[SI-LIST] Re: Capacitor surge current
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Christopher.Jakubiec@xxxxxxx
- Date: Tue, 09 Mar 2004 10:28:00 -0800
Chris, I don't know where you are with the design, but would urge you to
put some sort of controlled impedance between any 50V supply and
capacitors. A 1uF 100V 1812 MLCC cap has only 26milliohms ESR. If you
connect that to 50V, but for other parasitics, theoretically, there are
almost 2000 peak amps available. There are lot's of little discrete
circuits you can make with a couple of transistors to control turning that
FET on in a controlled manner. An alternative is the tried and true NTC
thermistor. The problem with those is that they need to cool down to
reset. So, if this is a fan tray that someone might remove and reinsert
before it cools, you will have lost your protection. If you are blowing a
2A fuse in under 1ms, then you must have a current on the order of 100A.
Steve
At 10:16 AM 3/9/2004 -0800, Christopher Jakubiec wrote:
>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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