[Amps] Equalising resistors with HV diodes

[peter.chadwick at Zarlink.Com]

Rich's argument appears to me that because the current is the same in each
diode, and each diode goes into avalanche, the voltage distribution on any
one diode can't be such as to exceed the the reverse leakage current rating
of that diode. Now that leads to an interesting question: suppose you have
a 1 kV diode in series with a 200v diode, and that the capacitances are
equal. Then as the reverse volts increase, the 200 volt diode will get half
the applied voltage (no leakage in the 1 kV diode) and so we will have the
apparent anomaly of the 200 volt diode having far in excess of 200volts
across it (the voltage across 2 series capacitors is inversely proportional
to their capacity, even on DC)  but unable to break down and avalanche
because no leakage current can flow - the 1 kV diode is reverse biased.

Reverse leakage is very temperature dependent too, and I still say that you
need to use diodes from the same batch to be certain - ceratinly, different
manufacturers are very likely to have differing capacitances, which will
affect voltage distribution. Reverse recovery time may not appear to be a
factor at 50/60Hz, but variations can lead to a transient reverse voltage,
which if repeated often enough, can cause problems. Ask two semiconductor
process development guys about reverse breakdown and the time allowable for
some difference of opinion! The argument runs: If you're at 2,4GHz on a
process witha 3v breakdown, can you run transformer coupled with the drain
hitting 6 volts for half the cycle, on the basis that the  voltage rating
is only exceeded for 1ns, and breadown can't inititate in that time. I
know, I'm deviating from the subject.

I have another business trip, and I won't have email access, so I'm afraid
there's no point in replying to this if you want an answer before next week
at the earliest......

73

Peter G3RZP