Re: Rectifier/NST Protection

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: ",I= by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Steve-at-g8cyerichmond.freeserve.co.uk>

Another thing to remember with the idea of parallel protection resistors is
that some modern HV rectifiers are already made up of a series of PN
junctions, and they don't have individual protection resistors.

----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, December 21, 2000 7:03 PM
Subject: RE: Rectifier/NST Protection


> Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<pkomen-at-zianet-dot-com>
>
> Jim,
>
> Thanks, I won't rely on my OLD ARRL handbook for semiconductor advice.
>
> Pete
>
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <jimlux-at-earthlink-dot-net>
>
>
>
> Tesla list wrote:
> >
> > Original poster: "Pete Komen by way of Terry Fritz
<twftesla-at-uswest-dot-net>"
> <pkomen-at-zianet-dot-com>
> >
> > Josh,
> >
> > According to my old ARRL manual, diodes in series need both a swamping
> > resister and a bypass capacitor across each diode.  The resister
balances
> > voltages across the diode stack.  The capacitor protects against
differing
> > turn off times and passes HF spikes around the diodes.  If HF bypasses
the
> > diode, it must end up at the NST.
>
>
> The operative word here is "OLD"....
>
> In fact, (although opinions vary) putting capacitors across the diodes may
> actually make it worse.  Modern diodes, bought in a batch, will be very
> closely matched (far more closely than any set of resistors or capacitors
> you are likely to find...). If you were getting your diodes by unsoldering
> them from scrap equipment, then the whole RC equalization network thing
> would be worth considering. The 1995/6/7 Radio Amateur's Handbook explains
> why rectifier 'equalization' is prone to cause premature rectifier
> failure.[page 11-9, middle column, top]
>
> Consider the capacitors, for a moment.  They form a voltage divider.
> Typical ceramic caps have a tolerance of 20%.  I haven't actually measured
> any, but the piece to piece variation might be a lot smaller.  However, if
> you string them up in series, the voltages will divide inversely in
> proportion to the capacitance, and hence, could vary up to 20%.  Not great
> if you are running on the ragged edge for your diodes.  Same applies to
the
> resistors.
>
> Another thing about capacitors is that they store energy, which, in
general
> with HV gear, is a "bad thing".
>
> A much better approach is just to overdesign the diode string.  Put a
bunch
> extra in series.  Say you need a 20 kV rectifier and you have 1000 PIV
> diodes.  Use 30 diodes not 21...
>
> Also, consider your application. if you are rectifying a sine wave, who
> cares if they turn off at different speeds.  The voltage doesn't
> instantaneously reverse on the string.  As the voltage crosses through
> zero, the diodes will start to turn off.  If one turns off a bit faster
> than all the rest, sure, it takes the entire voltage, but, the voltage
> isn't coming up all that fast (in terms of the diode recovery time.. a few
> nanoseconds), so by the time the voltage gets big enough to cause
problems,
> the rest of the diodes will also have recovered.  Commercial HV stacks are
> made with soft recovery diodes by the way, just so that if one starts to
> turn off early, it isn't as big a deal. (Note that if you have capacitors
> in the stack, when that first one turns off, the capacitors across the
> other caps actually slow their recovery down, aggravating the process).
>
> Another thing to consider is that modern diodes have fairly good avalanche
> characteristics, so, if one DOES turn off fast, it just breaks down (like
a
> 1000V zener), and conducts a bit of current while the other diodes
recover.
>  Sure, for that few nanoseconds, the power dissipation shoots up, but the
> number of joules isn't all that big (unless you have all those bypass
caps,
> which is why stored energy is bad).  Likewise, if the stack is momentarily
> overvoltaged, and one of the diodes goes into reverse breakdown and starts
> to conduct, you don't want stored energy in the cap aggravating things.
>
> A nice discussion is at: http://www.vcnet-dot-com/measures/D.amplifiers.3.html
> >
> > I was pondering something similar using 1N4007s with a 3 megohm resister
> and
> > a .01 microfarad ceramic capacitor in parallel with each diode.  3
megohms
> > gives a current about 10x the leakage current. of the diode.
> >
> > Pete Komen
> >
>
>
>
>
>
>