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?

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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.

enter image description here

  • \$\begingroup\$ 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? \$\endgroup\$ – Vladimir Cravero Mar 22 '19 at 14:49
  • \$\begingroup\$ Added some extra information. \$\endgroup\$ – Fat Diode Mar 22 '19 at 15:02
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    \$\begingroup\$ 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. \$\endgroup\$ – Spehro Pefhany Mar 22 '19 at 16:27
  • \$\begingroup\$ 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? \$\endgroup\$ – analogsystemsrf Mar 22 '19 at 17:00
  • \$\begingroup\$ Sounds like what you want is a DC link or snubber capacitor. \$\endgroup\$ – Robert Endl Mar 23 '19 at 3:25

Cap surge is defined by the impulse or step response as V/ESR. Caps with the lowest ESR tend to be able to handle more RMS ripple current and X and Y rated poly caps with self-healing properties are the best at this but do react with high surge currents.

TVS's are designed to withstand a certain Joule or watt-second rating as well as MOV's but these degrade after each event.

Without knowing your SPECS on interface surge standards or ESD test, that you wish to meet for protection, e.g. rise/tail time and voltage, we cannot make any explicit recommendations.

However, in general, the best approach is to use a line filter or LCL filter or use ferrite beads to raise the impedance where your surge current spectrum BW exists up to 10 MHz.

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