[SI-LIST] Re: Capacitor surge current
- From: steve weir <weirsp@xxxxxxxxxx>
- To: bmgman@xxxxxxxxxx, dimiter.popoff@xxxxxxxxxxx
- Date: Thu, 11 Mar 2004 21:45:53 -0800
Mike, this is exactly the problem. People forget that 50V is a lot of
potential. Connect it to a low impedance source and all sorts of things
fry easily. Your FET smoke release problem is a common one, and it is
often worse with high transconductance devices feeding big thirsty
capacitor banks. That is why some of the hot-swap controllers monitor
current flow to regulate the current during charge up.
People who build converters, particularly flybacks, frequently miss the
fact that at start-up, the output looks like a dead short through the
magnetics and the rectifier. Pressman covers these topics pretty well in
his books going back to the early '80s when a 555 was still considered
useful as a control element in an SMPS.
Regards,
Steve.
At 11:25 PM 3/11/2004 -0600, Mike Brown wrote:
>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There's an I^2*t limit on the component - probably unpublished - that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
>
------------------------------------------------------------------
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:
