[Amps] Re: Equalizing resistors with HV diodes

[R.Measures]

On Sep 21, 2004, at 9:36 AM, Dave Haupt wrote:

> I've been watching the resistors and diodes thread
> with some interest.
>
> I see one point of misunderstanding, and that is about
> reverse leakage current.  The more accurate term is
> the reverse saturation current, and it is usually
> assigned the mathematical symbol Is.  Most rectifier
> diodes are not manufactured to very tight tolerances
> on this specification.  Further, as pointed out, it is
> temperature dependent.  It's not an unknown
> dependence, it is predicted by fundamental diode
> equations.  You will find them in any textbook on
> solid state devices.    Is is critical; it is the
> multiplier in front of the exponential expression for
> forward curent also.
>
> In the reverse-biased mode, a rectifier diode acts as
> a constant current device.  If the external circuit
> (the rest of the diode string) is not supplying enough
> current, then the diode reverse voltage will be close
> to zero.  If the external circuit is attempting to
> supply more current than Is, then the diode terminal
> voltage will rise to the avalanche potential.
>
> Placing two or more constant-current devices in series
> is the "dual" of placing two more more zero-impednace
> voltage sources in parallel.  What happens if you
> place a 1.5 volt battery and a 3 volt battery in
> parallel?  You get huge currents.  What happens if you
> place a 5uA and a 10uA current source in series?  Huge
> voltages.
>
-  My present high-pot tester is only capable of 9kV.  I already have 
5uA and 10uA diodes.  How would I utilize them to get some huge 
voltages?

> Therefore, if you place diodes in series, and the two
> are mismatched in Is, it is not simply likely that one
> of them will operate at avalanche - it is guaranteed.

Are you saying that there could be avalanche current in one diode and 
none in the other diode?
>
> My employer makes lots of things, among those things
> are diodes.  Our diodes are hot carrier, microwave
> switch, GaAs detector, PIN, switching, etc - no power
> diodes.  However, in the past we have made power
> diodes.  I brought this topic up to one of our diode
> designers.  He responded at some small length, and
> ultimately said this: a diode should not be routinely
> operated at avalanche threshhold unless it was
> specifically designed for that operation.  High
> voltage diodes, he advised, are designed with the
> expectation that the circuit designer will take steps
> to prevent the diode from reaching avalanche voltage.
> It is usually the case that Is at avalanche voltage
> will cause metal migration, which will substantially
> reduce the mean time between failures (MTBF).
>
> Running the diode all the way to the avalanche
> voltage, but limiting the current will almost never
> cause an instant failure.

How does one limit current in the reverse direction without limiting 
current in the forward direction?

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