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Old 01-13-2013, 02:08 PM   #5957 (permalink)
jyanof
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Quote:
Originally Posted by FieroChris View Post
any thoughts on going with the CD4E caps vs the SBE Part 700D10896-348 film cap???

SBE price is $233 for 1000uf at 600v.
CD4E (3x) $89*3 for 660uf at 600v.

there is a 500uf SBE for $172

thoughts ??
I've been helping 64jeep with his controller design. The SBE part is superior from a performance perspective, but is quite a bit larger - we were going for packaging and low parasitic inductance, which kind of go hand-in-hand.

I think the current 500A controller design demonstrates that the capacitor bank only has to handle the ripple current at 'sustained' conditions. 16 of the panasonic electrolyics result in a ripple current capacity of 60A straight off the datasheet. Throw on a multiplier for operating at 16khz instead of 10khz and another for cooler ambient temps, and we could probably double that to 120A.

So, these caps had very little heat generation (not enough to measure any temp difference between them and the rest of the controller parts) when cruising around at 200 motor amps. (an aside - we observed the caps getting a few deg C hotter than the other controller parts when the switching frequency was set to 8khz instead of 16khz - this would essentially double the ripple current demand on the caps, (or halve the cap ripple current rating)).

With my motor/battery voltage configuration, it turns out that cruising happens around 50% duty cycle which Adam pointed out is the worst case duty cycle for ripple current. The cap bank probably wouldn't be sufficient at 500A, but the rest of the controller can't sustain that output anyway. The caps would just heat up a little, then cool back down.


Anyway, the motor cruise amps aren't going to really change, so as long as the CDE caps have a total ripple rating greater than 100A (which 3 of them will), I think we're ok. Nevertheless, the simple thing we plan to do is to throw a temp probe on them and measure if they get hot.
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