If, for example, a Silicon diode rated at 100V is reverse biased at 200V (long term, not just very briefly) but in series with a few MegaOhm resistor such that the reverse current is limited to one microAmp (or a few uA), would that change the characteristic of the diode. I know, from experience, that the diode won't become faulty but, rather, I am trying to inquire whether this is an acceptable practice. Presumably the diode works in Avalanche region but can it recover without any issues?
This question can also be applied to other pn junction devices e.g. transistors (C-E rating exceeded but current limited to a microAmp or two via MegaOhm resistor).
31May'18: WhatRoughBeast, Elliot Alderson and BimpelRekkie have all really made me think harder. Thanks all. I think you all have raised useful points. This question is not a theoretical one since I actually noticed a somewhat similar scenario in one of the circuits used successfully over a number of years, the series (collector) resistor being 470k but the device in question was an NPN transistor (BC547B) rated at 45V, Common-Emitter config. The positive rail could surge well beyond the 45V. Sorry, I can't post the full circuit due to strict NDA with various companies I worked for.
A diode (pn junction), once it reaches its breakdown voltage level cannot "allow" voltage across it to rise further (the I-V graph slope is very steep beyond the Breakdown voltage) so the series R will keep absorbing any further rise in supply voltage (like in a normal Zener diode). So, basically, my question effectively boils down to: "Can a diode (or pn junction in Transistors etc) be damaged, or deteriorate in its standard characteristics, if it sees Breakdown voltage repetitively over long term, keeping in mind it doesn't venture far into the Breakdown region, only slightly i.e. a few uA. If power dissipation is the only consideration then the answer is "No, it won't get damaged", but I had been suspecting a possible problem elsewhere, hence my question. BTW, other external considerations, such as Safety, Creepage/Clearances can be assumed to cope with higher voltage surge.
5 June'18: I have given it further thought and I think the answer also depends upon how rapidly the reverse voltage is applied. For a "very fast" rising (reverse) voltage, when the current is initially zero, all of the voltage will appear across the pn junction. This raises alarm since there is (internal) di-electric breakdown consideration too (perhaps, at the back of my mind, this is what I had been worrying about). In practice, it may not be a problem since such adverse conditions are not so realistic (?). Anyhow, if anyone of you does manage to carry out relevant tests, perhaps on a Fast Recovery and on a slow recovery diode, then please do enlighten us too. If I ever get an opportunity and time, I'll endeavor to do likewise…keeping in mind, a failure will provide us with a conclusion but a "pass" won't (necessarily….since it's just a one-off test!)
Best Answer
Can..?
Yes you can, as long as no damage occurs. If you limit the current and voltage such that then no damage (to the component) can occur then it won't.
It depends on the power rating of the component how much it can handle. For example a 1 A diode like the 1N4001 has a forward voltage (at 1 A) of 1.1 V so that gives a total of 1.1 W dissipation. In reverse at 200 V at 1 mA that would result in 200 mW dissipation, far below that 1.1 W. So you do not have to limit the current to a few uA, one mA might be safe as well.
A 100 V rated diode is not guaranteed to show breakdown at for example 200 V. It can be at 101 V or 1000 V. The manufacturer only guarantees that breakdown occurs above 100 V.
Basically almost all diodes can be used as a zener diode however a zener diode will exhibit breakdown at a specified voltage. Using an NPN's BE junction as a "poor man's zener" is indeed an option. You just should not rely on an accurate breakdown voltage.