Thanks to all who responded.

Florian got to the heart of my question: does the temp spec refer to
capacitor case temp or ambient temp. The particular caps I was using
are Panasonic FC series. Maybe I am blind but I couldn't find any
definition in that datasheet of where the temperature was to be
measured.

I've seen higher-end cap datasheets but they usually go into a lot
more detail (which is good), but still doesn't tell me how Panasonic
chose to spec their temp data.

Actually, I discovered this morning that I don't need nearly as much
capacitance in this circuit (high current motor drive) as I had
thought, so it looks like I can get rid of the electrolytics and just
use many MLCC chip caps (which I already had present in the design,
sprikled about the PCB  to take the fast current edges from the FET
switching). If I simply double the quantity of these little caps, I
will be able to eliminate the electrolytics and save a lot of board
space, too. The MLCC chip caps are rated for something like 3 A rms
ripple each(since their ESR is so low), so they should be able to
handle the current (in the neighborhood of 80 to 90A rms) when spread
over the large number (more than 100).

Thanks,

Sean


On 4/14/07, Florian Voelzke <fvoelzke@gmx.de> wrote:
> For a better understanding of the temperature specs of electrolytic caps
> I recommend looking at some high power high voltage cap datasheets.
> There you usually find graphs with useful live versus ripple current and
> temperature and more. Evene if your cap datasheet does not have this
> info, it gives you a general clue how the different parameters work
> together. And be careful how the manufactor specifies the temperature
> value! Is it body temp, surface or ambient? How is ambient defined and
> so on...
>
> One quick example:
> Look at the B43511 series from epcos
>
> Regards,
> Florian
>
> Sean Breheny wrote:
> > Hi all,
> >
> > Can anyone tell me exactly what the temperature rating for aluminum
> > electrolytic caps means? When the datasheet says 105 deg C max, which
> > of the following (if any) does this mean:
> >
> > a) the capacitor will operate within specs (including max ripple
> > current) when the ambient temp is 105 deg C (so the case could
> > actually be 20 to 30 deg C hotter)
> >
> > b) the capacitor will operate within specs when the case is 105 deg C
> > (so the ambient may need to be 20 to 30 deg C cooler if running at max
> > ripple current)
> >
> > c) the capacitor will operate within specs when the maximum internal
> > temperature (hot spot) is 105 deg C (very difficult to determine
> > without more data from the manufacturer)
> >
> > Thanks,
> >
> > Sean
>
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