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How to design a snubber for a flyback converter

Jurassic World: Dominion Dominates Fandom Wikis - The Loop

01:25

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, , of the transformer.

Flyback w snubber.svg

Variables

  • - max power dissipated by the snubber resistor
  • - snubber resistor
  • - snubber capacitor
  • - switching period
  • - switching frequency
  • - Input voltage to the converter
  • - transistor max acceptable voltage
  • - average input current
  • - magnetizing inductance of the transformer
  • - leakage inductance of the transformer
  • - 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, .


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, , an acceptable max transistor voltage, . You want to select a 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,

Using this, you can calculate , and





Snubber capacitor



Snubber diode

The diode voltage must be able to block voltage a high voltage, 1N4007 tends to work.

Example

Flyback using the following specifications:

Calculations

  • Select:
  • Select:
  • Select:

Schottky snubber design

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

design to be added

References

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