[SI-LIST] Re: Capacitor surge current
- From: Mike Brown <bmgman@xxxxxxxxxx>
- To: dimiter.popoff@xxxxxxxxxxx
- Date: Thu, 11 Mar 2004 23:25:15 -0600
It seems to me that the capacitor explosion phenomenon is likely a peak
power problem.
The stored energy is Cv^2/2. That's also the energy dissipated in the
process of charging the cap. Where does this dissipation happen? In
the Z of the charging supply, and in the ESR of the cap. If the ESR is
large compared to the supply impedance, the dissipation is mainly in the
cap. The power is E/t, and t can be pretty small.
There's an I^2*t limit on the component - probably unpublished - that
lets the smoke out if it is exceeded.
We were popping inrush limiting FETs a while back, due to an unannounced
change (a so-called process improvement, putting a smaller die in the
package, raising the thermal resistance and decreasing the thermal
capacity) in the I^2*t rating of the FET we were using. I see no
fundamental reason why the cap shouldn't pop due to similar overstress.
Mike
Dimiter Popoff wrote:
> Quite recently I had a related problem. I designed for a guy a device
> which included a +/- 100V , apr. 10W DC-DC convertor. No rocket science,
> things went just as expected.
> I had put a number of 1206 ceramic caps (like a stripe under the
> electrolythic
> one), which was 100% overkill but I just repeated what I usually did
> when noise is a real issue (like 14 bit precision @ 10 MSPS, which was
> worlds away from this particular case).
> The 1206 caps were rated at 200V. Well, after a few days of usage the first
> of
> them exploded. Another few days later the next one; and then another one...
> The common was that they did it on power on.
> A closer look revealed I had messed up the soft start circuit and the
> convertor would start abruptly; then, the coil was 30W capable (I know
> this from other designs) and the secondary was really low impedance.
> Frankly, I would not have believed this were possible if I had been told
> that before - but the inrush current really blasted those caps (which
> we tested to truly sustain 200V DC over time and which never saw more
> than 100V in the actual circuit, no peaks etc.).
> Has anyone had similar experience? I cannot think of another reason
> why the 1206 ceramic caps would explode upon power up without any
> overvoltage spikes, but then I did not really measure/record the
> inrush currents I suspect, so the issue - while practically solved - is
> not quite closed for me.
>
> Dimiter
>
> -----------------------------------------------------------------
> Dimiter Popoff ++359/2/9923340
> Transgalactic Instruments, Gourko Str. 25 b, 1000 Sofia, Bulgaria
> http://tgi.cit.bg tgi@xxxxxx dimiter.popoff@xxxxxxxxxxx
> -----------------------------------------------------------------
>
>
>
>>To: si-list@xxxxxxxxxxxxx
>>From: Hal Murray <hmurray@xxxxxxxxxxxxxxx>
>>Subject: [SI-LIST] Re: Capacitor surge current
>>Date: Thu, 11 Mar 2004 09:10:44 -0800
>>
>>
>>Suppose I was trying to design a good/safe circuit using large ceramic caps.
>>Is there anything in the data sheet that tells me how much current they can
>>take?
>>
>>Most data sheets for electrolytic caps have ripple current specs. I don't
>>remember seeing anything like that for ceramic caps. I just scanned a few
>>data sheets that I had handy. I didn't see anything, but maybe other data
>>sheets are better.
>>
>>What sort of low duty cycle peak currents are reasonable? The particular
>>case that started this discussion seems to be due to a turn-on surge. I'm
>>surprised that an occasional event like that would harm a capacitor. How
>>would I calculate a safe rise time? Or is that even the right question?
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
