High voltage constant (low) current load - Page 1
Forum section: electronics / projects

author: inse
author_rank: Super Contributor
timestamp: December 31, 2023, 09:54:54 pm
content: I got myself a Electrophoresis power supply capable of 3kV output voltage, which I want to use as a HV source for experiments as it allows control of voltage, current and power.The unit has been made by the former Swedish brand Pharmacia in the early 80‘s.No documentation available so far…It is working fine but has a feature that is limiting its use: the output requires a minimal load of ca 1mA to activate.I don’t know whether it’s a kind of safety feature to avoid high voltage on exposed electrodes or just a technical limitation.The behavior could be fixed by adding a constant current load of 1mA to the output, with only 3W of max. power dissipation.The voltage range is in principle from 10 to 3000V (I plan to use it above 100V), the circuit would have to cover a huge range.As MOSFETs are only available to max 1.6kV breakdown voltage, my idea was to make a cascode circuit of four 1kV MOSFETs and a simple current regulator with an OPAMP and a sense resistor.Unfortunately I don’t have experience with cascode arrangement and wonder if this idea is reasonable.Can anyone give me advice on HV designs?

author: moffy
author_rank: Super Contributor
timestamp: December 31, 2023, 11:24:02 pm
content: This might suffice:https://au.mouser.com/datasheet/2/240/media-3321478.pdf

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 01, 2024, 12:08:17 am
content: 3W and 1 mA? Why don't you want to test it with just a simple passive resistor load?What do you want from it so you need active electronic load?Why 6x 510k 2W resistors in series are not enough to load it?

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 12:29:57 am
content: I need to sink 1mA also at lower voltages, the power supply is adjustable from ~10V upwards.The power supply won’t start even at lower voltage settings unless a minimal current is drawn.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 12:37:20 am
content: @moffy:hefty price tag

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2024, 12:39:06 am
content: Quote from: inse on January 01, 2024, 12:37:20 amNot for what you want to do.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 01, 2024, 12:50:00 am
content: That MOSFET is way cheaper than my first thought of doing it with tubes.If you want cheap get reverse engineering and hack it to work how you want it to work.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 12:58:28 am
content: Thanks for the suggestion.Above the price of the MOSFET, do you think I will be able to get the regulation stable over such a huge range?

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2024, 01:07:30 am
content: Quote from: inse on January 01, 2024, 12:58:28 amThere is no reason not to as long as the current regulation loop is moderately fast, and for current that is quite easy.

author: Slh
author_rank: Regular Contributor
timestamp: January 01, 2024, 03:11:10 am
content: Heads up, that MOSFET is only rated to around 2000V at DC . Look at the SOA curves. The curves don't go as low as 1mA so you might get away with it but you won't know until you test it.

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2024, 03:23:10 am
content: Quote from: Slh on January 01, 2024, 03:11:10 amGood point.

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2024, 03:47:59 am
content: Maybe a cascoded pair of SiC, JFETs they are certainly cheaper:https://au.mouser.com/datasheet/2/412/UF3N170400B7S_Data_Sheet-3177149.pdfbut the biasing network would be interesting.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 06:43:02 am
content: „The Art of Electronics“ also mentions the cascode for high voltage trick, but no further details on the bias network.Have a happy new year, everyone!

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 07:50:24 am
content: Quote from: Slh on January 01, 2024, 03:11:10 amThe curve drops vertically, I don’t think it would recover below 10mA - there must another reason for that limit.

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2024, 08:56:07 am
content: Quote from: inse on January 01, 2024, 06:43:02 amI did a similar thing for a HeNe laser gyroscope a number of years ago. The final laser current was controlled by a 200V JFET in a constant current feedback loop and the voltage had to be trimmed by placing a series resistor with the JFET drain to limit the maximum voltage. To aid this I designed and built a test jig with about 4 of the JFETs cascoded so that the optimum current for the laser could be set, you wanted max intensity without multi moding. The trick is to share the voltage equally across the JFETs, even in transient conditions.Included is a rough template, but should not be used as is, it is just an illustration to be improved upon. You would probably want to consider some sort of gate clamping.P.S. A happy new year to you also.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 09:23:33 am
content: Interesting application- how did you design the D/S snubber network?

