Summary:

I would be interested in knowing if anyone is aware of a formal 
specification for fail-Short-Circuit behaviour for zener diodes or any 
discussion on the subject.

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Use of zener diodes as "fail-short" protection on power supply rails 
is common.
If the rail voltage exceeds the zener voltage adequately the zener 
overheats and fails short-circuited, thereby clamping the rail to near 
ground and blowing the fuse which you had better have provided. The 
intention is to protect equipment which may otherwise be damaged by 
severe fault over voltages.

I have used 6 Watt SOD-64 glass bead Philips BZW03Cxx zener diodes for 
this purpose for many years. These notionally will (in the right 
circuit) clamp excess voltages in the sub-nanosecond timeframe. My 
experience is that when used on a supply with a nominal 6 amps 
capability these zeners will reliably fail short-circuit. On every 
occasion when I have seen these fail they invariably go SC (short 
circuit) and never OC (open circuit) .

        http://www.vishay.com/docs/85602/85602.pdf

The range of conditions under which a zener will fail SC and not OC 
and, very importantly, the range of subsequent currents over which a 
failed zener will REMAIN SC does not seem to be part of any 
specification I have ever seen.

I note that much smaller zeners (eg 500 mW) also seem to always fail 
SC under realistic provocation. It is not obvious what certainty I can 
have that this will always be the case for any given circuit.

I would be interested in knowing if anyone is aware of a formal 
specification for fail SC behaviour or any discussion on the subject. 
Google knows about lots of people who mention the phenomena but 
doesn't seem to know about anyone who discusses it usefully.

The sort of thing that would be useful is knowing that you can take a 
given Vvolt Wmilliwatt zener of a certain construction and that if you 
destroy it with less that Iamps/Pjoules or whatever (mainly a complex 
function thereof) then you can expect it to go SC rather than OC with 
a given degree of certainty.

That there are upper limits is demonstrated by considering a 5V1 500 
mW zener placed across a 12V car battery. The zener may go SC 
initially, but not for long. The fused junction will have a different 
current carrying capacity than the leads and any interconnects. 
Something will fail and rapidly. In a few informal tests of 500 mW 
glass zeners across a 30V 5A lab supply failure was always SC with the 
supply then sitting on the internal current limiter. I haven't pushed 
this to see how repeatable it is or what the upper limits of reliable 
SC are.

In my target application I have 1360 uF at 150V behind a FET which may 
itself fail SC and connect this supply to a 16V rail. The 3W zener 
does a good job of stopping any damage if this happens. 500 mW zeners 
are much smaller and cheaper, but .... .



        Russell McMahon

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