Path: utzoo!attcan!uunet!comp.vuw.ac.nz!canterbury.ac.nz!phys169
From: phys169@canterbury.ac.nz
Newsgroups: sci.electronics
Subject: Re: How to make 600VDC from 800VDC
Message-ID: <1990Oct29.152553.9568@canterbury.ac.nz>
Date: 29 Oct 90 02:25:52 GMT
References: <1990Oct18.134252.4635@mlb.semi.harris.com> <1990Oct23.151030.5176@engin.umich.edu>
Organization: University of Canterbury
Lines: 63

From article <1990Oct18.134252.4635@mlb.semi.harris.com>, by jws@thumper.mlb.semi.harris.com (James W. Swonger):
[circuit using "stack of diodes with breakdown of 600v"]

In article <1990Oct23.151030.5176@engin.umich.edu>, ssave@srvr1 (Shailendra Anant Save) writes:
>            The stack of diodes to total a breakdown of 600V, would be
> a LOT of diodes, unless you mean zeners....
> 
[goes on to use 78xx series regulator well beyond it's voltage rating]
 
The first circuit was best, but the reference voltage can be derived in better
ways. One way is a bunch of neon bulbs in series (with 1Mohm resitors
across each one - about 90volts/bulb from memory) but you need another
transistor to get enough current into the base of the transistor, and a
capacitor to smooth out nasty jumps in base voltage as the neons fire. If great
regulation isn't required, the regulator could simply chop off 200v (less
zeners or VR tubes, etc). Or a resistor divider could feed the base of the
transitor (still better regulation than a potential diver straight into the
load). But my favourite (avoiding a 600v reference) is...

       800V+ --------------|------|
                           |      >
                    R3=    >     <  R2= 160V/Imax (limit power in transitor)
                   R2*HFE <       >  (approx 50W)
                           >      |
                           |    | /
                           |    |/     TV flyback (horizontal
                           +----|      output) 1500V BVceo,
                           |    |\     at least 40W. 
                           >    | V    
                   R6=    <       |   
              R3*550/200   >      +----+-----------> 600V+
                           |  C1__|__  >
                           \ |  _____ < R4 ~ 100K/5Watts
                            \|    |    > ( <<R3*HFE2*579/200)
                             |----+----|
                            /|         |
                           V           < 
                           |            > R5=R4 * 579/21
                         -----         <
                           ^ ' 20v     |
                          /_\ zener*   |
                           |           |
                         __|__       __|__
                         /////       /////
Notes:
(1) Use 2*10Volt low power zeners in series if you like. 
(2) C1 is optional; try 200nF/800Volt - larger capacitor for slower start-up,
    if you omit it, put at least a 1uF capacitor across the zener to reduce
    noise in the power supply line - zeners can be noisey, and we're going to
    magnify any noise by a factor of 30 otherwise). C1 is essential if you
    have a big power supply capacitor on the 600v side of the circuit.
(3) Second transitor needs to be able to withstand at least 800V, 12Watts.
(4) remember an insulated heatsink for the main power transistor.
(5) R2 & R6 are there basically just to reduce heat in the transitors, and to
    limit currents.
(6) Be careful with the high voltages.
(7) If the 600V load has a large capacitor, but the 800v supply doesn't (but
    has some other equipment sucking current, be careful when switching off
    the power that the main transistor doesn't become temporarily reverse
    biased - you may need a protection diode.
(8) E&OE (I haven't actually tested the circuit, but I've built similar ones)

Mark Aitchison, University of Canterbury, New Zealand.