Hi Bill,
I called Motorola, and no one there had any data to support this
claim one way or another.
It was all mostly rumor.
When I finally reached a semiconductor engineer who actually was
involved in diode design and testing, he said he'd think about the
problem and call me back.
About a week later, he called and said he had measured some
diodes and he recommended using both capacitors and resistors
across the diodes.
The reason he gave was there is no guarantee the diodes are of the
same characteristics for reverse recovery time, junction
capacitance, or resistance when they go into cutoff. He said there
was very large differences from batch to batch, and that the only
test was a minimum acceptable rating for each characteristic. He
said they were two cent parts, not 35 dollar parts and there was no
guarantee they would be even close to the same specs.
Since the proper components cause no harm, since they also aid
in RFI protection, and since there was no professional engineering
support available that said they were NOT necessary, I always use
them.
> Hi Vic, from 1996 ARRL Handbook, DIODES IN SERIES, page 11.9:
>
> "There used to be a general recommendation to place a resistor across each
> diode in the string to equalize PIV drops. With modern diodes, this
> practice is no longer necessary." It goes on to explain why then
> concludes with: "In fact, shunt resistors can actually create problems..."
> and why again.
While it is true the WRONG type of resistor can be harmful, the
correct type of components can do no harm. Going by what the
semiconductor engineer in charge of diodes at Motorola told me, I'll
continues to use some form of equalization. The minimum he
recommended was a capacitor to slow the recovery time down on
faster diodes. The shunt reactance he recommended was a
minimum of 500k ohms at the switching frequency.
73, Tom W8JI
w8ji@contesting.com
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