[SI-LIST] Re: Capacitor surge current
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Christopher.Jakubiec@xxxxxxx, HaroldLSJ@xxxxxxx
- Date: Mon, 15 Mar 2004 15:08:22 -0800
Chris, first 63V is an unacceptable Al electrolytic capacitor rating for
use with a -48V supply. Do they have a safety agency approval with such a
low voltage rating cap? Even without derating, 63V on an Al electrolytic
won't cut-it in a -48V circuit. NEBS as I recall lists 5msec surge well
above that, I think 80V. 125V is more like the required rating. Did they
put two 63V caps in series?
I would concentrate on fixing this problem with the ceramic caps first. I
hadn't thought about the resonant peak that occurs during initial capacitor
charging without some damping resistance, which of course a soft-start
circuit provides. That overcharging is used to advantage by Vicor.
Anyway, I expect the peak charging voltage to be 1.41Vin, not 2.0Vin as
Harold states. Basically, while the capacitor is charging to Vin, current
builds up in the interconnect inductance. For a lossless system, the
energy in the inductor when the capacitor reaches Vin is the same as the
cap: 1/2C*Vin^2. That energy subsequently transfers to the capacitor,
resulting in Voc = sqrt( 2 * Vin^2 ) = 1.41 * Vin. Losses, including
capacitor ESR will limit it a bit lower. That is going to get you pretty
close to 100V but should not take you over, unless Vin >= 70V.
Do you have a clamping device to protect the assembly from overvoltage
and/or reverse battery? A 75V Transzorb type device would be reasonable.
Regards,
Steve.
At 02:42 PM 3/15/2004 -0800, Christopher Jakubiec wrote:
>Harold,
>
>Thanks for the great information. I'm learning more and more about the
>physics of capacitors with the
>help of the list members. One other thing that I left out of my original
>posting, is that we use
>2 independent manufacturers for our fans. Their designs are similar,
>however they use different
>components. With regards to the capacitors that are shorting, we are
>observing failures from
>both fan vendors. One vendor uses a 100V X7R ceramic capacitor as I have
>already stated, and the
>other vendor uses a 63V aluminum electrolytic.
>
>Regards,
>
>Chris
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