# Formula of snubber capacitor capacitance for SCR dv/dt protection

I have a formula here for finding the suitable value of shunt capacitance to be put on an SCR snubber.

$C=\frac{1}{2L}(\frac{0.546V}{dv/dt})^2$

where L is the snubber inductor, V is the DC supply voltage and $dv/dt$ the maximum allowable surge voltage. My question is about the derivation of the said equation$-$if it is a correct one. The formula looks bizarre to me when considering the equation for getting the right inductor, $V = L\frac{di}{dt}$, which is more straightforward. Moreover, I'm puzzled why choosing the capacitor value would depend on the series snubber inductor.

• L is the load inductance – Marko Buršič Aug 24 '17 at 9:01
• Based on my experience I think it's the snubber inductor. – DorkOrc Aug 24 '17 at 9:07
• There is no snubber inductor, just RC snubber. The snubber circuit is connected in parallel with SCR. – Marko Buršič Aug 24 '17 at 10:28
• Yes, and an inductor would be connected in series to protect the SCR from di/dt. So essentially it is also in series with and becomes part of the load, as you said it is load inductance. – DorkOrc Aug 24 '17 at 12:39

## 1 Answer

Interesting question.

The inductor stores energy in the current: 0.5 * L * I^2

The snubber capacitor needs to store that same amount of energy, as voltage across the capacitor: 0.5 * C * V^2

Equate the two energy equations, and have fun.