[SI-LIST] Re: Capacitor surge current

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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