[SI-LIST] Re: Capacitor surge current
- From: "Michael Poimboeuf" <Michael_Poimboeuf@xxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 9 Mar 2004 14:30:02 -0800
Your design may be in production, but it clearly has a safety problem
with surge current. This class of problem (turn on surge current in
caps) has been at the root of many product recalls - especially laptops.
I worked on one computer recall myself. We found that in additon to
surge current as the proximate cause of failure, the caps were weakened
by overheating during reflow (very common) and sometimes during rework.
If you ship a product like this to Japan you're asking for serious
criminal charges, they have strict regulations regarding
fire/overheating. Many companies in Japan power cycle their equipment
every day as a standard operating procedure - this componds surge
current problems.
Those who live in paper houses should not experience surge current
failures. - mkp@sgi circa 1997
It's your duty to give your production/operations folks notice of this
failure mode.
--
Michael Poimboeuf
-----Original Message-----
From: Christopher Jakubiec [mailto:Christopher.Jakubiec@xxxxxxx]=20
Sent: Tuesday, March 09, 2004 10:39 AM
To: steve weir
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Capacitor surge current
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:
>=20
> Chris, I don't know where you are with the design, but would urge you=20
> to put some sort of controlled impedance between any 50V supply and=20
> capacitors. A 1uF 100V 1812 MLCC cap has only 26milliohms ESR. If=20
> you connect that to 50V, but for other parasitics, theoretically,=20
> there are almost 2000 peak amps available. There are lot's of little=20
> discrete circuits you can make with a couple of transistors to control
> turning that FET on in a controlled manner. An alternative is the=20
> tried and true NTC thermistor. The problem with those is that they=20
> need to cool down to reset. So, if this is a fan tray that someone=20
> might remove and reinsert before it cools, you will have lost your=20
> protection. If you are blowing a 2A fuse in under 1ms, then you must=20
> have a current on the order of 100A.
>=20
> 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=20
> >controller. A power MOSFET is used to switch in the 48V return path=20
> >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=20
> >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=20
> >of 16A.
> >
> >Thanks for your input,
> >
> >Chris
> >
> >
> >steve weir wrote:
> > >
> > > Chris, generally the current is limited by power dissipation=20
> > > capacity determined by ESR. 16A destroying a big ceramic=20
> > > capacitor doesn't seem right. Are you sure it isn't more like=20
> > > 100A?
> > >
> > > It may be too late for your particular design right now, but=20
> > > usually on power entries, I try to use absolutely minimal=20
> > > capacitance on the line side of the hot-swap switch to prevent=20
> > > 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=20
> > > >susceptability of ceramic surface mount capacitors to=20
> > > >surge/inrush currents? I have a 48V circuit that uses a 1uF,=20
> > > >100V, X7R SMD ceramic capacitor and it appears that the=20
> > > >capacitors are shorting out in some cases. I have measured the=20
> > > >inrush current during turn on, and I am finding a current spike=20
> > > >on the order of 16A for approximately 40uS, and then the current=20
> > > >decays for another 160uS or so. The voltage at turn on is=20
> > > >peaking at about 58V. Most capacitor manufacturer data sheets=20
> > > >that I have looked at do not really specify this specifically.
> > > >
> > > >Thanks,
> > > >
> > > >Chris Jakubiec
> > > >Sun Microsystems
> > > >-----------------------------------------------------------------
> > > >-
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