[SI-LIST] Re: Capacitor surge current

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: Christopher.Jakubiec@xxxxxxx
  • Date: Tue, 09 Mar 2004 11:02:43 -0800

Chris, using Miller capacitance in a HV circuit can be very dangerous at 
turn-off time.  It is very easy to destroy the gate of the MOSFET if you 
are not careful.  Something else you could try is a sizeable powdered iron 
inductor.  The cores are cheap, and the material is lossy, something that 
you want.  The ideal solution will still be to effect a slow-start.  If the 
couple of bucks for a soft-start controller is too much you can get 
reasonably close with a couple $.02 transistors and a few discretes.

Steve.
At 10:38 AM 3/9/2004 -0800, Christopher Jakubiec wrote:
>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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