High Voltage Bench Power Supply Design - Page 1
Forum section: electronics / projects

author: H713
author_rank: Regular Contributor
timestamp: January 25, 2019, 04:11:56 am
content: One of my hobbies is designing and building tube-based audio equipment. I'm at the point where my Eico 1030 tube-based high voltage power supply isn't really cutting it anymore for a few reasons:1) At this point it needs a fairly hefty investment in parts and labor (and that means my time), including about $50 worth of tubes and way more time than I want to put into a piece of equipment that doesn't really fit my needs.2) The 150 mA maximum current is a limitation when prototyping output stages of larger amplifiers. I'd really like to have closer to 300 mA continuous.3) It's big in an awkward way (tall), and bench real estate is at a premium.This has led me to the decision to build a new power supply that is better suited to my requirements. In a move that is likely to be controversial, I have decided to go with a solid-state based design using MOSFETs as the series pass element. Unlike many a threads on this, I have no illusions that a 500V 10A MOSFET can handle 10A while dropping 500V across it (I have yet to see a TO-247 device that can dissipate 5 kW). I will likely need multiple devices in parallel in order to get enough current with a comfortable safety margin.In the interest of simplicity, I have decided to use an LR8 as the voltage reference. This should get me a maximum output of around 450 volts. A few key design specs here:-It doesn't need to be completely idiot proof. It's a high-voltage power supply, and that in it of itself is anything but idiot proof.-It should go from 80 to 450 volts (more is better, but the LR8 is limited to 450).-It should be able to handle 300 mA continuous.-It should be very quiet (electrically, that is. There will be a fan).-It should be current limited. If a device under test were to short, It should current limit it so that it doesn't immediately blow the pass elements. In this situation it would dump the full 500V output from the rectification stage into the shorted load- not a good situation, creating lots of smoke and sparks. The filter capacitors will not be very small, so blowing the MOSFETs to pieces would not be hard without some form of protection.The power transformer in question is a Tektronix 120-037. Information can be found on the web for those who are interested, however for me to get about 500V rectified output from it at sufficient current is easy, and I already have the filter capacitors and rectifiers on hand. This will likely be a cheap project, as aside from the FETs and the LR8, I should have most of what I need in my "junk box".Looking at the DC SOA, I think that a pair of these FQA8N90C-F109 should be sufficient for the series pass element with a reasonable safety margin.https://www.digikey.com/product-detail/en/on-semiconductor/FQA8N90C-F109/FQA8N90C-F109-ND/1954558I know that at a lower voltage output (say 100V), this will be dissipating a lot of heat. I'm planning to mount the FETs to the heatsink with a clamping bar. I also plan to take the time to ensure the heatsink is actually flat (I will put it on the mill and skim-cut it with a fly cutter if need be). In order to maximize heat transfer, there will not be any mica spacers. This means extra care will have to be taken in chassis design since the heatsink will be hot (both electrically and physically).That pretty much leaves my current limiting circuit, which I'm not quite sure how I should go about. It needn't be adjustable, I was planning to leave it fixed at around 500 mA. I have a distinct feeling that an LM317 is not going to be able to take the voltage drop necessary. Most of the designs I've seen for high-voltage current limiting circuits use a BJT, but I'm concerned that I will run into issues with secondary breakdown. If I could use the same MOSFETs as for the pass element, that would be great, but I haven't seen a whole lot of designs that do this.I would like to keep this reasonably simple. I do not plan to design a PCB for this. Perf-board and point-to-point is the plan.I'm sure I'm crazy, but this is so far what I've come up with. I have thought very hard about using tubes for the pass element- there are certainly advantages (usually they don't fail short like a FET) but at the desired output it would simply take up too much space. I'd love to hear suggestions on how I should proceed. Hopefully I'll have time to draw up a schematic with my idea this weekend.Sorry for the long post.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 25, 2019, 05:24:17 am
content: What about use a variac to vary the voltage?

