[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 20, 2004, at 6:08 AM, peter.chadwick at Zarlink.Com wrote:

>
>
>
>
> Rich said:
>
>> Please explain why equalizing resistors have merit.
>
> Because if the reverse leakage differs between diodes,

-   Is not current equal through each element in a series circuit?  
Does it make sense to fix something that is not broken?

> 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.

-  As the avalanche potential is approached. reverse leakage-I 
increases and tends to limit V.  Thus, the reverse potential that 
appears across each rectifier in a series string is proportional to its 
reverse potential capability -- provided that the reverse current 
maximum is not exceeded.  ***  note -- for 3A units - such as the 
1N5408, this is c. 5uA.

> With
> resistors, the voltage distribution across the diodes is fixed by the
> resistor ratios.

-   Which un-equalizes the previously equal reverse current in each 
diode in the string.
>
> 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-equalization?

> Another is the reverse recovery time.

-  probably not significant for 50-60z?
>
> Experience also suggests that for best reliability, buy diodes from
> reputable manufacturers - some of the far Eastern ones are distinctly 
> dodgy
> froma reliability viewpoint.
>
>
-  Which is why is why I measure the PIV of the diodes I buy.

  cheerz, Peter

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org