[Amps] Re: Equalizing resistors with HV diodes

[Ian White, G3SEK]

RZP wrote:

>G8WRB said:
>
>>the rate of metal migration would decrease
>>exponentially with increasing thickness
>
>There is a limit to how thick you can deposit metal. The Unitrode process,
>which uses the end of the lead in direct contact has some advantages here.
>At least in diodes, you don't tend to get the step coverage problem that
>you get in ICs. However, metal migration is also a temperature related
>phenomena, so depending on the diode construction, and the temperature and
>the operating time, it might or might not be a problem.
>
>Operating in avalanche mode is not desirable for a semiconductor junction
>that hasn't been designed to so do, at least if you want reliability.

That final sentence was also Dave, W8NF's point. However, we do need to 
be clear that this discussion is not about routine operation in the 
avalanche region.

The power rectifier problem is to ensure that the diodes will survive 
very occasional avalanche events, provoked by incoming voltage spikes on 
the mains. These can come in a very wide range of durations (time) and 
also a wide range of voltages, so it's a game of chance. The aim of 
engineering design is to make sure that all the frequent, smaller spikes 
will be survivable, and most of the rarer, larger events too.

This is best done by ensuring that all the diodes in the rectifier 
strings are operating far, far away from their avalanche region at 
normal mains voltages. In other words, definitely use lots of 
well-matched diodes in series to give a large excess of PIV capability.

BTW, I don't believe it's helpful to talk about extreme cases such as 
wildly mismatched diodes (like 1000V and 200V PIV) in series, or to 
assume that reverse-biased diodes behave as ideal current sources (they 
don't).  Sometimes extreme cases can throw a useful light on what 
happens in real life, but they can also be very misleading.



-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek