[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 21, 2004, at 12:30 AM, peter.chadwick at Zarlink.Com wrote:

>
>
>
>
> Rich's argument appears to me that because the current is the same in 
> each
> diode, and each diode goes into avalanche, the voltage distribution on 
> any
> one diode can't be such as to exceed the the reverse leakage current 
> rating
> of that diode.

-  Only if the total PIV of the diode string exceeds the actual voltage 
encountered on the job
.
> Now that leads to an interesting question: suppose you have
> a 1 kV diode in series with a 200v diode, and that the capacitances are
> equal.

-  If the junction capacitance at 200v on one diode equals the junction 
C of another diode at 1000v, the die of the 200v diode must be much 
larger than that of the other diode.

> Then as the reverse volts increase, the 200 volt diode will get half
> the applied voltage (no leakage in the 1 kV diode) and so we will have 
> the
> apparent anomaly of the 200 volt diode having far in excess of 200volts
> across it

-  However, if the capacitances of the diodes are small, the allowable 
avalanche current will be capable of charging the unequal capacitances

>  (the voltage across 2 series capacitors is inversely proportional
> to their capacity, even on DC)  but unable to break down and avalanche
> because no leakage current can flow - the 1 kV diode is reverse biased.
>
> Reverse leakage is very temperature dependent too,

When testing the 56 diodes for the current project's FWB/FWD anode PS, 
they exhibited less piv-dependency on temperature than I have seen with 
older diodes.

> ... ... ...

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