I've been watching the resistors and diodes thread
with some interest.
I see one point of misunderstanding, and that is about
reverse leakage current. The more accurate term is
the reverse saturation current, and it is usually
assigned the mathematical symbol Is. Most rectifier
diodes are not manufactured to very tight tolerances
on this specification. Further, as pointed out, it is
temperature dependent. It's not an unknown
dependence, it is predicted by fundamental diode
equations. You will find them in any textbook on
solid state devices. Is is critical; it is the
multiplier in front of the exponential expression for
forward curent also.
In the reverse-biased mode, a rectifier diode acts as
a constant current device. If the external circuit
(the rest of the diode string) is not supplying enough
current, then the diode reverse voltage will be close
to zero. If the external circuit is attempting to
supply more current than Is, then the diode terminal
voltage will rise to the avalanche potential.
Placing two or more constant-current devices in series
is the "dual" of placing two more more zero-impednace
voltage sources in parallel. What happens if you
place a 1.5 volt battery and a 3 volt battery in
parallel? You get huge currents. What happens if you
place a 5uA and a 10uA current source in series? Huge
voltages.
Therefore, if you place diodes in series, and the two
are mismatched in Is, it is not simply likely that one
of them will operate at avalanche - it is guaranteed.
My employer makes lots of things, among those things
are diodes. Our diodes are hot carrier, microwave
switch, GaAs detector, PIN, switching, etc - no power
diodes. However, in the past we have made power
diodes. I brought this topic up to one of our diode
designers. He responded at some small length, and
ultimately said this: a diode should not be routinely
operated at avalanche threshhold unless it was
specifically designed for that operation. High
voltage diodes, he advised, are designed with the
expectation that the circuit designer will take steps
to prevent the diode from reaching avalanche voltage.
It is usually the case that Is at avalanche voltage
will cause metal migration, which will substantially
reduce the mean time between failures (MTBF).
Running the diode all the way to the avalanche
voltage, but limiting the current will almost never
cause an instant failure. But it does lead to
alterations at the crystal and metallurgy level that
reduce the diode's lifetime. The rate of damage is
accelerated at higher temperatures.
This effect is so well-known that military diodes are
routinely subjected to lifetime tests known as "high
temperature reverse bias" or HTRB testing.
The summary is this: Unless the diode's data sheet
explicitly indicates that the diode is designed to
operate routinely in avalanche mode (for instance:
voltage regulator diodes), then you should design your
circuit, one way or another, to prevent any diode from
reaching the avalanche voltage in the reverse
direction.
73,
Dave W8NF
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