I believe any component can breakdown once a certain
point is reached. In this case a zener can burn up
after the maximum temperature is reached. 

http://www.futurlec.com/Datasheet/Diodes/1N746-1N759.pdf

Look at maximum junction temperature. 175 c. After
this temperature well........ I haven't seen a lab
test on this and can't fully backup my claim.

john

--- Russell McMahon <apptech@paradise.net.nz> wrote:

> 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.
> 
> _____________________________
> 
> 
> 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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