[Amps] Rectifier diode strings

[EVonvaltie@aol.com]

Well, so far the only significant reason that has been offered, by Steve K., 
is that for fast transients, the rectifier string "appears to be a string of 
small capacitors." True enough, and indeed small they are. Down in the 10pf 
range. 

Now. lets consider this in context. These rectifiers will typically appear on 
the secondary side of a large transformer that is not exactly a shining 
example of high-frequency transformer design. Although I have never gone 
looking for it, I would not expect to see much high frequency energy on the 
secondary side. Especially peaks in the range of many kilovolts.

In order to get anywhere with this line of reasoning, we need to adopt a (or 
some) generic models of transient pulses and look at the possible results. I 
think it is futile, however, because of another practical factor. In a 
typical full-wave rectifier, one leg of the rectifier is always in 
conduction. This provides a path from the transformer, through the conducting 
rectifier (very low R) through the filter caps (low Z for all except UHF 
transients) and back to the transformer. Hence, any high-voltage transient 
that would happen to make through to the secondary during the 'off' period of 
the other rectifier would likely be highly suppressed. I can't see how it 
could ever even develop any kind of energy-bearing waveform across our string 
of diodes/capacitors.

The basic technical issue at work here is still hanging wide open, and I 
maintain my challenge to anyone to offer something we can chew on. That is, 
how do you get all the rectifiers in the string into simultaneous conduction? 
Bear in mind that when reverse conduction happens, it does so at the nominal 
PIV+. The diodes do not go into an irreversible SCR-type avalanche where the 
voltage drop essentially goes to 0. So even if a transient happened to kick 
the string into conduction, as soon as the transient dies away, we have 
sub-PIV conditions again.

Enough for now. Let's try to put this myth to rest once and for all, or else 
give it a dignified birth certificate.

Eric von Valtier K8LV


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