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Old 04-13-2009, 12:18 PM   #904 (permalink)
Roger Heuckeroth
EV Converter
Join Date: Mar 2009
Location: Saugerties, NY
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Originally Posted by edown View Post
Anyway, I have a question.. Paul uses 24, 200v, 470mf caps. Is it OK to use 3 large, 4400mf 300v caps? Is it the total “mf” that is important?
I want to just screw mount 3 of them straight to the buss bars omitting the PC board.. I can get 10 of them for $30 on ebay right now..

That's a really good question. Paul already mentioned the importance of low ESR. It actually also comes down to heat dissipation. A bunch of smaller cylinders have more surface area for heat exchange than three larger ones, and the life of a capacitor is directly affected by temperature. The hotter they run, the shorter they last.

There is also another reason for going with smaller caps, and it has to do with geometry of the controller. You want to minimize the inductance between the capacitors, the mosfets and the diodes to avoid voltage spikes. Ideally, the connections between the capacitors terminals, the diodes terminals and the Mosfets drain and source are as short as possible. This is because of the massive amounts of current flowing between them.

Otmar, the brains behind the Zilla controller, has been quoted calling this the holy trinity. If you look at his design, these components are physically very close to one another. BTW, his 1000 Amp controller uses 12 - 330 uF caps, which is almost 1/3 of the total capacitance Paul is using. So, I believe Paul's design is very conservative.
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