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Peter,

Minor change, I noticed after I posted the
note below that I had not specified the second BJT in the
Darlington. The enclosed schematic is corrected and now the
simulation predicts about dead-nuts on for the tempco of the
output. Other than that, same comments as below. I think this is
the best "good enough" cascoded shunt regulator.

rogerw

Peter,Enclosed is a schematic of my
suggestion below. That is an 8.2V Rohm Zener chosen for tempco
to cancel the negative tempco of two Vbe's. It is slightly too
positive in the result but quite "good enough." You willr just
get much flatter regulation with the Darlington pair and that
can be had in a single package.

Of course, depending on the Vcemax spec of
your cascode MOSFET, you might have only one cascode instead
of two..... That will not make a noticeable difference except
to breakdown.

rogerw

On 4/6/2022 8:32 AM, Roger Whatley
via groups.io wrote:

Peter,

I realize that circuit described below
might be "good enough," but realize that making that one BJT
a Darlington pair is a significant improvement to the
regulation. You can even get the Darlington in one
transistor package, of course, cheap.

As UnRegHV increases, the current in the
BJT necessarily increases in order to regulate the loop. For
every extra 0.1uA of extra base current required, the
voltage drop in the feedback resistors (66M + 66M + 1M) goes
up 0.1uA*(66M+66M+1M) = 13.3V and thus regulation suffers.
The BJT current easliy goes from less than 10uA to over
200uA, thus the base current might go up by a worst-case of
as much as 1uA, giving a 133V change in the output. So that
is the motivation for the Darlington.

(In case anyone wonders, why not an FET
instead of a BJT, that WAS the original circuit and it
suffers worse circuit errors.....)

The next big error is that as the BJT
current increases with UnRegHV increases, so does Vbe and Vz
increase. Those are the motivations for subsequent circuit
complications and resulting in the Penultimate Way-Improved
version..... it is overkill, I know, to the requirement, but
by simulating those improvements I gain insights as to just
what the remaining errors were. And it was fun!

My take is that your present circuit, but
with a Darlington Pair, will be "good enough" and if it were
a purpose-built Darlington in one three-lead package, no
more expensive or even board-space than a single BJT.

Of course, a single Zener voltage may be
chosen to match the negative tempco of two Vbe's. Again, no
extra expense or board space. (Off the top of my head, I am
thinking about 8V Zener for +4mV/degC?)

rogerw

On 4/4/2022 10:33 PM, peter via
groups.io wrote:

Roger:

?back to the circuit you posted in message #300

Simple cascode, with only 1 npn tr + 2 mosfets.

Since I found the 2SK3265, I removed the 2nd mosfet so its
very basic circuit.

66Meg, two 1 Meg, 100k+ 2.2M trimmer , 5.1V zener, 2SK3265,
2SC3311

Set trimmer for 900V, Vary input from 950~ 1150, output only
varied 5V or less.

Heated the circuit with a hair dryer, got it up from 25C to
about 60C, voltage rose may be 5 or 6 volts. Good enough for
me!

Time to make it small enough to stuff inside my CDV-700 !

It on the LHS of the attached jpg, circuit in message #300 is
on the RHS

P

-- 
rogerw
On the Catclaw
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Virus-free.

-- 
rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.

-- 
rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.