[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 20, 2004, at 7:38 AM, Steve Thompson wrote:

> On Monday 20 September 2004 14:08, peter.chadwick at Zarlink.Com wrote:
>> 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.
>>
>> 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.
>> Another is the reverse recovery time.

> If I understand it correctly, equalising resistors might hurt you only 
> if you
> get a reverse voltage spike that exceeds the specified breakdown of 
> the diode
> chain. In many, if not most, power supplies will a modest mains filter 
> all
> but eliminate that likelyhood?

Steve -  In the example I gave, adding equalizer resistors would cause 
the lesser piv diode to fail and take out the other diodes in the 
series string without any spike having come along.
>
> Steve
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>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org