There are many mechanisms that impact this, but first is that the ripple
current rating is typically based on internal heating, so it depends on the
operating temperature and also harmonic content. The ripple current rating is
typically for a single frequency and I suspect if you calculate the equivalent
heating using an FFT you aren't close, but way above the rating. The capacitors
are also sensitive to overvoltage and negative voltage as well as surge
current.
It would seem that you didn't adequately derate the part. For long life, I
would suggest never exceeding 75% voltage or 50% ripple current and this would
be at the highest operating temperature. I don't know where the part is used,
but if it is the output of a regulator, slowing down the soft-start will
reduce, but not eliminate the inrush.
If you can tolerate the very low ESR, you might try to stack a few ceramics in
a 1206 or 1210 case size. They will take more punishment. I don't know your
application, so perhaps you can also live with reduced capacitance, so that you
don't need to stack them. If there is a reliability concern over stacking the
caps, some companies can assemble them into a lead frame for you.
It's good you found this now, as it would have been a time bomb. It would be
exacerbated in time due to capacitor aging.
Charles Hymowitz - Managing Director
AEi Systems
Charles@xxxxxxxx
(310) 216-1144
(310) 863-8034 (M)
http://www.aeng.com - Analytical Heavy Lifting
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Chris Belting
Sent: Thursday, February 04, 2016 11:09 AM
To: telegrapher9@xxxxxxxxx; buenoshun@xxxxxxxxx
Cc: lwrbakro@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Tantalum explode
Perhaps a little caution is worthwhile if changing to a ceramic. There are some
nice write-ups regarding how small sized ceramics rapidly lose their rated
capacitance as the voltage approaches the rated value. This was printed in the
Jan 2013 issue of EDN: "Temp and voltage variation of ceramic caps, or why
your 4.7-uF part becomes 0.33 uF", (also available on Maxim's website). Just be
sure to check the capacitor manufacturer's DC bias curve.
The capacitor vendors recommendation you mentioned for including series
resistance reduces the instantaneous current but that can also be managed by
controlling the dV/dt. As many have alluded to during start-up, hot swap, etc.
instantaneous application of voltage results in impressive currents:
100uF*12V/100us = 12A. For a high-reliability Tantalum (solid: CSR, CSS, CWR)
the recommendation is to add resistance to keep this at 0.1 ohm/V or 10A max. 3
ohms/V is only 0.33A. If the input comes up faster than 3.6ms you are exceeding
the vendors guidance.
Chris Belting
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Dave Cuthbert
Sent: Thursday, February 04, 2016 10:24 AM
To: buenoshun@xxxxxxxxx
Cc: lwrbakro@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Tantalum explode
I would look at replacing it with a ceramic cap (unless you are relying on the
Tantalum cap ESR).
A quick look at Digikey in the 1206 package shows X5R ceramic caps of 47 uF
rated for 16V and 25V.
Dave C
On Thu, Feb 4, 2016 at 9:47 AM, Istvan Nagy <buenoshun@xxxxxxxxx> wrote:
Hi,
What do you mean by "first power on"? Connecting or reconnecting the
power cable, or pressing the power button?
When the board is still hot from the previous run (e.g. you didn't
connect fan or heatsink on your prototype), and you reconnect the
power cable, this can easily happen.
Make sure your board does not get too hot during normal run.
Only reconnect the power cable after it cooled down.
For electrical/functional reasons it is also good to wait 5s...30sec
before reapplying the power, for letting registers clear and do hard reset.
Istvan Nagy
-----Original Message-----
From: Bni I
Sent: Thursday, February 04, 2016 7:50 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Tantalum explode
Hi experts
I am having issues with my 100uF 20V 1206 tantalum capacitor for 12V input.
The ripple is lower, but close to the limit (said in the DS) [~150mA]
I am facing 20% failure.Most of the time, it explodes on the first
power on.
Sadly I have a lot of PCB without any hope to change the PCB.
I haven't found much on the application of the tantalum.
I found that there is some condensation/crystallization time on the
first power up.
The article suggest to use 3xVin [Ohm] serial resistor.
I am not satisfied with this solution. I think it does not help on the
problem.
It only mitigates the problem and it will blow up in my customer hands.
Any experience or artical/book are welcome.
Thanks
Steve
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