[SI-LIST] Re: Capacitor surge current
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Michael_Poimboeuf@xxxxxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 09 Mar 2004 18:48:51 -0800
Michael, well said. What many people overlook is that a 50V supply is
2500Watt-ohms. Hook that up to a thirsty low ESR, low ESL capacitor and
massive, dangerous surge currents occur. A device that routinely blows
fuses should be recalled as a basic fire hazard.
Regards,
Steve
At 02:30 PM 3/9/2004 -0800, Michael Poimboeuf wrote:
>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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