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Old 05-16-2011, 06:54 PM   #2 (permalink)
DJBecker
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Picking capacitors is really, really hard. It's all about trade-offs.

The primary task is to clamp the voltage spike from drive turn-off. If you can't absorb the energy, the voltage spike will avalanche the MOSFETs or IGBTs. So you need at least a minimum capacitance to handle the inductive surge.

We can't tell you how much that will be, since it's highly dependent on the batteries, the battery condition, and the details of the battery cable runs.

If you have the bare minimum, the capacitors will have to deal with a high ripple voltage/current. They'll need a low ESR to not overheat from the current, and a high ESR to limit the current. You can't have both, so you'll need more capacitors to both lower the per-cap current and spread the heat out.

If you are designing for this regime, you pretty much need to use polyester (Mylar) film capacitors.

If you increase the capacitance beyond the minimum, with capacitors that can handle the current, the ripple voltage will decrease. That will decrease the dV/dt, which will widen the range of capacitors you can use. At this point you can start using aluminum electrolytic capacitors.

Continuing to that corner of the design space, you can minimize the ripple voltage rather than just barely handle it. The means much more capacitance, generally a dozen or more high density, moderate ESR capacitors. Generally these are connected close to the battery cable input to the bus bar, with low ESR film caps connected close to each switching device itself to handle the inductive spike from the bus bar.

A side note about this: mixing capacitors with different characteristics can be effective at reducing noise and the peak voltage. Or it can create its own ringing as current "sloshes" back and forth between capacitors. Sometimes you just have to built it and measure.

In general, when picking capacitors you should avoid physically large surplus ones. They are usually designed for power supply filtering, especially handling brief power flickers. They can handle a small ripple voltage at 60Hz, and a partial discharge every once in a while. If you have them absorb the same ripple voltage at 15KHz, that's 250x the power they were designed for. They will heat internally and they don't have the surface area to volume ratio to get rid of the heat.
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