I've been playing with a couple of circuits for driving LEDs with short, high-current pulses (so far up to about 10 amps per LED chip). At microsecond pulse lengths, this isn't the kind of current you get through wires from your supply; this is the kind of current you draw from local capacitors, planning the current path in tight loops to minimize inductance. So, I need some capacitors. I found some surprising things while researching them. Here are the three types I've looked into:

On the left is a 10uF 25V MLCC (1206 package). Center, we have a 40uF 320V photoflash capacitor. You don't find too many with smaller capacitance than this. Finally, we have a 10uF 250V metalized polyester capacitor. These things each have their drawbacks.

MLCC

The multi-layer ceramic capacitor is a modern wonder - how do they pack 10uF of capacitance into that tiny case? They use lousy dielectrics that lose most of their capacitance with applied voltage. Here's a curve I pulled from one of the datasheets for a 10uF 35V cap:

The curve shows the change in capacitance vs the applied voltage. If you use the capacitor at the rated voltage, you lose more than 90% of the capacitance - the 10uF is now less than 1uF. Of course, you always de-rate components, but even if you de-rate this voltage by a factor of 2.3 and use the cap at 15V, you've still lost 80% of the original 10uF. This graph is for an X7R dielectric, which is specified for +/-15%, but that has to do with changes over temperature, not applied voltage.

They're not all this bad. I couldn't find this graph for the capacitors I've been using, but the loss of capacitance at higher voltage is clear from the waveforms produced (detailed in the next log). A possible (expensive) workaround is to parallel many of these capacitors so that you end up with the desired capacitance at the working voltage. Even with 5 of them paralleled on my test board, I still see problems.

The second problem with these capacitors is that the dielectric is typically piezoelectric. In sensitive circuits you can get microphonic noise introduced into your signals. In my case, I have the opposite issue - the capacitors issue a troubling "snap" noise each time the LEDs are pulsed. I can only wonder if this mechanical stress will eventually damage the capacitors. I've had MLCC's "sing" before in circuits, and while it's annoying, I had never feared for their safety. This time, I'm not so sure.

Finally, these caps seem to top out at around 50V ratings. I had hoped to keep the LED flash circuits strictly low-voltage to make them safer than xenon flashtube versions, but driving high currents into strings of LEDs looks like it will take some higher voltages.

After experimenting with theses caps for a while, I've decided to look at the other two.

Photoflash Electrolytics

The second suspect is the capacitor behind the typical xenon flashtube. These high-voltage electrolytics are designed to have low ESR (equivalent series resistance) and ESL (equivalent series inductance) so they can deliver high-current pulses in the millisecond range or so. Normal electrolytics, even higher voltage types, typically aren't rated for the high pulse currents involved. I haven't tried this particular cap in the flash circuit yet, but I worry that the ESL will be too high to generate very short pulses. I can't find any specs on ESL for these, so I'll just have to try them. Electrolytic caps also have a lousy lifetime, but they're cheaper than the film types below.

Another drawback is that these specialized capacitors aren't typically stocked by the usual, reliable distributors. I was forced to find some on ebay. Who knows what I actually got.

Metalized Polyester

The last cap is probably electrically the best. I chose some 10uf 250V caps made by Illinois Capacitor - I won't be using voltages this high, but chose this unit for the current rating. These capacitors are rated for a maximum dV/dt of 15 V/us. For a 10uF capacitor, that translates to a current of 150A. I figure I can safely use them for 100A pulses or more while maintaining a healthy de-rating. As far as ESL, the datasheet states less than 1nH per mm of body and lead length - probably around 40nH mounted on a PCB - not bad at all. Finally, the ESR can be estimated from the dissipation factor, which is rated less than 1%. This document explains dissipation factor, which is:

where Xc is the capacitive reactance at the test frequency. At the 1 kHz test frequency, a 10uF capacitor has a reactance of:

So, the ESR should be less than 1% of 15.9 ohms or 159 milliohms. A 100A pulse would drop a maximum 16 volts across the ESR - with a maximum of 250V to play with, we can just crank the voltage to compensate.

The downsides are that these capacitors are huuuuge and expensive.

EDIT 20161115

I forgot to mention that some of my thoughts about capacitor selection were influenced by Don Klipsein's notes about bullet photography with traditional flashlamps. His advice is to use the foil types for the kV-level voltages involved in that kind of ultra-short xenon flash driver. My thought was that if they can deliver microsecond pulses to flashlamps, they can do the same for LEDs. Of course, with LEDs, you can easily control the flash duration by switching off the driver, instead of relying on a small capacitor to fully discharge.