[AMPS] In diodes we trust

[Radio WC6W]

On Thu, 29 Jul 1999 09:27:48 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>Marv says:
>
>>After that time, all the diodes will return to their blocking state
>>and will share the reverse voltage...  
>
>In inverse proportion to their individual capacities......which for
>rectifier diodes are not usually controlled.
>


On Thu, 29 Jul 1999 08:32:43 -0400 "Tom Rauch" <w8ji@contesting.com>
writes:
>Now let's see if we can sort through the rectifier thing in an equally 
>professional manner.
>
>> >After that time, all the diodes will return to their blocking state
>> >and will share the reverse voltage...  
>
>It appears Peter and I have the same question. What mechanism 
>is it that causes this "shared reverse voltage"? My concern is 
>some of the statements made in the Motorola Rectifier and Diode 
>Handbook that seem to conflict with the idea the diodes won't 
>suffer increased failure if voltage doesn't happen to divide equally.
>
>So maybe someone can explain, hopefully in a nice sane way, why 
>the voltage divides equally.
>

On Thu, 29 Jul 1999 07:30:04 -0700 Rich Measures <measures@vc.net>
writes:

>  It seems to me that there are two things that are being 
>overlooked:   
>1.  The division of the reverse potential is controlled by the avalanche

>voltages of all the diodes in a series string. .  2.  The capacitance 
>of a P/N junction is not fixed, but decreases considerably as reverse 
>potential increases.  
>>

Good Morning (at least here in California) Peter, Tom & Rich, and diode
fanciers everywhere,

   Keeping in mind my previous caveat limiting this discussion to 60 Hz
operation... and neglecting the contribution of any stray inductances...

1.  The capacitances which are already smallish at 60Hz, though
uncontrolled as Peter commented and varying over perhaps a 5-1 range,
plus diminishing with inverse voltage, as Rich notes, reduce to a level
where they become a secondary factor in real world voltage division.

2.  The reverse leakage current, which may vary over a 100-1 range, at
rated PIV in a non-avalanche part, as distinct from the well controlled
reverse breakdown of an avalanche diode, will be the determining factor
in reverse voltage sharing.


  In a real world string, more voltage will be sustained across the
"better" (lower leakage) diodes and less across the "worse" (higher
leakage) parts.  This is somewhat self equalizing in practice since as
the applied inverse voltage approaches the rated value there is an upward
knee in the leakage current. 


   Therefore, as long as the diodes in a string are reasonably of the
same pedigree (same manufacturer & vintage), matched in the field by
diode trainers or happenstance, they will tend to share the inverse
voltage, allow marginal engineering to escape into the world, and
engender lengthy globalwide discussion as to why it seemingly works in
practice.  :-)


  Whether a particular string of diodes works long and reliably or
self-destructs in a fiery display will ultimately depend on whether any
of the individual parts undergoes permanent breakdown at its particular
reverse operating point.  This is a problematic situation with
non-matched, unequalized, non-avalanche rated diodes which are NEVER
supposed to be operated above their PIV rating.  A string might work for
hundreds of hours and then (does this sound familiar): "just explode for
no apparent reason".


  In order of advisability, the current state of the art seems to favor
using avalanche rated parts in strings.  Matching non-avalanche diodes
would be the second choice, and incorporating equalizing components with
unmatched diodes would be the third option.


73,
   Marv  WC6W









*


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