author: moffy
author_rank: Super Contributor
timestamp: January 01, 2024, 09:43:06 am
content: Quote from: inse on January 01, 2024, 09:23:33 amNot really a design just an indication, the capacitance is for sharing transients equally between the FETs, the resistance just to limit transient current.

author: ArdWar
author_rank: Frequent Contributor
timestamp: January 01, 2024, 10:46:13 am
content: Quote from: moffy on January 01, 2024, 03:47:59 amYou can control N-JFET with the same opamp-based N-MOS control scheme, just give the opamp more negative supply headroom. The control "sign" is still the same after all.

author: inse
author_rank: Super Contributor
timestamp: January 01, 2024, 07:42:14 pm
content: I will keep you informed once the components arrive…

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2024, 12:38:10 am
content: Quote from: inse on January 01, 2024, 06:43:02 amUsually it comes down to series connected resistors to control the voltage at each gate or base. A high voltage zener diode may be placed across each resistor to limit the maximum voltage of each stage under dynamic conditions. Then the bottom stage is configured to control the current.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 03, 2024, 04:04:12 am
content: Quote from: inse on January 01, 2024, 07:50:24 amIt also varies with HV mosfet type and vendor.I see the Mouser G2R1000MT33J MOSFET 3300V has a SOAR slope, and allows ~ 1A @ 80V and 100mA @ 800V, so should be fine for 1mA at 3.3kVQuoteDid you measure that 1mA across voltages ? Maybe you can modify that limit ? (possibly they have poor regulation at very light loads, so added that to stay in spec ? )

author: inse
author_rank: Super Contributor
timestamp: January 03, 2024, 04:38:46 am
content: Maybe I will have to do a deeper analysis of the circuit.So far I found out that the HV drive is shut down in case of open circuit.A bias voltage of ca. 12V is present across the output and once a minimum current is drawn, the driver starts up.I haven’t figured out what the actual converter topology is, the transformer is driven by two transistors, possibly in half bridge configuration.But I don’t know what topologies the controller TDA1060 actually supports and whether it needs a minimum load for stability or if there is a safety feature to avoid high voltage on open connections.

author: moffy
author_rank: Super Contributor
timestamp: January 03, 2024, 04:52:23 am
content: Quote from: PCB.Wiz on January 03, 2024, 04:04:12 amThe data sheet I have shows those points as on the 100ms pulse width curve, is that considered the same as a DC load? I thought they were different.https://genesicsemi.com/sic-mosfet/G2R1000MT33J/G2R1000MT33J.pdf

author: inse
author_rank: Super Contributor
timestamp: January 03, 2024, 05:16:20 am
content: Shouldn’t the difference between DC and pulsed operation only be thermal constraints?

author: moffy
author_rank: Super Contributor
timestamp: January 03, 2024, 05:20:25 am
content: Quote from: inse on January 03, 2024, 05:16:20 amIf you look at the SOA curve for:https://au.mouser.com/datasheet/2/240/media-3321478.pdfThe max DC voltage is below the max voltage for the pulsed 100ms, it's probably avalanche related.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 03, 2024, 05:21:31 am
content: Quote from: moffy on January 03, 2024, 04:52:23 amIt varies with vendor, but 100ms is usually close to DC params.The Zth curves give a clue, as the DC and 100ms values are essentially the same.

author: inse
author_rank: Super Contributor
timestamp: January 03, 2024, 05:35:17 am
content: I don’t have experience in this operation mode, I have to confess.Though I am dealing with high power switching semiconductors, they are only used for switching, linear operation is considered devils work.I have added a photo of the board in reply #21

