Re: Organic Aluminum Polymer capacitors in series for higher voltage?


"Paul E. Schoen" <pstech@xxxxxxxxx> wrote in message
news:4861f7eb$0$2727$ecde5a14@xxxxxxxxxxxxxxxxxxx
I need a low ESR high ripple current capacitor for the output of a
switching DC-DC boost converter, and I am considering using several organic
aluminum polymer capacitors in series. What I need is at least about 50 uF
at 60 VDC or more, with ripple current at 100 kHz of about 3 amps RMS. The
closest I could find in standard type low ESR capacitors was a Nichicon UPW
series 470 uF 63 VDC with 55 mOhm ESR and 2090 mA in a 12.5 x 31.5 mm
radial package, which is about the largest I can fit. I can get a 270 uF LF
series polymer cap in a 16 VDC rating that is good for 4800 mA, with 9 mOhm
ESR, in an 8 x 12 mm package, and I think I can fit four of them in the
space I have available.

My concern is that the applied voltage may not be distributed evenly
enough among the four capacitors, and may cause an overvoltage condition
on one or more. The same charging current will be going through each, but
the voltage will depend on the actual capacitance, which can vary 20%, so
I might see 12 to 18 VDC on them with 60 VDC maximum total voltage. The
other concern is during unloaded discharge, where leakage current will
vary and some will discharge faster than others. However, if leakage
increases with applied voltage, and does not vary too greatly, it may be
self-correcting, as those capacitors with more voltage will bleed off
faster than others. I would rather not add bleeder resistors across each
one to equalize the voltage, if possible.

I have looked at large multilayer ceramic capacitors and they are
ridiculously expensive for more than 2 or 3 uF at 100 VDC. I saw some when
I checked stock at Mouser that were nearly $1000 each! Obviously intended
for military, aerospace, or audiophools.

Err, I get a few like this:
http://www.mouser.com/Search/ProductDetail.aspx?qs=hnsWAVXnr6FyB9xGVTFjpA%3d%3d
From $7 in 1-off to under $1 in QTY. 4.7uF 100V 2220 case.

Digikey have ones up to 15uF:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=565-3242-1-ND
But they are more pricey.

What is your space concern - height?, area?, both?
There are ways to reduce both.

Dave.


.

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