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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

VariablesEdit

  • $ 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.

ExampleEdit

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 $

CalculationsEdit

  • $ 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

ReferencesEdit