[Prev][Next][Index][Thread]

Re: MMD (not another acronym!)

Hi Mike,

At 03:03 PM 03/07/2000 -0800, you wrote:
>    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?  

The voltage blows holes in the die layers causing cracking and general
destruction.  This is usually a short but if they blow apart, they are open.

>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.

Best to use extras, although if one fails, what ever caused it will toast
the others in the blink of an eye.

>    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).

Diodes are NOT over rated.  Be sure never to go above their rated voltage
for an instant or failure is very likely.  If RF is present there may be
concerns about inductances and such but probably not too much of a worry.
R-C across each one is highly recommended.

>     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 ; )

One string should easily take the current.  However, when the gap first
closes, you make see large spikes but the diodes "should" handle that.  Be
sure to fuse and guard against diode failure so if it lets loose, it will
fail gracefully.

Cheers,

	Terry


>      Thanks for any, and all responses.
>                    --Mike
>
References: