Changes: How to design a snubber for a flyback converter

Revision as of 09:04, 5 May 2015

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 specs. $V_g=150V, V = 30V, n = 0.2, f_s = 50kHz, L_m = 320uH, I = 5A, V_{t-peak} = 500V$

Calculations

$V_{t-peak} > V_t > Vg + V/n$
$400V > V_t > 150 + 15/0.2$
$400V > V_t > 225$
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 design

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

design to be added

@@ Line 53: / Line 53: @@
 ===Example===
 flyback using the following specs.
-<math>V_g=311V, V = 30V, n = 0.2, f_s = 50kHz, L_m = 320uH, I = 5A, V_{t-peak} = 500V</math>
+<math>V_g=150V, V = 30V, n = 0.2, f_s = 50kHz, L_m = 320uH, I = 5A, V_{t-peak} = 500V</math>
 ====Calculations====