How To Wiki

How to design a snubber for a flyback converter

1,795pages on
this wiki
Add New Page
Talk2 Share

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. Flyback w snubber


  • 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 designEdit

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 snubberEdit

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.


flyback using the following specs. V_g=311V, V = 30V, n = 0.2, f_s = 50kHz, L_m = 320uH, I = 5A, V_{t-peak} = 500V


  • V_{t-peak} > V_t > Vg + V/n
  • 400V > V_t > 150 + 15/0.2
  • 400V > V_t > 225
  • Select: V_t=325V
  • L_k=0.03*L_m=(0.03)(0.001)=30uH
  • P_s = 1/2L_fI^2f_s = (0.5)(30uH)(1.5A)^2(100kHz)=3.375W
  • V_s = V_t - V_g = ?
  • R_s = V_s^2/P_s = 9074\Omega
  • Select: R_s = 10k\Omega, 5W
  • C_s >> T_s/R_s = (10uS)/(10k\Omega) = 1nF
  • Select: C_s = 47nF, 500V

Schottky snubber designEdit

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

design to be added


Ad blocker interference detected!

Wikia is a free-to-use site that makes money from advertising. We have a modified experience for viewers using ad blockers

Wikia is not accessible if you’ve made further modifications. Remove the custom ad blocker rule(s) and the page will load as expected.

Also on Fandom

Random Wiki