How to calculate snubber components for half and full bridge smps

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1) Put your oscilloscope probe across the LOW SIDE fet (Drain source) or BJT (collector emitter) be sure to put the ground lead
to the negative side of the bulk caps (source of mosfet) (Emmiter of BJT).
2) Isolate your power supply if it is an off line one to be tested with an isolating transformer otherwise there is a good chance of
burning your oscilloscope and your smps.
3) Depending on the voltage across each fet select a suitable probe for your oscilloscope (X10 or X100) If it is an off line psu
240 or 110 volts use (X100 probe)
4) Load your smps enough so that ringing can be clearly seen and focus on it with your oscilloscope
5) Bring out the vertical cursors and measure the ringing frequency
6) Select a small high voltage ceramic or film capacitor around 1nf or so and put it across the primary winding of the
transformer if only one snubber is to be used or across the drain source of each fet if you are using a snubber for each
6) Determine the value that brings the ringing frequency around 3 times the original ringing frequency Cー (ex 1Mhz becomes
7) The resistor that should be placed in series with the capacitor is approximately calculated as per equation.

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Ringing Frequency R =____1_______
measurement 2π FーCー

Loaded psu Where Fー is the ringing frequency and
Cー is the selected capacitor.

Frequency measured = 1Mhz
Frequency with snubber capacitor (1nf) = 3Mhz

So R = _________1______________ = ___1___ = 53 Ohms
2 x 3.142 x 3 x 10^6 x 1x10^-9 0.01885

However the nearest value should be selected in this case a 52 ohm resistor is close enough.
Other combinations are also possible and the best result will be achieved by trial and error

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  1. Silvio's Avatar
    The text was perfect before posting but everything moved where it wanted as soon as I posted this blog and I could not edit this post. Can the administrator arrange it for me please?