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 03, 2024, 09:18:58 am
content: Quote from: inse on January 03, 2024, 05:16:20 amThat also depends on the pulse width.At long times (100ms+,) yes, DC is close to thermal-behaviour following.However, at short pulses, (10us to 1ms) you can find that a 100:1 change in pulse width, only gives a 10:1 gain in power, due to localised failure effects.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 04, 2024, 07:55:35 pm
content: You could go for a cascode tree with medium voltage MOSFETs but watch out for dissipation.See my mad idea for a floating source attached. It simulates ok .The MOSFET types aren't crucial but they to be able to manage a couple of watts.The SOT223 p-ch MOSFET I've used here would need propper (pricey) heat sinks so you might be better off with some TO-220 types.An IMA0105S12 would do nicely as the DC-DC converter. The voltage supplying the current source circuitry isn't critical but needs to be at least Vgsth+Vset(max) to work properly.9-15V say.I'd go for an isolated digipot in the final version to keep the HV away from your knob.Else a pot with a long plastic shaft. You can derive Vset with a resistor from HT and a control current sink.A high voltage N-ch in the sink will help if you want to go down the current programmed route. A lot of the volts on the current path can be dumped in a large (hot) resistor.A good idea I thought to explore was to derive Vset usingpwm. That would be much cheaper and simpler to isolate from the grounded world.I'll dwell upon it.For those who think there's not much difference between a source and a sink, think again.

author: inse
author_rank: Super Contributor
timestamp: January 04, 2024, 10:37:10 pm
content: Thanks for your suggestion, Terry.I have already gone through my collection of DC/DC converters and found one that is capable of 1.5kV isolation.That would be just sufficient for my power supply as it has in principle a bipolar output with center connected to low voltage GND.Is there a specific reason you went for PMOS?I would have tied the control to HV minus and used a stack of NMOS.I have already ordered a couple of good old BUZ50A‘s, capable of 1kV each and SOA rated for 1kV DC.I would not need to adjust the current level, I‘d simply go for ~1mA, which is sufficient to start the HV circuit as proven by experiment.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 14, 2024, 05:32:43 pm
content: There is a much easier non-mad way to build a ccs. Make a floating source/ sink. Invest in a big ass 3.3kV SiC MOSFET. eg G2R1000MT33J ($15 Mouser).Because the source is floating, either side may be earth referenced if required. To the outside world it acts like a 2 terminal device.The DC-DC needs to provide enough headroom for the source. I'd go for a 12V output. The opamp can be any single supply type that can swing its inputs and ouput all the way to ground (for zero current). It has to be comfortable operating at 12V too. The DC-DC conveter could be replaced with a battery controlled with reed relay.You will need to use an iso DC-DC of >3kV, you don't want to kill your control circuitry.Its easy to control the set current with PWM via an optocoupler.Death is optional.

author: inse
author_rank: Super Contributor
timestamp: January 14, 2024, 07:31:52 pm
content: No progress so far as I am still waiting for the MOSFETs.Thanks for your valuable reply.I plan to use a DC/DC converter module 15V/3W ( is at hand) with 1.6kV isolation.This should be ok as the HV is bipolar with center tapped to the LV side GND, so the DC/DC won’t see more than -1.5kV.It is supposed to supply an OPAMP which drives a Cascode of four 1kV MOSFETs - need to work this out experimentally.As the HV is limited to 150mA max/ 50mA at 3kV, failure of the cascode dimensioning hopefully won’t destroy the MOSFETs.Death is optional - I know

author: inse
author_rank: Super Contributor
timestamp: January 14, 2024, 07:37:10 pm
content: -

author: inse
author_rank: Super Contributor
timestamp: January 14, 2024, 07:43:17 pm
content: -

author: inse
author_rank: Super Contributor
timestamp: January 14, 2024, 07:46:41 pm
content: -

author: inse
author_rank: Super Contributor
timestamp: January 25, 2024, 07:54:11 pm
content: So finally I got the right amount of MOSFETs, the ebay seller mixed up different types.The cascode circuit works fine, it keeps the current absolute stable up to 3kV.The voltage is shared between the MOSFETs equally and the load current is shared between the resistors and the MOSFETs.Unfortunately the circuit does not work down to 12V - initially I wasn’t regarding this as problematic but…The power supply has some kind of sense voltage, which is used to detect open load condition.Once the sense voltage is loaded, the HV generator is activated.The load current is also monitored and if it drops below a certain level, the HV is turned off.So with my constant current load, the HV is kept alive as supposed, only the startup is problematic.When flipping the standby switch repeatedly, the HV starts up after several attempts if the voltage setting is not too low…So either I switch over to a single, costly HV MOSFET or find another way to trick the sense voltage circuit.As I am located in Bavaria/Germany, I shy away from ordering the proposed part from Digikey or Mouser and Farnell would only do BtoB.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 25, 2024, 11:55:04 pm
content: Quote from: inse on January 25, 2024, 07:54:11 pmIt must get quite close to 12V min ? What does it need to work down to ?You could try 1200V or 1500V MOSFETS, but their VGSth edges up

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2024, 12:08:49 am
content: Quote from: inse on December 31, 2023, 09:54:54 pmTAoEX-chapterssection 3x.6 is relevant here - "Floating High Voltage Current Sources". That outlines designs going to 3kV.Key concept: use an HV depletion mode MOSFET in the same way you would use a JFET.

author: inse
author_rank: Super Contributor
timestamp: January 26, 2024, 04:36:27 am
content: @PCB.wiz:The G2R1000MT33J you proposed has a gate threshold of 3.5V, so this should work well - my problem is sourcing.@tggzzz:I have the German copy of the 1989 (7 th?) edition of TAoE, the chapter you mentioned is not included.I got my inspiration from chapter 6.20 high voltage regulators.Which edition are you referring to?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 26, 2024, 07:21:06 am
content: Quote from: inse on January 26, 2024, 04:36:27 amI see IXYS make depletion mosfets to 1000V, for not too much premium. Microchip have them up to 700V.The depletion ones will work down well below 12V

author: tggzzz
author_rank: Super Contributor
timestamp: January 26, 2024, 09:08:21 am
content: Quote from: inse on January 26, 2024, 04:36:27 amThe supplementary X-chapters.https://x.artofelectronics.net/gives the contents.13 pages in total, example circuits shown

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 26, 2024, 07:29:51 pm
content: Yes, G-S Zeners seem a good idea. You would need care to avoid 'live connect' this, give the device capacitances.Sounds like the power source does ramp carefully.

author: moffy
author_rank: Super Contributor
timestamp: January 27, 2024, 02:03:18 am
content: Using a high voltage NPN (1.8kV) for the first stage would alleviate the voltage needed at low voltages, you would also only need 2 of the BUZ50A e.g.

author: inse
author_rank: Super Contributor
timestamp: January 27, 2024, 07:39:23 am
content: Interesting part, let me see whether I can find it at a reasonable price, but looks better than the 3kV SiC.Did you have an actual model of it for LTSpice?Can you do a ramp on the HV source to see when the current kicks in?If I took two of them, I would not need the BUZes anyway.

author: moffy
author_rank: Super Contributor
timestamp: January 27, 2024, 08:00:01 am
content: Quote from: inse on January 27, 2024, 07:39:23 amYou need the BUZes to keep the gate driver network high impedance, especially for the low voltages, Ib times the divider resistance voltage drop. I couldn't find a SPICE model for it. Since I don't have a model for it or the BUZ50a, I chose a MOSFET with a Vto=3.5V, I can't say when it turns on, if you want to test the low voltage performance you could use any NPN( just keep in mind the low hfe more base drive) with your BUZ50as as you have the actual devices it should give reasonable results.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 27, 2024, 08:24:11 am
content: Quote from: inse on January 27, 2024, 07:39:23 amThe HV NPN part looks to be EOL. Sanyo to onSemi, and now only at Rochester - unusual specs, high voltage and low current. Not many customers ?I'd be wary about series use of dissimilar parts. The NPN has very low Cfb, so a rapid power ramp will have less drop across the higher-C MOSFETS.If you are confident the ramp up/down is slow enough, try it ?

author: inse
author_rank: Super Contributor
timestamp: January 27, 2024, 08:54:42 am
content: @ Moffy: You are right, of course - I was shortsightedly only thinking of the high voltage case.But I would not get enough base current at low voltage

author: inse
author_rank: Super Contributor
timestamp: January 27, 2024, 09:03:22 am
content: @PCB.wiz: the circuit would be installed inside the power supply, so ramp-up would be quite limited (need to measure the dv/dt with my newly acquired HV probe).But if I short the output, the negative slope will be dramatic.I will take account of a G-S diode to limit negative voltage.I already fried an OPAMP when I made ‚scratchy‘ shorts so I switched over to the robust, primitive and absolutely sufficient transistor circuit.But the test setup was far from ideal then, everything connected with long wires…

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 27, 2024, 10:25:51 pm
content: Quote from: inse on January 27, 2024, 09:03:22 amWow. Shorts are a tough ask.You will need large resistors ( R7 will have 3kV spikes) and a clamp diode on the NPN as well, and make the gate-source zeners serious ones too.

author: inse
author_rank: Super Contributor
timestamp: January 28, 2024, 09:15:16 am
content: For the moment I will keep to my 4 BUZes, I made a photo of the hodgepodge setup.Rise and fall times are not too dramatic with 0.1V/us, but I noticed a ‘funny‘ overshoot to 4kV =:oThis thing is designed for electrophoresis, it will never become a electronics lab graded supply…

author: Wolfgang
author_rank: Super Contributor
timestamp: January 28, 2024, 10:51:24 pm
content: Quote from: inse on December 31, 2023, 09:54:54 pmHave you ever considered a tube based load ? A PL519 would easily do the job, even up to higher voltages. And a GP5 would even allow 10kV or more at 1mA. Both tubes are cheap, and robust.

author: moffy
author_rank: Super Contributor
timestamp: January 28, 2024, 11:44:07 pm
content: Quote from: Wolfgang on January 28, 2024, 10:51:24 pmNot that familiar with tubes but they are an excellent HV solution, the problem being encountered is not the HV end but working down to the 12V or lower end. Would tubes work at that voltage?

author: Wolfgang
author_rank: Super Contributor
timestamp: January 29, 2024, 12:28:16 am
content: 12V is a bit low, but if you spend enough grid current this could work for a sweep tube like the PL519, why not. In this case you should check for Barkhausen instability.A guy on microcontroller.net did operate the tube at 12V/500mA at the anode, so 1mA should not be a problem. I would try this out.

author: moffy
author_rank: Super Contributor
timestamp: January 29, 2024, 12:36:13 am
content: There is a solution for your low voltage dilemma, use the 15V supply as a negative supply and reference the current source to the -15V then the minimum voltage across the current source at 12V is 27V.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 29, 2024, 12:45:29 am
content: Should work as well, especially at only 1mA of max. load current.

author: inse
author_rank: Super Contributor
timestamp: January 29, 2024, 04:59:08 am
content: This is really getting creative, all this brainstorming- amazing, I appreciate it.The tube idea triggered me, but it would be hard to mount this 12cm beast inside the case…Looks like I brought up an interesting topic :-)I still am looking to trick the startup protection circuit, so far I only found out how it’s not done.The power supply controller is surrounded by 14(!) OPAMPs, part of them presumably for the measurement display.Also an interesting device: an analog multiplier IC, presumably for the power measurement as there is no microcontroller involved.By the way, did anyone notice the 2N3055? Made me smile…

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 30, 2024, 12:49:49 am
content: I came across a 1700V depletion mode mosfet, looks prelimhttps://www.ark-micro.com/upload/day_230817/202308171413008670.pdf

author: Wolfgang
author_rank: Super Contributor
timestamp: January 30, 2024, 01:35:53 am
content: IXYS has some with up to 4500V, but about for 100€ each. Tube is large, but a fraction of the cost and probably a lot more robust.

author: inse
author_rank: Super Contributor
timestamp: February 01, 2024, 08:29:04 pm
content: Dear contributors, finally I made it: after partial reverse engineering the power supply, I added a RC combination at the right spot that makes the HV turn on and even keeps it alive.Nevertheless I keep the current sink as it improves stability and discharges the filter capacitors.The overshoot issue hasn’t been resolved yet.Thanks for all the inspiration and good or crazy ideas.I enjoyed the discussion and hope you did, too.and off I go to the next project: overcurrent shutoff/ torque limitation of an electric screwdriver that has been supercharged with LiPo cells (salvaged from eVapes) :-)