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How to design a snubber for a flyback converter

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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

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