Re: Tantalum cap failures
- From: John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 09 May 2005 08:27:22 -0700
On 9 May 2005 07:13:05 -0700, edward.wilson@xxxxxxxxxxxxxx wrote:
>We have a design in which we are using a parallel bank of 10 Arcotronic
>SFE solid tantalum capacitors, 47uF/63V, (case size 'D'), as a
>reservoir/filter for a 3-phase motor PWM supply opperating at nominally
>80kHz. Normal opperating supply voltage in our application is 28V.
>
>We have had problems with some capacitors becoming short circuit during
>high voltage testing, sometimes in conjunction with operation at limits
>of temperature, (all voltages and temperatures within the specified
>rating for the components). Sometimes the short circuit disappears on
>removal of the applied voltage, sometimes it results in disintegration
>of the offending component.
>
>These failures occur in around 2% of the capacitors, which means it
>occurs in around 20% of assembled units and this rate is too high.
>
>We have reduced the maximum voltage at which we test from 56V to 44V
>and also reduced the rate of change of supply voltage during
>transitions to around 1ms, to reduce the inrush current, but with no
>significant reduction in the failure rate.
>
>The bank of caps is there to sink/source short-term PWM current
>requirements, so it is not practical to introduce a series resistor on
>each capacitor, but Arcotronic specifically state that this measure is
>not required with these parts. Total peak ripple current is almost 20A
>and lasts for 10 to 20ms, with a maximum duty cycle of around 1%.
>Total steady-state ripple current is around 0.5A.
>
>Rated ripple current for the caps is 2.7A rms @20C,derating to about
>1.75A at our upper temperature. Given that the caps cannot be
>guaranteed to share equaly, we are exceeding the caps ripple rating,
>particularly at high temperature. However, since the issue of ripple
>current rating is purely one of capacitor heating, this is at such a
>low duty cycle, with such a low background level that this was not
>expected to be a problem. This view seams to be endorsed by the fact
>that failures never occur during the high currnet transients, but
>always occur during increased supply voltage.
>
>Arcotronics have been unable to throw any light on the problem and have
>stated that they consider our application to be reasonably benign, in
>terms of the stress placed on the capacitors.
>
>I wondered if anyone on here has any experience of this or similar
>problems and is able to offer a solution, or alternatively suggest an
>alternative part that would be more reliable whilst still meeting our
>basic requirements in terms of ESR and CV product?
>
>TIA for any help.
>
>Ted Wilson
>
>edward.wilsonatbaesystems.com
Tantalums hate AC ripple current. It causes local heating of tiny bits
of the sintered pellet. If the contact media is MnO2, which standard
solid tants are, the tantalum becomes fuel and the MnO2 becomes
oxidizer and it explodes. It's an instant-ignition effect, not slow
heating. It's very erratic from mfr to mfr, or even between lots.
Replace them with aluminums, or niobiums, or with the new polymer
tantalums, none of which has the same detonation mechanism. The
polymer tants have very low esr, too.
I'm selling VME modules to BAE, for a critical radar application, and
we used all aluminum caps! I only use tants in slow RC-type circuits,
never as power bypasses; I've learned that lesson!
John
.
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