Please find attached file and help me to calculate the value of capacitor & Resistor for R-C Snubber. For the frequencies of 20KHz & 50KHz. Please also provided a simple formula.

Attached Images

Snubber.jpg (31.4 KB, 34 views)

 

1. THe capacitor should be much greater than the capacitance of the MOSFET to properly dampen the oscillations and absorb the voltage spike (the specific capacitance is the one which the parasitic inductance will oscillate). As the capacitance gets bigger your have less overshoot but you also dissipate more power in the resistor and there are diminishing returns for larger and larger capacitors. The optimal value where the diminishing returns starts to really kick in and the power dissipation starts to increase too much for what you get is around 2,3,4x the MOSFET capacitance.

So just pick a capacitor value based on what I said above (there are ways to optimize but...yeah you need to actually measure and know the inductances in the circuit etc).

2. THe resistor's purpose is to stop the transistor from burning out when the capacitor discharges it's stored energy when the transistor switches on by limiting the capacitor's discharge current. So it is preferably it is as low as possible so that it interferes as little as possible with the capacitor's absorbing of the spike (the inrush current through the resistor will cause the voltage spike to be higher).

So use R=VI where I is the operating current and V is the maximum voltage spike you can tolerate.

3. To figure out the resistor power rating you need use:
P = fCV^2

And of course, make sure you use resistors and capacitors with minimal inductance and that the capacitor can handle the current spikes or dv/dt (depending on the manufacturer's datasheet). This is just two different ways to rate basically the same thing since I = C*dv/dt and you can jump back and forth between the two.

This is all from:
http://www.cde.com/tech/design.pdf

 

dknguyen, would you please , if you have, send other works/books by Rudy Severn.He has recently published one about snubber circuit.

 

Sorry half of everything I know about snubbers is in that one document. THe other half is in one other article. I'm just an end-user of snubber circuits. I didn't even notice that the article even had an author! Let alone that he wrote books. What's the name of the book?

 

Another good source is Marty Brown, Power Supply Cookbook, 2nd ed., EDN Series, ISBN-13:978-0-7506-7329-7, pp. 145-148.

John

 

The book's title is "Snubber Circuits for Power Electronic" by Rudy Severn, published at the end of April 2008 and it's all about snubber circuit.