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2019, 05:45:52 am
content: A few threads on HV power supply design have crash and burned here. Good contributors get into a flame war, edited/deleted posts, banned etc. I may have soot on my feathers.https://www.eevblog.com/forum/projects/looking-for-yours-opinions!-hv-stabilized-power-supply/https://www.eevblog.com/forum/projects/200v-200300ma-power-supply/https://www.eevblog.com/forum/beginners/a-good-homebrew-500v-1a-power-supply/https://www.eevblog.com/forum/projects/planning-to-build-500v-tube-psu-help-needed/https://www.eevblog.com/forum/beginners/transistor-vs-mosfet-for-linear-power-supply/Vacuum tubes are economical, robust and much simpler to design-in for pass elements compared to solid-state. This digresses into a religious war of sorts. Cheap, easy and antique is a couple pentodes.Modern/elegant is all solid-state but unfortunately "trial by fire" so design work is tedious. Very few MOSFETS are rated for power/decent SOA in the linear region. You'll have to series or parallel multiple parts or forgo a completely linear design and make a HV SMPS or a hybrid with a pre-regulator or transformer tap switching/variac. Or look for an off-the-shelf old BK Precision 9185 series, or Xantrek (Sorensen/Ametek) or Kikusui etc.It all depends on your budget, patience and expectations. A HV linear supply is never going to be compact. This is not a simple or cheap project.In old threads, requirements were not nailed down and a few hundred volts or mA here or there can add 10x the cost/complexity.There's no shortage of theoretical designs and optimistic Spice simulations. I did not see a design get carried to fruition.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 25, 2019, 08:08:07 am
content: Some people reported success with electrophoresis psu's ??

author: Gyro
author_rank: Super Contributor
timestamp: January 25, 2019, 09:19:06 am
content: Quote from: coromonadalix on January 25, 2019, 08:08:07 amIFyou can find one that isn't center grounded, without ground leakage, no load sensing etc. You probably need to do something about their voltage regulation response too (mine is still waiting for that experiment!):https://www.eevblog.com/forum/projects/300v-dc-supply/msg1926577/#msg1926577

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 09:30:57 am
content: Hi,some comments:- The FET you have chosen is not made for linear operation. You need an IXYS linear MOSFET made for this type of load, andprobably you need to run some in parallel. IXTH2N150L would be a choice.- current limiting is fine, but in case of a short circuit you will dissipate ca. 200W. Maybe some preregulator is a smart idea,or transformer tap switching if this is possible with your transformer.- I have made a 500V supply with 30mA. You could change my circuit to 300mA by replacing the pass MOSFETsLink here:https://electronicprojectsforfun.wordpress.com/power-supplies/high-voltage-lab-power-supplies/a-500v-30ma-variable-linear-power-supply/- Play safe!Wolfgang

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2019, 09:52:54 am
content: What's wrong with the FQA8N90C-F109? It's clearly specified for DC linear operation. Refer to figure 9 on page 4 of the data sheet.https://www.onsemi.com/pub/Collateral/FQA8N90C_F109-D.PDF

author: beanflying
author_rank: Super Contributor
timestamp: January 25, 2019, 10:12:14 am
content: There is a few of these on evilbay in the USA. Very unlikely you could buy or even scrounge the bits for this sort of price.eBay auction: #173757684650

