Diode Failure

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After about 1000 units all of a sudden two units used by the same customer
have failed.

The diodes are a full short  (FMKA140), understanding of a breakdown under
high reverse voltage was that it's non destructive and them the diode will
recover so long as it isn't damaged by heat.    There is no sign of heat
damage.

Has anyone ever seen a breakdown like this?  Any ideas what would cause it?

Thanks for any help

Ralph



Re: Diode Failure

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I can't answer your question.  If you are exceeding
the absolute maximun rating of the part, you're asking
for trouble.


Re: Diode Failure
On Wed, 10 Dec 2003 12:57:03 +1300, the renowned "Ralph Mason"

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Got a schematic?

Best regards,
Spehro Pefhany
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Re: Diode Failure

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customer
under
will
it?
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It's pretty simple.  Used for an automotive supply.

+12v

--------~~~------|>----------~|~---
                         |
IGN                      |
                         |
--------~~~------|>-------

The always on and a switched are feed into one via the diodes.  It's the ign
diode that is dead.  The fuses before the diodes are 5A and the polyswitch
is 1.3A.

The only other thing not shown here is a high impedance voltage divider
before the diodes on each inputs used to measure the incoming voltage.

That's about all there is to it.

Thanks for any thoughts

Ralph




Re: Diode Failure
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I'm not surprised.  The IGN circuit is feeding something
inductive, and depending on dI/dT effects to generate large
voltages.  You probably need kilovolts of PIV for that diode.
Automobiles are noisy places.

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Re: Diode Failure

in message news:J78Cb.26555$>
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ign

Is this connected to the ignition relay, perchance?



Re: Diode Failure
On Fri, 12 Dec 2003 13:40:00 +1300, the renowned "Ralph Mason"

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Maybe a field-decay transient. These can have a lot of energy. You
can't stop this sort of thing, just put in parts with enough rating to
avoid the issue (or suppress it, but that may be more expensive). On
automotive supplies it's possible to see ~ -400V /+150V transients
with various amounts of energy depending on the source. You can't get
Schottky diodes with that high reverse breakdown voltage.

If you're going to be making thousands of these things whoever is
doing the design might want to get ahold of the relevant standards so
they won't be guessing as to automotive requirements.

Best regards,
Spehro Pefhany
--
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Re: Diode Failure
In comp.arch.embedded, Ralph Mason
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I am not sure about the details, but car supplies are notorious for very
dirty voltages. Big transients that last longer than your usual spike etc.

Did all the failures happen on the same type of car or even the very same car?
Might be that this particular car has a badly filtered ignition or something
as simple as a weak battery that is not as good at absorbing spikes as it used
to. Also cable routing and bad grounding (rust) are common problems.

But as you probably can not control how the device is installed and in which
car, your safest option is to use more rugged diodes en maybe even some
filtering and/or transzorbs.

IIRC, I read somewhere in this thread that de reverse rating of these diodes is
only 40V, which seems very low for this application.

--
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Programming is an unnatural act.

Re: Diode Failure
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Where did you ever get such an idea?  Your design is faulty.

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Re: Diode Failure

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I don't operate them outside the design voltage, I am just trying to think
of any possible reason for the failures.

Ralph



Re: Diode Failure
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Then I'm confused.  If you never exceed the absolute maximum
specs, they should never fail.  Your first post implied that
you thought you could get away with exceeding the reverse
voltage specification and get away with it if there were
no heat damage.

Correct me if I'm wrong, but if you apply a high enough
reverse voltage to cause them to breakdown, aren't you
exceeding the abs-max reverse voltage specification?

Time to ask a better question.




Re: Diode Failure
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think

Perhaps: Diode broken, why? Is that a good question?

I don't knowingly run the diode in reverse break down or exceed the voltage
spec of the diode.  I do know that all of a sudden the diode is failing and
so am trying to figure out why.  My understanding leads me to believe that
reverse breakdown is non fatal to a diode.

Ralph



Re: Diode Failure
On Wed, 10 Dec 2003 12:57:03 +1300, the renowned "Ralph Mason"

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You may not be able to see the damage from the outside. A brief very
high current pulse can have enough energy to damage the P-N guard ring
or the junction. That particular part does not have a reverse
avalanche energy rating on the datasheet.

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Got a schematic?

Best regards,
Spehro Pefhany
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Re: Diode Failure

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40V is not a very high voltage rating  -- you can easily get
that much when you turn off a coil.

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it?

a) defective part
b) incorrect design that failed to account for something
    such as in-rush current
c) environmental damage





Re: Diode Failure
On Thu, 11 Dec 2003 07:57:12 GMT, the renowned "aurgathor"

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d) (possibly included under hazards of c)) something unforseen that
this particular customer is doing to the product. Never underestimate
the creativity of customers in finding design or instruction manual
weaknesses.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Diode Failure

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I think this should really belong to b) -- a good design
should take "creative" (or stupid) customers into account,
too.  ;-)



Re: Diode Failure

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"Should", maybe, but that would neglect one of the oldest theorems of
engineering: trying to design things to be fool-proof is futile ---
nature's response to such efforts is to evolve better fools, and you
can't beat evolution in the long run.

--
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Even if all the snow were burnt, ashes would remain.

Re: Diode Failure
On 12 Dec 2003 14:16:57 GMT, the renowned Hans-Bernhard Broeker

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Also, by the time you've figured it all out, you're overqualified and
should have moved on..

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Diode Failure

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It's impossible to make anything absolutely foolproof.  Somewhere out
there is a fool who's smarter than you are...

Regards,

                               -=Dave
--
Change is inevitable, progress is not.

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