The diode, resistor, capacitor parallel combo has been with us since the
1960s when 866s were starting to wane i popularity. Westinghouse,
Amperex, International Rectifier all had it in their application
literature. Everyone knows that the resistor is a DC grading resistance
to force all the diodes to have similar reverse voltage when they are
not conducting. This was more important when diodes PRV were not
consistent. It was discussed here that controlled avalanche diodes make
this unnecessary. The capacitor is there for the transient part of the
commutation in the diodes. All of them switch on and off together, but
any small variances due to the intrinsic capacitance in the die and
package of early diodes could cause a tiny delay in the back bias to
turn on in different diodes. It the package is swamped with the larger
external capacitor, this helps dominate the commutation to level the
playing field so that to the rectified voltage, the diodes all appear to
change at the same time when it crosses zero. But more importantly, in
very large rectifiers, you may find yourself replacing only a few of
hundreds of diodes and want to help match their characteristics even
though they are from the same company at different times. I have seen
controlled avalanche diodes used as well as RC networks with diodes. I
still see more of the compensated networks than not, in the HV power
supplies I deal with at work. These days, the use of carbon comp
resistors is over since they tended to drift over time, so very good
metal film high power Resitors (3watt), and ceramic capacitors are used.
We have a bunch (8) three phase 86 kV DC power supplies rated ~ 1 MW
each, using a full wave bridge in oil. These look like power substation
units and are outdoors of course. Each powers the capacitor bank for 6
or 7 klystrons running Megawatt pulses. The original 50 year old GE
units had many many diodes, rated 1 kV PRV. International Rectifier
85HF100 rated 85 Amps and 1 kV PRV. Now they are made by Dean
Technologies in NJ. I can't remember the count, but its under 200. They
are on phenolic boards that are raised out of the transformer tank with
a crane. Each diode has a RC, with ancient 2 Watt carbon comb, and a 0.1
uF 400 Volt Vitamin Q metalized polyester cap, hermetic seal mil spec.
We hate to have to test these things, takes several days to do it right.
We have one refurbished rectifier set from a company that only uses six
modern rectifier sticks in series with each phase. each stick or module
having 32 x 1.6 kV stud mounted diodes. Then there are 6 boards with
this arrangement. So the PIV rating of a stick is 51 kV, and there are 6
in series to get ~300 kV PIV per leg of the rectifier. This is a
significant safety factor but those things run for years, 24/6,
recharging the capacitor bank of 120 uF, 120 times a second. There are
capacitor and resistor compensation across every diode, and I am happy
with that design. It all depends on your tolerence for risk. In our
machine where downtime is very costly, we use RC networks on the
rectifier diodes, not for noise but to protect the diodes from worst
case events. This is my take on it, to skip the RC if you have only a
few diodes and they are modern, matched in characteristic and you feel
they are running well under their PRV and peak current rating.
73
John Lyles
K5PRO
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