Electronics > Projects, Designs, and Technical Stuff
High Voltage Bench Power Supply Design
duak:
With RV1 connected that way it will have a non-linear output voltage versus position and will be quite sensitive at high output voltages/low resistance settings. You might want to consider adding a trimmer resistor in series with RV1 to adjust the maximum output voltage.
I would not be surprised if this circuit oscillates. I don't see any capacitors to roll off the opamp's response. The voltage amplifier composed of Q1and R2 is adding a pole to the open-loop response as well as a nominal phase inversion. The opamp inputs should also be swapped.
I don't see any ballast resistors in the source leads of Q2 - Q4 to help with current sharing.
I would add a resistor in series with Q5's base to limit current under transient conditions. With high voltage supplies, the current can increase faster than the current limiter can act. eg., connecting a 1 uF cap to the output with the supply set to maximum.
H713:
--- Quote from: duak on June 16, 2019, 07:20:01 pm ---With RV1 connected that way it will have a non-linear output voltage versus position and will be quite sensitive at high output voltages/low resistance settings. You might want to consider adding a trimmer resistor in series with RV1 to adjust the maximum output voltage.
I would not be surprised if this circuit oscillates. I don't see any capacitors to roll off the opamp's response. The voltage amplifier composed of Q1and R2 is adding a pole to the open-loop response as well as a nominal phase inversion. The opamp inputs should also be swapped.
--- End quote ---
Op-amp inputs are an error in my schematic- the inverting input is connected to the 6.8V voltage reference. There is a .1uF capacitor from pins 8 to 5 on the 5534, which if I understand correctly serves to roll off the frequency response of said op-amp to enhance stability at low gain.
Just out of curiosity, any suggestions for how to make the voltage output track more linearly with the pot?
The 47K load resistor on Q1 was chosen to enable a less costly FET for Q1, but the slew rate issue is a fair concern and I may be better off dropping that value to more like 10K.
peteb2:
I recently re-committed myself to revisiting an old obsession with vacuum tubes, purely from a hobby standpoint. Ridiculous it would seem because i last had anything to do with the things was back almost 40yrs when i started out with a career in electronics to become a tech. The broadcast station where i trained was fully on valves running but here and there were little islands of transistorized stuff slowly making an inroad and then suddenly the entire place was updated, all tube gear simply dumped. My training courses however covered vacuum tubes with more emphasis than semiconductors probably because the curriculum hadn't been modernized quickly enough...
So i too am now in the HV bench power supply need and have been looking at options for a while.
Certainly having something i can scratch-build, switch-mode design using 'mosfets' etc would be ideal. Sadly over the years i've actually had to make such things work again be they low voltage of high that have failed in the real world, i have seen some of the most expensive devices simply have gone BANG rendering a real repair impossible because of the cost of the actual spares.
So my approach for FWIW will be a "Valves Design", to hunt down a few of those old vaccum tubes i once used (that seem to have ebay increasingly awash with) to do this task of a High Voltage, lowish current bench supply shouldn't be too much of a job.
So far the most frightening cost will be a big mains transformer but we'll see what comes. I can't see the point of building something with very high cost semiconductors that could all be so easily be destroyed for a silly mistake at the hobby bench...
H713:
I was originally going to use a big transformer out of an old 500 series Tektronix oscilloscope, but that transformer was commandeered for another project. Two suggestions for your mains transformer:
1) A 120:240 with a voltage doubler. Big control transformers that are a 120/240:240/480 are reasonably common and I see them for less than $50 on a regular basis, but they are big and awkward.
2) Have a look at Antek toroidal power transformers. They are good quality and difficult to beat for the price.
Personally, I think I can design a FET based power supply that will be reasonably robust. We shall see whether or not I am correct.
I thought very hard about doing a tube supply, but the space is a big issue, and I'm going to be employing forced air cooling in order to keep the size of my SS version down. I would probably need four or five 6L6s to deliver sufficient current. There is presently another design thread for a tube-based HV bench supply that has a lot of good information in it and which you may want to look at.
duak:
H713, here are two ways to solve the voltage control non-linearity in the existing circuit. The first is to go back to a fixed resistor where RV1 is and varying R12 instead. The second is to fix the ratio of R12 to RV1 and linearly vary the reference voltage.
Reducing the value of R2 will increase the slew rate somewhat but have you calculated the power dissipation of R2 & Q1? Also, the drain to gate capacitance of Q1 will have a significant effect due to the Miller effect. ie., voltage changes on the drain will pump current into the gate circuit and tend to slow things down. You may consider tying the gate of Q1 to +24 V and driving the source instead. ie., Q1 in common gate configuration.
Although I don't want to derail you from building this circuit, are you aware of something called the floating regulator? If I were to design and build a variable HV supply, I'd consider using that topology. If I can sum up the concept behind it, it's that it considers the +Vout terminal as the common rather than the -Vout. The opamp can drive the gates of the pass transistors directly without using a high voltage span driver stage. It's really just an artful re-arrangement of the existing parts. A complication is that you'll need an additional 24 V or so power supply isolated from the main raw supply to operate the reference and op-amp. hp, Lambda and others have been using this topology for years. There's a description of floating regulators at the end of this document: https://booksite.elsevier.com/samplechapters/9780750686266/Sample_Chapters/02~Chapter_1.pdf
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