[SI-LIST] Re: Capacitor surge current

Steve,

The design that I am working with does not really have a hot-swap controller.  
A power MOSFET is used to switch in the 48V return path based on a couple of 
digital logic signals, so power does not come up as softly as one might prefer. 
 The 48V is powering cooling fans that are rated at 48V, 800mA.  It is a single 
1uF, 100V capacitor on the 48V input to the fans that appears to be shorting 
and in turn blowing a 2A series fuse as well.  I am using a current probe on 
the positive 48V side just
before this capacitor to get an inrush current reading of 16A.

Thanks for your input,

Chris


steve weir wrote:
> 
> Chris, generally the current is limited by power dissipation capacity
> determined by ESR.  16A destroying a big ceramic capacitor doesn't seem
> right.  Are you sure it isn't more like 100A?
> 
> It may be too late for your particular design right now, but usually on
> power entries, I try to use absolutely minimal capacitance on the line side
> of the hot-swap switch to prevent arcing and deplating problems.
> 
> Steve.
> At 08:37 AM 3/9/2004 -0800, Christopher Jakubiec wrote:
> >Group,
> >
> >Does anyone have experience and/or knowledge of the susceptability of
> >ceramic surface mount capacitors to surge/inrush currents?  I have a 48V
> >circuit that uses a 1uF, 100V, X7R SMD ceramic capacitor and it appears
> >that the capacitors are shorting out in some cases.  I have measured the
> >inrush current during turn on, and I am finding a current spike on the
> >order of 16A for approximately 40uS, and then the current decays for
> >another 160uS or so.  The voltage at turn on is peaking at about 58V.
> >Most capacitor manufacturer data sheets that I have looked at do not
> >really specify this specifically.
> >
> >Thanks,
> >
> >Chris Jakubiec
> >Sun Microsystems
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