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Old 10-19-2015, 09:02 PM   #2206 (permalink)
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Regarding sizing those SBE ring capacitors, I found this:

*** edit ***
Yes, this paper is too good to be true. Best to avoid it - I'll keep looking.

It is not by SBE. Again, I'll keep looking.

Anyway, the numbers I come up with are quite small for a 50kW inverter. In fact, all that matters, according to the paper is ripple current and ripple voltage.

After running through their numbers, the very worst I could get, using 650VDC bus and the 1.4mH inductance of the motor @ 5kHz switching frequency is 23A peak for the ripple current. That's nothing for these capacitors. They can handle 100A>300A all day long.

For the ripple voltage calculation, I spec'd 1% regulation. Again, the worst case was 5kHz switching frequency, leading to 178uF for the bus capacitor.

If the switching frequency is increased to 20kHz for the SiC FET's, the numbers would be tiny and I would need less than 20uF.

This seems too good to be true. I mean, for the design I showed, I would need the 8.5" diameter capacitor just to physically fit things. The 8.5" 1200V/220uF capacitor would be overkill in about every way one could look at it.

Am I missing something? ?

- E*clipse

Last edited by e*clipse; 10-20-2015 at 02:29 PM..
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