To all,
This discussion has so far pointed to voltage and ripple current as being the
main causes for
tantalum capacitor mortality. There is, however, another feature of these caps
that so far has not
been discussed. That is one of temperature, soldering temperature to be exact.
We have learned over
the last few months that tantalum caps, when subjected to high temperatures
(soldering, especially
hand soldering), tend to change internally on a physical basis. A few capacitor
manufacturers have
verified this. After soldering, when a voltage is applied across the tantalum
cap, the tantalum cap
tries to self-heal itself. In doing so, the tantalum cap appears to re-rate its
voltage rating to
the applied voltage. The cause of the self-healing appears to be a function of
the soldering
temperature and the length of time that the temperature is applied. This is
greatly egaserbated
(sp?) when a soldering iron actually touches a terminal on the tantalum cap.
For example and after
very hot soldering, a 15V rated tantalum cap could conceivably be re-rated to
about 5V if 5V is
applied. We learned this the hard way.
We are now actively looking for a suitable alternative to the tantalum cap
application (buck filter
needing low ESR).
I wanted to throw in my 2 cents.
Best regards,
Ron Pickard
rpick...@hypercom.com
emcconsult...@yahoo.co
m To:
72146....@compuserve.com, chris.maxw...@nettest.com, emc-p...@ieee.org
Sent by: cc:
owner-emc-pstc@majordo Subject: Re: Tantalum
Capacitor Reliability
mo.ieee.org
07/29/02 01:35 PM
Please respond to
emcconsultant
Nonetheless, inrush current aside, a 20V Tantalum is considered marginal for a
12V
circuit if reliability is desired. A 60% derating factor was and is a typical
max
for reliability circuits, i.e a 30V min rated cap is recommended. Tantalums
require
additional derating than Al-electrolytics. Furthermore, switchers are notorious
for
destroying Tantalums due to the large and fast V-swings. I don't recall seeing
too
many Tantalums on switcher designs.
--- Cortland Richmond <72146....@compuserve.com> wrote:
>
> Chris,
>
> The issue isn't voltage rating; low-ESR caps such as these are susceptible
> to excessive charging current at turn-on. At a former employer, we saw
> REALLY GOOD, expensive caps used on a computer's 5V bus exploding at
> turn-on, even ones rated at 50 volts. Replacing them with cheaper
> electrolytics (TEN volts!) took care of that. You might be able to
> alleviate turn-on stress by using a power-on monitor circuit to slow down
> the initial charge. But it'd be far cheaper to go to electrolytics.
>
>
> Cortland
>
>
> >> One of my colleagues is testing a new design. He has designed
> >> a buck-boost switching converter which has tantalum output capacitors.
> >> We have looked at his design and gone through the calculations.
> >> His output current is 4 A maximum. His output voltage is 12 VDC His
> >> caclulated ripple current is 800 mA. He needed a 120 mV ripple voltage,
> >> so he put 8 each of 68 uF, 20 V tantalum capacitors (with 150 mOhm ESR)
> >> in parallel on the output. Each cap is rated for approximately 800mA
> of
> >> ripple current.
> >>
> >> He has seen two failures of these capacitors during initial testing and
> >> demonstrations. Meanwhile, many initial units run fine. From what I
> can
> >> gather, he hasn't violated any design rules. He has 20V rated caps on a
> 12V
> >> circuit. He has a ripple current rating of 8 X 800mA (8 caps in
> parallel).
> >>
> >> It is tempting to just increase the voltage rating to 25V or 35V....but
> why?
> >> Even if he does, how do you prove that the problem is fixed. It would
> take
> >> months of testing the new capacitors to get the history that we have on
> the
> >> existing design.
>
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