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
 * $$V_t$$ - transistor max acceptable voltage
 * $$I$$ - average input current
 * $$L_m$$ - magnetizing inductance of the transformer
 * $$L_k$$ - leakage inductance of the transformer
 * $$V_{t-peak}$$ - transistor peak voltage, spec from datasheet

Transistor snubber design
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, $$L_m$$.
 * Leakage inductance

$$L_k \approx 0.03 * L_m$$

If a transformer is well designed, leakage inductance can be reduced to 1% of the magnetizing inductance.

RCD snubber
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 its datasheet. It still must be greater than the transistors blocking voltage, $$V_g + V/n$$
 * Snubber resistor

$$V_{t-peak} > V_t > Vg + V/n$$

Using this, you can calculate $$R_s$$, and $$P_s$$

$$P_s = 1/2L_fI^2f_s$$

$$V_s = V_t - V_g$$

$$R_s = V_s^2/P_s$$


 * Snubber capacitor

$$C_s >> \frac{ 1 }{ f_s*R_s }$$

The diode voltage must be able to block voltage a high voltage, 1N4007 tends to work.
 * Snubber diode

Example
Flyback using the following specifications:

$$V_g = 150~{\rm V},\ V_{out} = 30~{\rm V},\ n = 0.2,\ f_s = 50~{\rm kHz},\ L_m = 320~{\rm \text{µ}H},\ I = 5~{\rm A},\ V_{t-peak} = 500~{\rm V}$$

Calculations

 * $$V_{t-peak} > V_t > Vg + \frac{V_{out}}{n}$$
 * $$400~{\rm V} > V_t > 150 + \frac{30}{0.2}$$
 * $$400~{\rm V} > V_t > 300$$
 * Select: $$V_t = 325~{\rm V}$$
 * $$L_k = 0.03 \times L_m = (0.03)(0.001) = 30~{\rm \text{µ}H}$$
 * $$P_s = \frac{1}{2} L_f I^2 f_s = (0.5)(30~{\rm \text{µ}H})(1.5~{\rm A})^2(100~{\rm kHz}) = 3.375~{\rm W}$$
 * $$V_s = V_t - V_g = 175~{\rm V} $$
 * $$R_s = \frac{V_s^2}{P_s} = 9074~\Omega$$
 * Select: $$R_s = 10~{\rm k\Omega},\ 5~{\rm W}$$
 * $$C_s >> \frac{T_s}{R_s} = \frac{10~{\rm \text{µ}s}}{10~{\rm k\Omega}} = 1~{\rm nF}$$
 * Select: $$C_s = 47~{\rm nF},\ 500~{\rm V}$$

Schottky snubber design
If you choose to use a schottky diode its a good idea to have a snubber.


 * design to be added