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Re: MMD (not another acronym!)
Diodes tend to fail shorted if over reverse voltaged repeatedly. They fail
open if massively overcurrented (i.e. they act like little fuses). In HV
rectifier strings I've built, the typical failure mode is that the PIV
rating of the string gradually decreases (as one of the diodes fails
shorted). Your mileage and experience may vary.
Actually, most all the failures are essentially thermal in nature. Get the
silicon hot enough and it melts. Get it really hot and it completely
disappears.
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Wednesday, March 08, 2000 5:13 AM
Subject: MMD (not another acronym!)
>Original Poster: "Mike Nolley" <mnolley-at-mail.slc.edu>
>
> As preparation for the diode bulk buy, I have a few questions for the
>learned ones about diode failure. Do hv diodes fail open, or closed? If
they
>fail open, then a single-string MMD would not catastrophically fail, if it
were
>encapsulated-- It would just turn off. The problem with this is related
>to the
>encapsulation--the diodes would be hard to replace. If on the other hand
they
>fail closed, I think it would be fairly likely that a single-string MMD
would
>fail quickly, if it were rated close to supply voltage.
> My second question is, how much should the MMD be over-rated--*given*
the
>use of an R-C network (which I presume any diode-user would need by
default).
> Multi-string MMD's seem to be impractical for size reasons, and silly
to
>boot as the 1kv, 6A diodes would seem to be sufficient for any system under
84
>kva ; )
> Thanks for any, and all responses.
> --Mike
>
>
>