At 03:44 PM 07/04/2011, you wrote:
>I was just chatting with a co-worker about acceptable margin on rated
>voltages for
>capacitors, and I found myself effectively regurgitating the common wisdom
>on that:
>
>1. Tantalum capacitors are touchy and should be well derated (probably by =
to
>50%
>of their stated value).

Depends on the temp range and peak current. I've seen manufacturer's
recommendations of 3:1 or more.

>2. Aluminum and ceramic capacitors are less touchy and can be used at clos=
er
>to
>the stated value but still should be derated (say, to 75%).

You can use them at the full rated voltage, but you should make sure
that the rated voltage is never exceeded under any circumstances (ceramic)
or by only a bit and for only a very short time (aluminum).

You should also not exceed the maximum rated RMS ripple current, and should
be aware that self-heating will reduce the lifetime.

>3. "Name-brand" capacitors will perform better than no-name types.

Yes, generally. For example, consider this one:
http://bobg-nowadayswhat.blogspot.com/2008/10/nice-chinese-made-capacitor-c=
e-approved.html


>Note that this is effectively only for power supply bypass type
>applications.
>It occurs to me that this information has just kind of...appeared in my
>head,
>aggregated from many conversations over the years.
>
>It also occurs to me that capacitor manufacturers have likely heard these
>rules
>of thumb as well.

You can get lots of info from the applications information from the major
makers. Most of the suggestions apply to all makers' capacitors.


>So, I'm wondering- what sort of test data exists out there to back this up=
?
>Has
>there been any critical study of this? Any articles regarding it?

No idea. There's plenty of evidence that tantalums catch fire (and the
makers admit it), and the lifetime of various ceramic caps is well
documented. Aluminum parts have life formulae in the data sheets,
presumably based on some roughly accurate approximation of reality.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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