On 2011-01-16, Ignoramus25553 wrote:

[ ... ]

Guys, can I ask an ignorant question. I thought that voltage drop
depends on the maximum reverse voltage that the diode would withstand?
Is that true or not?

In other words, is the voltage drop on a 1,000V rated diode, the same
as on a 30v rated diode?

For a single junction -- no. Pretty much the same range as a
function of the current through the device.

However -- high voltage rectifiers can be multi-junction devices
-- junctions in series -- to get sufficient PRV from a device which can
handle the current needed.

These multi-junction devices will drop a nominal FV for each
junction in series. Yes, it is about 0.700 V at some nominal current
and temperature, but yes, the Forward voltage drop will increase as the
current increases, and as the temperature increases. (And the nominal
0.700 V is for silicon junctions. Germanium ones are down around 0.150
V, but you are not likely to find germanium diodes in use these days.
Various other semiconductor materials (e.g. GaAs) will each have their
own nominal forward voltage drop

Now -- when you stack a bunch of diodes in series -- you can't
just grab any old handful from a bin and stick them in series. They
need to be individually tested and matched for junction capacitance and
for leakage at various reverse voltages so when there is a high voltage
(steady state or a spike) the voltage is evenly spread across the
diodes. Back in the old days, such things often had a set of resistors
and capacitors in parallel with each diode, to swamp the individual
leakage resistance and junction capacitance variations. Sometimes,
these assemblies were potted to make them look like (and fit in the
sockets for) high voltage rectifier tubes. Four pins on one end (only
one of which matters here, since there is no filament to heat up) and a
cylindrical cap on the other end for the high voltage connection.

Enjoy,
DoN.

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