > Vg + V/n$