Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!samsung!dali.cs.montana.edu!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: How to make 600VDC from 800VDC
Summary: Some additions to simple follower schematic
Keywords: high voltage, power supply
Message-ID: <9510@milton.u.washington.edu>
Date: 18 Oct 90 21:26:48 GMT
References: <92@decabo.enet.dec.com> <1990Oct18.134252.4635@mlb.semi.harris.com>
Organization: University of Washington, Seattle
Lines: 51



In article <1990Oct18.134252.4635@mlb.semi.harris.com> jws@thumper.mlb.semi.harr
is.com (James W. Swonger) writes:
 (basically a bipolar emitter follower on a zener diode regulator)

It is important to diode clamp the base/emitter of the transistor, if the 600V
supply has a filter capacitor (or power-down will kill the transistor)

It is also a good idea to put a collector resistor into the circuit;
if you KNOW you'll never need more than 200 mA, let the resistor dissipate
the heat at full load.

Also, power MOSFETs are available that can take the voltage drop this
circuit will have; use them, they're cheap (and the bias resistors
will dissipate a lot less power-the original was about 4 watts).  
Typical bias currents for 100V zeners are relatively low, so
you can use them at much lower than 20 mA.
The clamp diode for a MOSFET might be a zener diode (circa 30V) 
for more complete gate protection.

The pass transistor is part of a 'floating' regulator, so there's NO need
to have full 800V rated components; anything over 200V should do fine.

Amended diagram:
      800V+ --------------|------|
                          |      |
                          >      >   power resistor; 1k ohm, 35W
   Limiting resistor     <      <    (estimated peak dissipation: 20W)
                          >      >
   2.2M ohms  1/8W        |    |-
   ~80 uA to bias zeners  |   ||      350V rated power MOSFET
                          +---||
                          |    |      at least 35W
                          |    |-+    (estimated peak dissipation: 10W)
                          +-|<---+
                          |      |____________ 600V+
                          |
                          .
                          . Stack of diodes with breakdown circa 600V
                          . OR resistor for correct divider ratio
                          |
                         ---
                          ^
                         /_\
                          |
                        __|__
                        /////


        John Whitmore