[Amps] Rectifier diode strings

[Steve Katz]

> 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. 
	[Steve Katz]  Not necessarily.  Don't know where you came up with
that, but Cj is inversely proportional to Vr and varies with waveform --
large junctions are >>>10pF when lightly inverse biased.
> 
> 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. 
	[Steve Katz]  Actually, fast transients propagate through and around
transformer windings just fine.  One of the reasons that "conducted
interference" is often a much larger signal than "radiated interference"
from equipment.  The inductance in the xfmr means nothing to a fast
transient, which sees the xfmr as a capacitance.
> 
> 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.
	[Steve Katz]  What???  No, it's not.  Opposite legs are biased on,
then off, at a 60 Hz rate.  Nothing 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. 
	[Steve Katz]  Geesh, you obviously haven't done much studying in the
field of solid-state power supply design.  The dynamics of fast transients
has been quantified for decades and is a leading cause of rectifier failure.
> 
> 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.
	[Steve Katz]  This isn't true, unless the rectifier fuses (shorts)
due to excess avalanche energy.
>  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