>
>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...
? yes
>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.
>
? indeed.
>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.
? agreed
>
> 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. :-)
? I would add that, at 60Hz, the reverse currents in each diode in a
string are always equal provided that no "equalizers" are used.
>......
>
> 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
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|