[SI-LIST] Re: Capacitor surge current

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: Christopher.Jakubiec@xxxxxxx, HaroldLSJ@xxxxxxx
  • Date: Mon, 15 Mar 2004 15:28:15 -0800

Oops, Harold is right, 2*Vin.
At 03:08 PM 3/15/2004 -0800, steve weir wrote:
>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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