krw writes:

On 23 Sep 2015 15:24:36 GMT, joe hey wrote:

On Wed, 23 Sep 2015 07:22:29 -0700, John Larkin wrote:

On 23 Sep 2015 11:06:49 GMT, joe hey wrote:

On Wed, 23 Sep 2015 00:40:00 -0400, rickman wrote:

On 9/23/2015 12:20 AM, joe hey wrote:
On Tue, 22 Sep 2015 19:02:33 -0700, John Larkin wrote:

On Tue, 22 Sep 2015 17:55:51 -0700, Jeff Liebermann

wrote:

On Tue, 22 Sep 2015 13:40:36 -0700, John Larkin
wrote:

Dry-slug tantalums across power rails are bad news. High dV/dT
literally ignites them; MnO2 is the oxidizer and tantalum is the
fuel.

That's what I've read everywhere. Yet, I spent 10 years shipping
marine radios that were literally crammed with dipped and molded
tantalum caps on power supply rails with never a problem. The only
ones I've ever seen go up in smoke were reverse polarized (which
produced an impressive red glowing piece of slag and plenty of
white smog). Mostly, these caps were 25V caps on the 12V (nominal)
power supply lines and 16V caps on the 8 and 10V regulated lines.
There were also a bunch used in audio circuits.

The tantalum thing is very erratic. Some batches blow up, some are
fine.



However, we never used tantalums on the output of a switcher, where
I would expect problems. I guess using a tantalum in this 3.3V
switcher would qualify. However, at the time (1970's) the
literature declared that high voltage spikes were the culprit, not
voltage slew rate. Since these often appear together, I can see
where there might be some confusion.

I know for sure that tantalums sometimes blow up at below their
rated voltages, with no overshoot spikes. It's dV/dT, namely peak
current, that can ignite tiny particles of tantalum, which then burn
in the solid MnO2 electrolyte.


That's why in those cases a series resistor might be recommended in
order to limit the current spikes.

Add series resistance to a tantalum cap and you have just created an
electrolytic replacement.

Sorry, I forgot to mention to put the resistance in between the power
line and the tantalum.


But than it doesn't bypass the power rail!

No, we did it locally, every IC that was uncoupled with a tantalum, we
put a small resistor from the power rail to the tantalum and the problem
was solved. I clearly remember reading this advice in some tantalum's
datasheet or application note. As those IC's consume very little, the DC
voltage drop over the resistance was negligible, but the reduction in
current spikes through the tantalums was considerable.
They didn't blow up anymore and neither did the IGBTs.

The point being that by adding the resistor, you've increased the
"ESR" of the cap(-resistor). You might just as well use an aluminum
cap in its place if ESR doesn't matter.

No there is still a low-ESR decoupling for the IC. I do that a lot since
it isolates the IC rail from spikes on the main power rail. (Not with
tants though).


--

John Devereux