Re: rectifier stack experts?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, October 19, 2002 11:53 AM
Subject: Re: rectifier stack experts?


> Original poster: "Crow Leader by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<tesla-at-lists.symmetric-dot-net>
>
> I've noticed mega rectifier assemblies are compensated.

Not really... High quality strings are made from matched (i.e. same mfr lot,
low mfr variability, too) sets of rectifiers.  The last few versions of the
ARRL handbook have had an explanation of why you don't want RC around each
diode. (you can also find a similar explanation if you look through the
archives).

As another poster pointed out, fast but soft reverse recovery time, and
consistency, are what you need.

This makes sense as
> switching transients easily diodes, and in a string, when one shorts out
for
> good, the stresses just go up on the rest of them. Then eventually they
all
> break down and get destroyed. I'm assuming the use of MOVs absorbs any
> transients that might otherwise permanently destroy a diode.

What transient might that be?? in the forward direction, the MOV won't
breakdown (because the diode Vf is much, much lower) and in the reverse,
I'll bet the MOV is slower than the diode avalanche (nanoseconds), and
besides, the MOV is just a lossy clamp.  Furthermore, if your getting
transients on every cycle, the MOV will quickly degrade....

 As for how to
> desing this properly, I have not clue at all. I was using large 1N5600
(5kV,
> 2A) diodes before they shorted out.
>

Rules of thumb:

1) design for 2xPIV
2) design for 3x current
3) get all diodes from same mfr lot (not just same date code!).. if you get
tape and reel, they're pretty much always going to be same lot