[SI-LIST] Re: Chip caps vs. Tantalum
- From: "Fasig, Jonathan L." <fasig.jonathan@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 20 Aug 2002 08:55:02 -0500
Martin-
There are several factors relating to this topic. Perhaps foremost is the
fact that even large ceramic caps have a higher self resonant frequency than
comparable tantalum caps, which means that the ceramics are typically more
effective at maintaining a low PDS impedance at high frequencies (see the
web sites for AVX & Kemet for details). Some have already mentioned
reliability issues with tantalum caps. These issues typically force the
designer to derate the operating voltage of tantalum caps by a factor of 0.3
to 0.2 (eg, you need a 20v cap for a 5v circuit) to achieve a reasonable
MTBF (see the AT&T Reliability Handbook or some similar text for details).
Of course, Murphy's Law ensures that some percentage of tantalums will be
assembled with reversed polarity and consequently a dramatic reduction in
their expected lifespan. Finally, for those who care, the use of tantalum
caps contributes to the extinction of African mountain gorillas because the
tantalum mines lure large numbers of workers into remote areas and then the
miners hunt the gorillas for food.
Unfortunately, large ceramic caps are not without their problems as well.
Once soldered to a board, these caps can experience enormous mechanical
stresses which can lead to cracking of the ceramic and dramatic reduction of
the cap's effectiveness. These failures can be extremely difficult to track
down because the cracks may not be visible except under extreme
magnification.
Other issues may contribute to the decision as well. Hopefully others on
the List will elaborate with their experiences.
Regards,
Jonathan Fasig
-----Original Message-----
From: Martin Euredjian [mailto:martin@xxxxxxxxxxxxxx]
Sent: Monday, August 19, 2002 12:38 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Chip caps vs. Tantalum
I've been trying to figure out what the differences might be between
large-value (1 to 100uF) chip caps and similarly valued Tantalums. This, of
course, as it pertains to PDS or switch-mode DC-DC converter design.
When/Why might one choose a chip cap over a Tantalum? What are the
advantages/disadvantages in comparison?
I've seen pictures of modern DC-DC modules that seem to use several
large-value chip caps. If that's the case, why did the designer go this
route?
Thank you,
===============================
Martin Euredjian
eCinema Systems, Inc.
voice: 661-305-9320
fax: 661-775-4876
martin@xxxxxxxxxxxxxx
www.ecinemasys.com
===============================
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