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[]

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, $L_m$ .

$L_k \approx 0.03 * L_m$

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

RCD snubber[]

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 its datasheet. It still must be greater than the transistors blocking voltage, $V_g + V/n$
$V_{t-peak} > V_t > Vg + V/n$

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

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

Snubber diode

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

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: $C_s = 47~{\rm nF},\ 500~{\rm V}$