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[AMPS] Screen regulators circuits

To: <amps@contesting.com>
Subject: [AMPS] Screen regulators circuits
From: John Fielding" <johnf@futurenet.co.za (John Fielding)
Date: Wed, 18 Nov 1998 23:38:33 +0200
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This discussion seems to be going around in circles and not really =
getting anywhere.

May I be permitted to add my $10 worth on what in fact is a straight =
forward problem.

1.    The problem regarding the break-down voltage of transistors or =
fets is really very simple to get around.  In the off-line switch mode =
power supply industry this problem was solved many years ago by the =
simple expedient of connecting 2 transistors in series to double the =
break-down rating, exactly the same as putting several zeners in series =
to achieve a higher voltage.  By utilising a low cost IC known as a =
"High-side / Low-side" driver the top transistor base / gate is operated =
at the required higher voltage (half-rail)and the bottom device is =
simply treated as a single transistor shunt stabiliser.  The two =
transistors are connected in series and the bases / gates are =
effectively controlled in parallel, except the top transistor has the =
higher required voltage to ensure correct operation.  High-side driver =
IC's are available with up to 600V operating capabilities and cost a =
couple of $'s.  The circuit operates as the classic shunt stabiliser, in =
my opinion the only suitable solution for a tetrode screen supply.

2.    On the question of the required dissipation rated transistor for a =
shunt high voltage stabiliser.  Take a look at TV line timebase =
transistors, here operating voltages of 1500V are common with power =
dissipations of 50 to 125W.  They are low cost and freely available.  =
They only need the appropriate heatsink and control circuitry to operate =
safely.

3.    Much has been said on the transient effects at switch-on before =
the control loop takes over the control of the output voltage.  A low =
cost MOV across the output catches the initial over-voltage surge and =
clamps it to a safe level.  In my opinion anyone who builds a screen =
supply for an expensive tube and omits to fit an MOV or similar device =
to catch spikes and surges, which happen more often than you think, =
especially during high negative screen current modes, is nuts and should =
be lead gently away to a padded cell!  Why risk a couple of hundred $'s =
of  tube for the sake of a couple of $'s worth of protection.

4.    Series-pass screen stabilisers are for the birds!  I have after =
many years of using them finally come to the conclusion that they cause =
more problems than they solve.  Play with a couple of 4CX250's with the =
maximum anode rating and you soon realise that no matter how much static =
bleed you provide across the output of the regulator you are only =
transferring the dissipation from one part of the regulator to another, =
and you will pay the price dearly the first time the tubes flash-over!  =
I have destroyed several good tubes using series stabilisers under these =
fault conditions.  Take a little time to read the excellent section in =
Ian Whites book on tetrode screen supplies and you will learn a thing or =
two!

5.    Finally let me say that I have learnt a lot from this discussion.  =
Some really bad ways to make a screen stabiliser and some promising =
ideas that need some more thought and experimentation.  To all who have =
participated, I thank you. =20

Now what is the next topic for the long winter nights?


John    ZS5JF

=20

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<HEAD>

<META content=3Dtext/html;charset=3Diso-8859-1 =
http-equiv=3DContent-Type>
<META content=3D'"MSHTML 4.71.1712.3"' name=3DGENERATOR>
</HEAD>
<BODY bgColor=3D#fff8f0>
<DIV><FONT color=3D#000000 size=3D2>This discussion seems to be going =
around in=20
circles and not really getting anywhere.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>May I be permitted to add my $10 =
worth on what=20
in fact is a straight forward problem.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>1.&nbsp;&nbsp;&nbsp; The problem =
regarding the=20
break-down voltage of transistors or fets is really very simple to get=20
around.&nbsp; In the off-line switch mode power supply industry this =
problem was=20
solved many years ago by the simple expedient of connecting 2 =
transistors in=20
series to double the break-down rating, exactly the same as putting =
several=20
zeners in series to achieve a higher voltage.&nbsp; By utilising a low =
cost IC=20
known as a &quot;High-side / Low-side&quot; driver the top transistor =
base /=20
gate is operated at the required higher voltage (half-rail)and the =
bottom device=20
is simply treated as a single transistor shunt stabiliser.&nbsp; The two =

transistors are connected in series and the bases / gates are =
effectively=20
controlled in parallel, except the top transistor has the higher =
required=20
voltage to ensure correct operation.&nbsp; High-side driver IC's are =
available=20
with up to 600V operating capabilities and cost a couple of $'s.&nbsp; =
The=20
circuit operates as the classic shunt stabiliser, in my opinion the only =

suitable solution for a tetrode screen supply.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>2.&nbsp;&nbsp;&nbsp; On the question =
of the=20
required dissipation rated transistor for a shunt high voltage =
stabiliser.&nbsp;=20
Take a look at TV line timebase transistors, here operating voltages of =
1500V=20
are common with power dissipations of 50 to 125W.&nbsp; They are low =
cost and=20
freely available.&nbsp; They only need the appropriate heatsink and =
control=20
circuitry to operate safely.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>3.&nbsp;&nbsp;&nbsp; Much has been =
said on the=20
transient effects at switch-on before the control loop takes over the =
control of=20
the output voltage.&nbsp; A low cost MOV across the output catches the =
initial=20
over-voltage surge and clamps it to a safe level.&nbsp; In my opinion =
anyone who=20
builds a screen supply for an expensive tube and omits to fit an MOV or =
similar=20
device to catch spikes and surges, which happen more often than you =
think,=20
especially during high negative screen current modes, is nuts and should =
be lead=20
gently away to a padded cell!&nbsp; Why risk a couple of hundred $'s =
of&nbsp;=20
tube for the sake of a couple of $'s worth of protection.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>4.&nbsp;&nbsp;&nbsp; Series-pass =
screen=20
stabilisers are for the birds!&nbsp; I have after many years of using =
them=20
finally come to the conclusion that they cause more problems than they=20
solve.&nbsp; Play with a couple of 4CX250's with the maximum anode =
rating and=20
you soon realise that no matter how much static bleed you provide across =
the=20
output of the regulator you are only transferring the dissipation from =
one part=20
of the regulator to another, and you will pay the price dearly the first =
time=20
the tubes flash-over!&nbsp; I have destroyed several good tubes using =
series=20
stabilisers under these fault conditions.&nbsp; Take a little time to =
read the=20
excellent section in Ian Whites book on tetrode screen supplies and you =
will=20
learn a thing or two!</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>5.&nbsp;&nbsp;&nbsp; Finally let me =
say that I=20
have learnt a lot from this discussion.&nbsp; Some really bad ways to =
make a=20
screen stabiliser and some promising ideas that need some more thought =
and=20
experimentation.&nbsp; To all who have participated, I thank you.&nbsp;=20
</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>Now what is the next topic for the =
long winter=20
nights?</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>John&nbsp;&nbsp;&nbsp; =
ZS5JF</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT color=3D#000000 size=3D2>&nbsp;</FONT></DIV></BODY></HTML>

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