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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.

1 Variables
2 Transistor snubber design
- 2.1 RCD snubber
- 2.2 Example
  - 2.2.1 Calculations
3 Schottky snubber design
4 References

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
$V t - p e a k$ - 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 it's datasheet. It still must be greater than the transistors blocking voltage, $V g + V / n$
$V t - p e a k > V t > V g + V / n$

Using this, you can calculate $R s$ , and $P s$

$P s = 1 / 2 L f I 2 f s$
$V s = V t - V g$
$R_s = V_s^2/P_s$

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. $V g = 150 V, V = 15 V, n = 0.2, f s = 100 k H z, L m = 1 m H, I = 1.5 A, V t - p e a k = 400 V$

CalculationsEdit

$V t - p e a k > V t > V g + V / n$
$400 V > V t > 150 + 15 / 0.2$
$400 V > V t > 225$
Select: $V t = 325 V$
$L k = 0.03 * L m = (0.03)(0.001) = 30 u H$
$P s = 1 / 2 L f I 2 f s = (0.5)(30 u H)(1.5 A) 2 (100 k H z) = 3.375 W$
$V s = V t - V g = 325 - 150 = 175 V$
$R_s = V_s^2/P_s=(175V)^2/(3.375W)=9074\Omega$
Select: $R s = 10 k Ω$ , 5W
$C s > > T s / R s = (10 u S) / (10 k Ω) = 1 n F$
Select: $C s = 47 n F$ , 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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How to design a snubber for a flyback converter

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