[Amps] .01uf caps across diodes

[John Lyles]

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