# Silvio

#### How to calculate snubber components for half and full bridge smps

by

, 07-20-2017 at 08:57 PM (381 Views)
HOW TO CALCULATE THE SNUBBER COMPONENTS FOR SMPS

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

transistor.

6) Determine the value that brings the ringing frequency around 3 times the original ringing frequency C° (ex 1Mhz becomes

3Mhz).

7) The resistor that should be placed in series with the capacitor is approximately calculated as per equation.

Ringing Frequency R =____1_______

measurement 2π F°C°

Loaded psu Where F° is the ringing frequency and

C° is the selected capacitor.

Example

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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