# How do select capacitor for 400A surge currents?

I have a 220nF capacitor in a series with my surge protection circuit.

As can be seen from my spice simulation, the capacitor gets a surge of current passing through it under fault conditions, and ideally, I don't want the capacitor to explode. From looking at capacitor datasheets, surge current and power capacities of capacitors are unclear.

How do I go about selecting a capacitor for my application?

For simplicity, I have cropped the schematic to show where the capacitor is located. The Surge comes from the primary of the transformer(left) which has some leakage attached (L4 & L5). The transformer is 1:1 and has two ohms series resistance.

When the surge comes through, the output TVS clamps through the capacitor.

• Hi, can you also post your schematic and give a bit of insight? Did you use an ideal cap in your spice simulation, or did you add some series resistance? – Vladimir Cravero Mar 22 '19 at 14:49
• Added some extra information. – Fat Diode Mar 22 '19 at 15:02
• Is that a theoretical SPICE current or is it realistic in terms of what might actually occur? 400A through 220nF implies dv/dt of almost 2000V/$\mu$s, rather high. – Spehro Pefhany Mar 22 '19 at 16:27
• 400 amps in a triangular waveform, Trise of 0.5 microseconds. If the circuit has 20 nanoHenry of inductance (about an inch of wire), the voltage will be V = L * dI/dT = 20nH* 400amp/500nSec = 8000/500 = 16 volts. Do you care? – analogsystemsrf Mar 22 '19 at 17:00
• Sounds like what you want is a DC link or snubber capacitor. – Robert Endl Mar 23 '19 at 3:25