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 10:18:40 am
content: Quote from: Zero999 on January 25, 2019, 09:52:54 amYou're right, I have seen the DC in the SOAR, only I have no trust in parts developped for switching purposes. Fairchild parts also had a DC SOAR, and they blew up nevertheless. In a linear HV PSU, the pass transistor is the most critical part of the circuit. If this shorts out, the full high voltage will appear at the DUT, plus the charge stored in the smoothing cap after the rectifier. This can be many Joules of energy, normally killing your DUT (and hopefully not you too) instantly. I would play extremely safe here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2019, 11:32:15 am
content: I have a few FQA9N90Cs, I'll test DC SOA sooner or later.Good choice, I found they had the least cost per watt of any transistor with DC SOA and the ratings needed for a project.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 11:47:19 am
content: ... testing the SOAR is a good idea. A test would run a normal load, plus then pulses of the current that would occur when the output is shorted. A very robust load is recommended, just in case the test fails.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2019, 12:39:23 pm
content: Quote from: Wolfgang on January 25, 2019, 10:18:40 amAs I said in one of those other threads: never rely on semiconductor or thermionic devices to protect against electric shock. Always assume the output of your power supply has the potential to be at a hazardous voltage, even when it's set to <50V.It should have overvoltage protection, but even so, it shouldn't be relied on to protect the user from shock, only the DUT.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 12:44:56 pm
content: Even if the PSU works just fine, its output voltages can be lethal, as with any HV tool. Use caution even when you set the output at low voltages. Reasons see above.Old HV slogan:Your first error is your last error.

author: H713
author_rank: Regular Contributor
timestamp: January 25, 2019, 06:16:12 pm
content: Perhaps the key aspect here is the fact that I have most of the parts on hand at this time. All of the costly parts (Transformer, filter capacitors, panel meters, chassis, connectors, etc) is stuff I have on hand. Yes, I am aware that this FET is designed for switching. That is why I'm planning to put at least two in parallel, if not more, which should keep it well within the DC SOA. The idea is that each device will be run conservatively enough that even though it's not really designed for linear operation, it should be fine. A circuit breaker or HV fuse on the output will be implemented as well to stop anything too stupid from happening.The current limit is probably the hardest, but I'll spend some time digging around for a reasonably robust way of doing it.I pretty much assume (from a safety standpoint, that is) that if the output is capable of producing 500V, then I'm going to treat it as if it always does.Also, the key aspect is that I recognize the fact that the pass elements could short. That will always be in the back of my mind while I'm using this power supply.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 06:20:05 pm
content: What you could do for a safety measure is to implement an adjustable overvoltage crowbar at the output.

author: H713
author_rank: Regular Contributor
timestamp: January 25, 2019, 08:30:12 pm
content: I was thinking about doing a crowbar, however I'm wondering if you had anything in mind in terms of implementation.The first idea that comes to mind is to use a two-gang pot for the voltage adjust, the second of which forms an adjustable voltage divider. That along with a zener could be used with a beefy triac if the output voltage goes noticeably higher than it's supposed to. What I'm not a huge fan of about this is that it's putting a fair bit of trust in the wiper of a potentiometer.The second idea is a fixed crowbar circuit for say 500V. If the adjustment is limited from 0-450V, and it goes up to say 500 (indicating a pass element short), then the crowbar would be triggered (again using a zener, but this time with a fixed voltage divider).Attached is the world's ugliest schematic to try and illustrate my idea.Is this what you had in mind?

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 08:37:04 pm
content: You got the idea.In the circuit below:https://electronicprojectsforfun.wordpress.com/power-supplies/high-voltage-lab-power-supplies/a-1kv-50ma-linear-power-supply/I have created an electronic "fuse" that switches off a preregulator output in case of *overcurrent*.You could do something similar for *overvoltage".

author: 001
author_rank: Super Contributor
timestamp: January 25, 2019, 08:38:37 pm
content: oh, manit is long taleno easy way to make it with transistorscheck Heathkit units at ebay or rebuild Your own clone

author: Wolfgang
author_rank: Super Contributor
timestamp: January 25, 2019, 09:06:26 pm
content: Correct. Reliable transistor (or MOSFET) high voltage supplies are not as simple as they look.But its an interesting subject, IMHO.

