[AMPS] Shunt regulated high voltage psu

[Radio WC6W]

On Mon, 28 Jan 2002 19:58:55 +0000 "Ian White, G3SEK" <G3SEK@ifwtech.com>
writes:
> 
> Radio WC6W wrote:
> >
> >  I've been running my 4CX1500B like that for... 15 years.  The low
end
> >of the plate supply bleed (50K from 3 KV) is the screen voltage
source.
> >I also have a 350V SCR crowbar, in the plate supply at the low end of
the
> >bleed, in case something goes awry with the screen regulator or the 
wires
> >between the plate and screen supply.  Works fine and the crowbar  has
> >never crowed... except for test.
> >

> Thanks, Marv - I'm pleased to learn it can be done reliably. Is this 
> with an active regulator or with zeners?

> 
> -- 
> 73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>                            'In Practice' columnist for RadCom 

Hi Ian,
   The shunt regulator consists of a venerable uA723 regulator IC driving
a FET.  The FET is connected to the screen through a series R which drops
most of the voltage and therefore dissipates most of the heat.  The '723
is powered from a +15V supply which is independent of the plate supply.  

   In the event that the plate supply is incongruously enabled without
the +15V supply or the +15V supply fails,  I have a 1M resistor connected
between the drain-gate of the FET.  There is an opto-isolator, with its
input diode connected through a 10V zener and a small R to the +15V, that
couples the 723 output to the gate of the FET .  So, in the absence of
+15V the opto' is off, the FET is turned on hard by the 1M R and the
screen voltage is clamped to a low value.

  There is addtional positive and negative screen current sensing beyond
the regulator which provides overload fault protection.

  Long, long ago (about 20 years), I designed a series-shunt regulator
for a friend who used it with a pair of 4CX250F's.  It used the same
shunt regulator as detailed above.  The series pass was a FET operated as
a current source. He powered this regulator from a stiff 400V supply so,
a small R inserted in series with the shunt leg reduced the drive to the
gate of the series FET and therefore the current sourced, as the shunt
regulator current increased to limit the total dissipation.  I don't
recall from what source what he powered the '723 in this implementation. 
Incidently, this regulator replaced a string of three 100V/50W zeners...
I forget how much idle current he was running in this previous
arrangement.

  I have recently been playing with a screen regulator design, for a pair
of 4CX600's, using a TL431 in a cascode arrangement with a FET for the
shunt leg plus the series input as noted in the paragraph just above.  It
is also foldback limited at about 10mA into a dead short.  This will be
sourced from a 400V switching supply and requires no other power source. 
There is an SCR crowbar on the output, in case this scheme departs from
the accepted laws of semiconductor physics.

  Note in context the thread this continues, the two designs just
described require parts in the series leg that can accomodate the full
source voltage, in the event the output is overloaded, shorted or
crowbar'ed.
  
   Incidently, the test I use to check stability (and regulation) of
these critters is to feed a square wave (at a KHz or so from a generator)
back into the regulator.   I typically run 40V pp through a suitable
(600V) coupling cap and a 600 ohm R.   This is +/- 30 mA test.

73 and probably good night by you,
  Marv  WC6W











*
 
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