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High Voltage Bench Power Supply Design
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H713:
I remember looking at floating regulators when I started this design process, but I'll revisit the idea. I can certainly see the advantage. Eliminated the HV driver is definitely welcome.

Yes, I did calculate the power dissipation of Q1. It will have to be fairly considerable, probably a reasonably tough TO-220 package device. The one specified in the schematic is sufficient with a 47K resistor for R2, but if I drop that much lower I'll have to choose something with a beefier DC SOA.

I'm probably going to breadboard up a sketchy prototype with some IRFP450s (I have them on hand) as the pass elements and see how it performs. If it performs as expected then I will proceed to lay out a PCB for it. LTspice simulations look pretty good, so we shall see how it behaves in the real world.

FWIW, the voltage adjust pot will probably be a 100K 10 turn. I might have some 10K helipots as well, I'll have to dig around.



duak:
H713, you might be able to use a trick called bootstrapping to improve the slew rate of the driver stage.  Basically, you split the drain load resistor in two and connect the junction between them to a capacitor that connects to +Vout.  What this does is force the supply voltage for the driver stage to track the output voltage and effectively alter the time constant.  From the AC small signal standpoint, the drain load resistance has been halved, so the slew rate has been doubled.  From the DC standpoint, the total series resistance is the same so the power dissipation doesn't change.

BTW, here's a link to a floating regulator project: https://www.neurochrome.com/high-voltage-regulator/


H713:
Alright, I've redrawn the schematic so that RV1 adjusts the reference voltage. I breadboarded the circuit and the voltage adjust works quite well, however there is one significant issue, which is overcompensation. At an output of say 200V, applying a 1000 ohm load (which makes for 200 mA) results in an output voltage of about 230 volts.

Edit: Ignore the V2 attachment, it contains a significant error. The V2a attachment is correct.
splin:
For a current limit all you need is a depletion mode MOSFET with a resistor between gate and source - very easy but then you have then same SOA/power dissipation issue as with your regulator devices when the output is shorted. The IXTH6N100D2 might fit the bill - 1000V, 300W 350mA @ 500V DC SOA but costs around $6.

Presumably your regulator cct will provide a variable current limit function so a better solution (for protection, should the regulator fail short cct) might be to use a Bourns Transient Protection Unit (TPU), which looks like it would be perfect for your application:

https://www.bourns.com/docs/product-datasheets/tbu-ca.pdf?sfvrsn=9e4a8a65_38

The TBU-CA085-300-W has 850V peak withstand, (450Vrms continuous) 13 ohms max on resistance, 300mA min current trigger, 600mA max. When the current exceeds the trigger it blocks within 1us, limiting the current to around 1mA.

Not surprisingly the trigger current varies with temperature. It resets when the voltage drops below a threshold voltage, Vreset of about 16V. Best of all they're cheap at less than $2.

A selected TBU-CA085-200-WH might be better with 200mA min, 400mA max trigger.

Bidirectional versions (TBU-DT085-200-WH) are also available for the same price but shouldn't be necessary for this application, but it might help if the PS bridge rectifier were to fail short circuit.
duak:
H713, I'll bet it's oscillating.  Do you have an oscilloscope to see what's happening?  If not, put an AC voltmeter on the output to see if there are any spurious signals.

The NE5534 opamp is not guaranteed to work correctly if the input voltages get too close to the power supply rails.  On the data sheet this is the Common-mode Input Voltage Range.  In the latest design, the reference voltage applied to the inverting input will certainly go out of range.  At 200 V out, the ratio of R13 to R14 will put the non-inverting input right on the edge as well.  If you don't want to use a rail to rail opamp, you could make the NE5534 work by providing  a negative power supply a couple of volts below GND.

This could be done simply by using four rectifier diodes in series with the 24 V supply circuit GND connection and the general GND connection, ie., rename the two GND nets in the 24 V supply as -2.4V and connect the cathode of the rectifier stack to it.   U1 pin 4 would connect to -2.4V.  U1 pin 4 should have a 0.1 uF to GND for AC bypassing.

I think R7 to R9 are too low to help share the load current much.
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