[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 20, 2004, at 8:53 AM, Radio WC6W wrote:

>
> On Mon, 20 Sep 2004 14:08:51 +0100 peter.chadwick at Zarlink.Com writes:
>
> Good morning Rich & Peter,
>
>>
>> Rich said:
>>
>>> Please explain why equalizing resistors have merit.
>>
>> Because if the reverse leakage differs between diodes, the reverse
> voltage
>> distribution also differs. The one with the least leakage ends up with
> the
>> highest voltage across it, which could exceed the breakdown volts.
> With
>> resistors, the voltage distribution across the diodes is fixed by the
>> resistor ratios.
>
>    Thus, creating a known operating condition.   Without the R's, one 
> is
> relying upon the random reverse resistances & the unspecified avalanche
> characteristics of common diodes to set the voltage distribution.
>
Equalizing resistors exacerbate the problem of using less than 
perfectly matched diodes.

>    Without the R's, a string of unmatched and/or non-avalanche rated
> diodes might, or might not, exhibit a higher reverse breakdown 
> capability

  The avalanche characteristic is built-in at the time of epitaxy.

> but, this figure would wander around with temperature as it effects
> leakage resistance.  One diode in the string (likely the best, lowest
> leakage, unit!) would be the most stressed and prone to early failure.

Stress is caused by reverse current.  If the reverse currents are 
equal, how can one diode be stressed more.
>
>
>> If the capacitances aren't matched, that will also affect the voltage
>> distribution. That's one reason why it is not wise to mix diodes of 
>> the
>> same nominal type from different manufacturers without some
> equalisation.
>
>    C's are used for transient protection in a low frequency rectifier
> application.  They should be selected to be large enough to swamp the
> diode's intrinsic C to but, not so large as to drive excessive 
> avalanche
> energy into the individual diodes during an "event".

Why wouldn't a C-filter store the rectified energy from a transient.
>
>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org