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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

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

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rogerw
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rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

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