Hi Paul,


I also like the "simple" current sink circuit approach, but it usually 
takes a few more parts to get a stable circuit... the most basic design 
with just a resistor, transistor and op amp tends to "sing"! And there's 
also some parts selection to be done.

 

See Linear Tech Application Note 105, figure 200 on page 108:  
https://www.analog.com/media/en/technical-documentation/application-notes/an105fa.pdf


Here's where I might start on adapting the Linear Tech design...

 

Change the 1 Ohm resistor to 10 Ohm for a 1 V  / 100 mA control voltage.

 

Will need a suitable MOSFET for the application… I filtered N-Channel 
MOSFET on DigiKey looking for the following:

 

Vdss between 250 and 400 VDC... I regularly use supplies at 180V and 
occasionally push 300V. Make sure you have some margin.

Power Dissipation between 25 and 40 W… I assume at least 100 mA at 200 V 
for the load… plus margin… plus a suitable heat sink!

Continuous current between 1.7A (lowest available value by the time the 
previous filters were in place) and 3A (provides a few more alternatives!)

 

I also suggest a 10V to 12V supply for the op amp to assure ample gate 
drive voltage. Some of these MOSFET need a bit of drive to get to higher 
currents… 5V might well be sufficient, but need to check the MOSFET data 
sheet. 


Select an op amp for operation to the negative rail for low current 
settings and a suitable V+ supply range.

 

 Anyway, that's a start!


Enjoy

Bob

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