Electronics > Projects, Designs, and Technical Stuff
100kV isolation transformers, high precision voltage and current sensing
(1/6) > >>
Spirit532:
So here's a few interesting questions.

I'm designing a power supply for a very low power(2W), but rather high voltage microfocus x-ray setup. However, due to how the tube is constructed, and partially what the datasheet says, I have to run the tube in a grounded anode configuration, where the cathode sits at -100kV, rather than supplying it with the full positive potential.
While the electrostatic focusing mechanism only benefits from this, being able to simply run through a pre-calculated resistor, I am facing an interesting problem, and that problem is filament supply. I need to somehow deliver 3.2A at 1.8V to the filament and monitor that current(in a feedback loop), all while it's floating at -100kV.

Question #1: Is there an easy, intuitive solution, or do I just have to wind a physically huge isolation transformer and place the entire monitoring circuit, including logic and ADC, at -100kV, all while feeding the data back via something like fiber optics?

Question #2: Since I'm going to be using a CW multiplier to generate the high voltage required, if I try to measure the output voltage at the transformer or one of the first legs of the multiplier, I'm going to get an inaccurate estimate of the output, and for my application the output voltage must be within +-0.5kV of the desired output throughout the entire range of -40 to -100kV.
What's the best way to measure the full output voltage directly without affecting it too much? Just a massive resistor divider with a filter, built out of long through-hole parts in oil or silicone?

Question #3: Since this is an ultra low power(2W) tube, I'm going to also need relatively precise current measurement from ~8 to 20uA with a resolution of at least 0.1uA and ~+-5% accuracy. Since the anode is grounded, I should be able to just do a low side(relative to the rest of the system) measurement, but are there general good practices to measuring tiny currents in high(er) noise environments, other than lots of shielding?

To calm the grumpy people that are terrified of x-rays: I'm very well educated in this field and I know exactly what I'm doing. Nobody will be exposed, it's all tightly controlled and measured.
jbb:
I haven’t done X-Ray work but I have done high V converter work which has similar auxiliary supply issues.  Am I right that 100KV is quite high and will produce fairly high energy X rays?

I’m guessing you’re going to have a (silicone) oil tank in any case which makes things easier. Do you have some kind of handbook on material compatibility?

On the filament supply: I think you’ll end up with a physically large transformer in oil.  The big question is how precise do you need to be? Does it need AC or DC excitation?  What comes to mind is a smart driver on the primary side and dumb transformer on the HV side. This will be simpler but offer less accurate control of the filament current (especially at low currents).

Another option to the conventional transformer Is kind of ‘inductive power transfer’ scheme (ie air cored low coupling factor transformer) which could consist of 2 pancake coils in oil. That means you’ll only need one clearance gap rather than several as in a conventional transformer. This would then require brains on the HV side for control.  Maybe it will need some attention to magnetic leakage so it doesn’t perturb the electron beam?

If you have brains on the HV I guess you could measure the cathode current itself and do the beam and filament current control on the same board. Communications could be via fibre, but the thought occurs that the oil tank is likely opaque and oil is fairly transparent, so maybe you could just use IR / red LEDs and photodiodes and beam the signal through the bulk oil? Does the communication need to be very fast?

On V sensing: I expect you’ll end up with a big resistor divider. You’ll need to look out for stability and temperature coefficients, and may need to add caps across each resistor to improve the frequency response. I think your V divider could wind up eating more current than the tube, so you’ll have to take that into account.
Spirit532:

--- Quote from: jbb on February 08, 2019, 08:30:23 pm ---Am I right that 100KV is quite high and will produce fairly high energy X rays?

--- End quote ---
Yes - the peak(!) x-ray photon energy is directly proportional to the voltage, so I'll get a normal distribution curve with 100keV photons at the top. It's relatively uncommon in a microfocus application, but the tube choice was driven mainly by price, not by exact specifications :)


--- Quote from: jbb on February 08, 2019, 08:30:23 pm ---I’m guessing you’re going to have a (silicone) oil tank in any case which makes things easier. Do you have some kind of handbook on material compatibility?

--- End quote ---
The tube has a beryllium window and isn't designed for submersion, it's going to operate in air with very large silicone doughnuts hugging it for insulation. The rest of the HV circuit will likely be either in oil or filled with silicone.


--- Quote from: jbb on February 08, 2019, 08:30:23 pm ---On the filament supply: I think you’ll end up with a physically large transformer in oil.  The big question is how precise do you need to be? Does it need AC or DC excitation?  What comes to mind is a smart driver on the primary side and dumb transformer on the HV side. This will be simpler but offer less accurate control of the filament current (especially at low currents).

Another option to the conventional transformer Is kind of ‘inductive power transfer’ scheme (ie air cored low coupling factor transformer) which could consist of 2 pancake coils in oil. That means you’ll only need one clearance gap rather than several as in a conventional transformer. This would then require brains on the HV side for control.  Maybe it will need some attention to magnetic leakage so it doesn’t perturb the electron beam?

