[SI-LIST] Re: Capacitor surge current
- From: Christopher Jakubiec <Christopher.Jakubiec@xxxxxxx>
- To: steve weir <weirsp@xxxxxxxxxx>
- Date: Tue, 09 Mar 2004 10:38:55 -0800
Steve,
The design is already in production, so I don't have the flexibility of making
major changes to the board. I have been experiementing with adding Miller
capacitance between the gate and drain of the FET, and it appears to be helpful
with controlling turn on transients.
-Chris
steve weir wrote:
>
> 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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