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I need advice in between MKT (metalized polyester film) and Ceramic capacitors . I am building a pulse discharging system which is required to discharge about 5KV through 10Ohm resistor in a time as fast as possible (say in pico seconds ~ us). Please can anyone advice me which type is best suited for such an application. The frequency of charging and discharging will be about 100Hz or so. I need this for experimentation with spark generation.

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  • \$\begingroup\$ Holy cow how big are these resistors? \$\endgroup\$ – scld Feb 17 '14 at 14:40
  • \$\begingroup\$ The rate at which the capacitor discharges is a function of its level of charge and the load. What is a MKT capacitor? What is the capacitance of your system? \$\endgroup\$ – HL-SDK Feb 17 '14 at 14:50
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    \$\begingroup\$ @scld For 2.5MW, I'd say they are about... cow sized :) \$\endgroup\$ – Lundin Feb 17 '14 at 14:52
  • \$\begingroup\$ @Lundin haha I didn't realize I actually asked a rhetorical question! \$\endgroup\$ – scld Feb 17 '14 at 15:41
  • \$\begingroup\$ Although, to be fair, the duty cycle of this pulse is (at least, by design) going to be pretty small but....I don't think that was actually considered. \$\endgroup\$ – scld Feb 17 '14 at 15:42
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Picoseconds to microseconds is a range of a million to one, which doesn't really narrow things down much. "I need some money". "How much?" "Oh, somewhere between a hundred bucks and $100,000,000.00, as much as possible".

If you really want to make the discharge time as small as possible, I suggest two copper sheets separated by a dielectric.

For only 100pF, you could use a few square inches of 1/32" (0.8mm) double-sided PCB material, preferably wide and short with the connections across the entire width.

To get the fastest discharge you need to minimize the series inductance (and resistance) which means short wide current paths. Even a few inches of wire has significant inductance, and that will limit how fast the current can change. Even 100nH of series inductance (not much) will resonate with 100pF of capacitance at 50MHz, so you're not going to see really fast edges (even assuming whatever you're using as a switch could operate infinitely fast).

From the added information, perhaps consider something like the AVX HQCEMM101GAH6A, which has very low ESR (< 0.1 ohm) and a SRF of ~350MHz. Not cheap.

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  • \$\begingroup\$ I need around 30A in short pulse of 1~2 nano sec . \$\endgroup\$ – Adi Feb 18 '14 at 16:01
  • \$\begingroup\$ @Adi see my edit above. \$\endgroup\$ – Spehro Pefhany Feb 18 '14 at 16:07
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To answer this, you look in something called the "datasheet" for each of the candidate capacitors. In particular, parameters like the maximum ripple current, maximum pulse current, and equivalent series resistance (ESR) will be most relevant.

You may not find all these specs in all datasheets. Maximum pulse current is what you really want to know, but that is rarely specified. Capacitors with low ESR will have a ESR rating and usually a ripple current rating. The RMS of the ripple current times the ESR causes heating in the capacitor, which is usually what limits the ripple current spec. Calculate your RMS ripple current and eliminate any caps for which it is too high.

The instantaneous pulse current may be limited by other effects. Capacitors intended specifically for this kind of application will probably have a maximum pulse current spec, but otherwise probably not. If you can't find this spec, then identify a few capacitors that have a suitable ripple current spec and otherwise seem appropriate, and contact the manufacturer. Unless you find capacitors specifically intended for high pulse applications, this is a sufficiently off-mainstream application where you should have a conversation with some apps engineers from a few manufacturers.

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  • \$\begingroup\$ Thanks for the reply ! . I am struggling to get good datasheets as most of the manufacturers dont mention most of the details. I need advice regarding which type of caps to shortlist , or is best suited, Metallized polyester type or Ceramic type. It can be seen Ceramic caps are very non-linear ... But maybe its non linearity wont be of much concern in fast discharge type application I dont know. \$\endgroup\$ – Adi Feb 17 '14 at 19:28
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Metalized polyester and polypropylene is better. Polyester for low repetition rate, polypropylene for high repetition rate. Many capacitors in parallel will give lower ESL. Ade Z.

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    \$\begingroup\$ I did not down-vote, but this was probably meant as a comment. \$\endgroup\$ – Rev1.0 Jun 18 '15 at 10:08
  • \$\begingroup\$ If you don't agree than please correct me if I'm wrong! \$\endgroup\$ – user79021 Jun 18 '15 at 12:57
  • \$\begingroup\$ I just was suggesting that the down-vote may have happened because this answer sounds more like a comment to Olin Lanthrop's answer. I have no clue if what you wrote is semantically correct. Nevermind ;) \$\endgroup\$ – Rev1.0 Jun 18 '15 at 14:29
  • \$\begingroup\$ No problem Rev. I have some experience with PFN (pulse forming networks) and I think the best option in this case is diy. \$\endgroup\$ – user79021 Jun 18 '15 at 17:09

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