If you have brains on the HV I guess you could measure the cathode current itself and do the beam and filament current control on the same board. Communications could be via fibre, but the thought occurs that the oil tank is likely opaque and oil is fairly transparent, so maybe you could just use IR / red LEDs and photodiodes and beam the signal through the bulk oil? Does the communication need to be very fast?

--- End quote ---
I need to be quite precise with the filament current, and the datasheet specifies that it has to be driven with AC. The electron beam is crushed electrostatically, then driven into a thin tube that has a ring magnet around it, to get the 20um focal spot. Fiber should work fine, but I've never really designed something that would float at such a high voltage relative to ground, so I'm not entirely sure on what kind of transformer I should be designing. I was thinking about stacking a couple Qi wireless chargers side by side, since I only need around 10-12W delivered through ~20mm of silicone rubber isolation for 100kV.
The cathode current can be measured on the other(grounded) end, so I could move the circuit away from the HV drive system.


--- Quote from: jbb on February 08, 2019, 08:30:23 pm ---On V sensing: I expect you’ll end up with a big resistor divider. You’ll need to look out for stability and temperature coefficients, and may need to add caps across each resistor to improve the frequency response. I think your V divider could wind up eating more current than the tube, so you’ll have to take that into account.

--- End quote ---
My original idea for the divider was to tap the first leg of the 8-10 stage CW multiplier and cut the AC component, then scale the voltage appropriately, but I'm not sure whether the output voltage will scale linearly with load(in theory it shouldn't), so I don't know if calibrating it that way would be possible. Another solution could be building a 1GOhm divider to reduce the load.
T3sl4co1l:
Most generally, you're doing isolation by converting static (DC) or quasi-static (AC, low frequency?) electrical power into E&M fields/waves, then back again.

Anything which can do this will work, and to this high-level view, is equivalent.

So, transformer: dPhi/dt == EMF.  Voila, we have E&M, and so we can say a transformer is a (near field) antenna.  We could go the other way, using dE/dt to induce displacement current -- coupling energy through a capacitor.  Both of these have the downside that they need to be fairly nearby, which puts a lot of pressure* on the insulation system.

*Electrical pressure, voltage. :P

If we open up the spacing in the transformer or capacitor, we can use RF tuning methods to cancel out some of the reactances thus created.  Instead of a nearly ideal transformer with undesirable leakage, we have a leaky transformer better described as a poorly-coupled pair of inductors (or capacitors).  This is very demanding on the Q factor of the components, and requires carefully balanced design to avoid generating interference, but can be done.

If we open it up even farther, we leave the near field entirely (depending on frequency), and move into a radiating mode.  Now we need good antennas to ensure low power leakage and losses, and we need efficient transmitters and receivers to avoid huge losses there as well.  Antenna size is acceptable in the microwave range, but the electronics aren't so great.  (A transmitter over 80% efficiency isn't unreasonable, but I'm not sure you can make a rectenna as good?)

You can continue going up in frequency, which doesn't really do you much good at first.  I guess inbetween, you could use, like, radiant heat on Peltier modules or something (or any suitably insulating fluid also carrying heat, like pumped and heated transformer oil?).

Once you get into significant quantum energy levels (namely, a few eV -- visible light), it gets interesting again. There, your transmitters and rectennas suck pretty bad (~20% each), but they are at least very cheap and available.  That is to say -- you may consider a stack of LEDs and solar cells for this application. ;D  (Or if that's a bit too bulky, a laser, fiber and cell can be smaller, if maybe not any more efficient, and definitely not cheaper.  Some isolated scope probes use this!)

In short, you can look at options literally anywhere across the spectrum.  Figure out a basic design -- and therefore capability and cost tradeoff -- for each typical case, then pick the best one. :-+

If you just want an easy suggestion, do lights and panels if you have the space, or figure out the insulation requirements of a probably resonant transformer system.  For data over the isolation barrier, consider putting ADCs local and clocking it out over fiber optic.

Tim
Spirit532:
In-air HV isolation would be quite painful, so I'm going to have to encase it in silicone. LEDs and solar panels are a no-go, especially because I need ~10-12W for the whole thing.
As for pushing data back and forth, fiber is easy and really cheap - the data rate needs to be maybe a few kBit/s tops, just to receive current settings and report back on the actual current once in a while.
S/PDIF modules are cheap and work with basically anything that transmits light - hell, I could probably just point them at each other through the clear silicone, but a 50mm piece of plastic fiber is peanuts.

Seems like there's plenty of existing solutions(like Qi 1.2, rated up to 45mm) that could provide the power I need for the filament and still be able to sit in silicone at the full isolation distance.
I've found a few rather large(9cm dia.) devices on Aliexpress that claim they can transmit up to 30W and 50W peak - if it can do 30W when the coils are stacked together, it might just be capable of delivering the ~12W I need for the filament and feedback circuitry at ~20mm separation.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod