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Overloading Film capacitors with ripple current, how well they tolerate it?
Miyuki:
Hi,
I have a question, do you have experience how long and how film capacitors fail under high ripple current ?
I am building for my own use a big adjustable load up to 3kW
It is designed as 3 phase interleaved inverting buck-boost converter at 50kHz, with bank of light bulbs at output, with 24V or 72V settings
It should be able to work at 10-15 minute intervals with total working time in lower tens of hours
It have not to last for ages and should be cheap
Now to capacitor question.
On both sides are film capacitor banks which sees huge ripple current
Is here only problem of heating and resulted overheat or some other failure mechanism ?
moffy:
The losses are typically around 1% of load current (tan(d) metal polyester film) so it depends what their rated power disipation is.
T3sl4co1l:
Hm, why film? Electrolytic will be fine at that kind of frequency. Interleave helps a lot too.
If you must --
1. Don't use PET, it's lossy. It also has runaway losses; once the dielectric starts breaking down, it melts, carbonizes and pretty soon you have a hole spitting goo out of your ex-capacitor. PP is often similar price, or more available anyways.
2. PP losses vary by type, of course. I suppose partly construction, and also how the metallization is applied, how much and where. EMI and DC link types tend to be very thin, so the current ratings are meager.
I've melted B32672s before, they tend to expand, cracking the epoxy fill around the base, extruding gray capacitor-goo that eventually breaks down, starting a spark and shorting the supply. Astonishingly, the breakdown isn't instant; caps can be swollen and oozing without shorting out instantly. I guess there isn't enough shear to mix the plates together, when this happens.
It's not like gooey goo, it's melted plastic, so it hardens once everything cools down again.
On-times of 15 minutes is probably enough for everything to come up to temperature, so I don't think you'll have any savings from thermal performance. You'll need to rate it as good as continuous.
3. Prefer high ripple or snubber types, if possible. More expensive, but more tolerant of abuse. Not always rated for much RMS current, by the way -- CDE 935C series for example heats up pretty quickly despite apparently being of good construction. I suspect some of these have a skin effect limitation, that high frequency currents only flow on the outside of the assembly, hence the current density is higher. The losses may be good at 1 or 10kHz (and this is reflected in the tan d), but not so great at 100 or 1000kHz.
One of the best I've used is Illinois Capacitor PPB series. Once used a bank of 10 x 0.33uF 630V's in parallel, as a coupling capacitor for a 70A, 400kHz load. They got toasty, but handled it no problem.
And TDK has some lines with similar performance. These are more expensive families, and also pretty bulky in large values (which somewhat defeats the current capacity as well -- it's a scaling problem), but can be worthwhile.
4. Consider a resonant topology. Takes as much capacitor and inductor capacity as power output (i.e., about 3kVA worth of each), but can have savings in efficiency, size and ripple. The higher operating frequency is the biggest bonus to size.
Tim
Conrad Hoffman:
+1 on the Illinois cap parts. You want the lowest dissipation factor you can get and that means polypropylene, but they wont' be small and/or you'll have to use several.
schmitt trigger:
Capacitor internal construction is paramount, and in this regard, not all manufacturers are equal.
The only failure I have thoroughly investigated, because it involved a very costly recall, that particular failure was related to the way the terminals were bonded to the film itself.
An uneven conductive epoxy application, which lead to current crowding, overheating and big sparks.
Afterwards, whenever we were considering a new capacitor in a high ripple situation, we would perform due diligence with a capacitor cross section.
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