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I'm seeking recommendations for an inexpensive capacitor that will handle fairly-large pulse currents. Maximum voltage is 15 Vdc applied to the capacitor and I'd like to see current pulses in the 100A range if possible.

I will increase the supply voltage if necessary.

Expected pulse width is determined by the total circuit resistance and is expected to be less than 20us. Repetition rate is about 500 Hz.

My current setup is using a IRF540 MOSFET (RDSon 44 milliOhms max) and a 2u2 250V MKT film capacitor from Epcos (TDK) discharging into a total resistance of less than 0.2 Ohms. Gate drive is about 12V with rise time better than 100ns and fall time better than 200ns.

This current setup is giving me a peak current somewhat more than 20 A and the film capacitor is running slightly above ambient temperature.

I can go to a much better MOSFET but I think that my main limiting factor is the capacitor.

My gut tells me that a ceramic capacitor is going to give me better results than the film capacitor. Question is: Which ceramic capacitor?

Or is there a better capacitor choice that I should be exploring?

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    \$\begingroup\$ It might be easiest to use multiple film capacitors in parallel (5 given your numbers) and pay careful attention to the parasitic inductance (and resistance) between the caps, mosfet and load in the high current path. \$\endgroup\$ – Dean Franks Apr 12 '18 at 0:10
  • \$\begingroup\$ I will try adding additional film capacitors in parallel and see what happens. Great suggestion! \$\endgroup\$ – Dwayne Reid Apr 12 '18 at 5:14
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Are you familiar with VDD planes and GND planes? Learn and use them.

For dI/dT of 100 amps in 100 nanoseconds, you need a TOTAL LOOP INDUCTANCE of 1 nanoHenry

Yet inductance of a wire is approximately

1uH/meter * log(1 + a bunch of stuff)

thus your total length must be only 1milliMeter. Unless you explore planes, and capacitors in parallel, and FET switches in parallel.

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  • \$\begingroup\$ I'm not expecting to get 100 Amps in 100ns. I'm expecting somewhere between 5us to 20us - or even longer. That's a whole lot easier. Please note that the only time that I mentioned the figure of 100ns is in reference to the gate signal driving the FET. \$\endgroup\$ – Dwayne Reid Apr 12 '18 at 5:11
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I believe that most high energy capacitors are ultimately film type capacitors. Your best bet is to put multiple film capacitors in parallel to get the required pulse current.

If you need fast rise/fall times on the output pulse you will have to very carefully manage the inductance and resistance of the entire discharge loop (capacitors to MOSFET to load including the ground return path). Short, wide wires or better yet bus bars if your components are leaded, wide traces and thick copper with no vias in the current loop if you are surface mount.

Also watch out for parasitic capacitance in the MOSFET -> load region, this will slow the edges of the output pulse as well.

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  • \$\begingroup\$ I did add another identical capacitor in parallel with the existing cap. Peak current increased by about 30% - perhaps more. Obviously current is limited by wire lengths and not-optimized wire layout. \$\endgroup\$ – Dwayne Reid Apr 14 '18 at 15:34

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