# 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:

$$C=\frac{1}{2L}\left(\frac{0.546 V}{\frac{dV}{dt}} \right)^2$$

L is the snubber inductor, V is the DC supply voltage and $$\\frac{dV}{dt}\$$ the maximum allowable surge voltage. My question is about the derivation of the said equation—if it is a correct one. The formula looks bizarre to me when considering the equation for getting the right inductor, $$\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 Commented Aug 24, 2017 at 9:01
• Based on my experience I think it's the snubber inductor. Commented Aug 24, 2017 at 9:07
• There is no snubber inductor, just RC snubber. The snubber circuit is connected in parallel with SCR. Commented Aug 24, 2017 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. Commented Aug 24, 2017 at 12:39
• @DorkOrc But even if what you said were true (and I've never heard of a snubber inductor before), this isn't that scenario. Just as you already said, the equation is using dV/dT, not dI/dT. The L here is the load inductance. Commented Aug 12, 2019 at 3:26