author: H713
author_rank: Regular Contributor
timestamp: January 26, 2019, 01:39:26 am
content: So I've drawn up a schematic with my idea as it stands now. I may choose to parallel a third transistor, though I do suspect that two would be more than enough. I still haven't worked out my current limiting circuit, so if anyone has an idea as to how to go about it I'm all ears.One thing I am planning on implementing (though I didn't draw it in the schematic) is to have a "50-250" and a "250-450" voltage switch. In the 50-250 mode, it shorts across R1 and R5. This does several things. Firstly, it means I don't need to use a 500K pot for the voltage adjust (more precise setting), and it will limit the amount of power dissipated by the pass elements. This combined with a current limit (if I can come up with something suitable) and the crowbar circuit makes me feel much better about the reliability of the FETs.Edit: Do NOT try to build this schematic as shown- Testing at a later date revealed that the FETs cannot meet their published specs and a pair of them in parallel will not be able to handle the load. More viable schematics are shown towards the end of page 3.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 26, 2019, 02:27:45 am
content: The crowbar is OK, maybe it needs a small cap across the resistor so it does not fire from just a small spike.Manuall switching transformer ranges is possible, but unelegant. You could do this electronically.Look here:https://electronicprojectsforfun.wordpress.com/power-supplies/preregulator-circuits-for-linear-power-supplies/for some ideas.

author: H713
author_rank: Regular Contributor
timestamp: January 26, 2019, 03:08:57 am
content: Thanks, I didn't even think of switching transformer taps. If I use the Tek power transformer which has a lot of windings, that could work quite well. The voltage doubler on the 2.5 ohm 155V winding (which would be put in series with two of theseven6.3V taps this thing has) was just the easiest way to get around 500 volts- but this is definitely a better way to do it.Electronic switching would be great, but it wouldn't solve the potentiometer issue. I could, of course, just use a fine and coarse adjustment system, which would be pretty easy to implement.Regardless of whether I go for the electronic switching or doing it manually, I do like the idea since it reduces the stress on the FETs.One thing that should be considered with this design is that if the wiper on the potentiometer were to go open, the power supply would jump to it's highest output setting. It wouldn't be high enough to trigger the crowbar circuit, however, because it would still be within the power supply's acceptable output range. This is a problem I've seen on most of these designs (including many tube ones). Potentiometer failures aren't super common, but they do happen and I would hazard a guess that it's more likely than having the FETs blow without triggering the crowbar. The dangers of this could be mitigated however by having the crowbar circuit change whenever the transformer taps are switched. While an open pot wiper would still be a bad situation, at least the power supply wouldn't dump 500 volts into a load intended for 100.A current limit fixed at 750 mA would help mitigate the damage done if something were to fail. 750mA is still way more than enough to kill you, but a short probably wouldn't be quite as spectacular.Edit: Forgot to attach schematic.

author: duak
author_rank: Super Contributor
timestamp: January 27, 2019, 12:46:26 am
content: Background: I have a Kepco ABC-425 power supply (400 V @ 50 mA) that has the difficult to get 8068 regulator tube. It was built in 1973 and except for the tube is all solid state. I've been toying with the idea of replacing the tube with FETs should it expire. I'd sort of known that switching FETs weren't the best choice for linear applications even though in the late 70s and 80s the SOA graphs said they should work well. Apparently, the problem is Electro-Thermal Instability. I've attached some files showing this a bit better. NASA plus the FET manufacturers have more info on this phenomenon.To get back to the OP's design: When I look at the attached graph, I see that ETI is more likely at low current and high voltage. This tells me that it's probably better to put the pass FETs in series rather than in parallel. It's not quite as easy as paralleling them but it can be done with a voltage divider.As others have said, a short circuit is bad news. This can happen if a capacitor fails. When I look at the latest schematic, I don't see any current limiting. If the output is shorted or the crowbar is fired, C6 will discharge through the 15 V zener and will probably short it out. Depending on the zener's series resistance, it may momentarily increase the drive to the FETs, causing them to dump the DC bus into the load as well.I work with Variable Frequency Drives using IGBT modules that incorporate cycle by cycle current limiting, quite often by monitoring the IGBT's source current. Most of these drives can handle a direct short to ground without damage. Applying this to the OP's design, I would add a fast current limiter that would monitor the voltage across the FET source resistors, and reduce gate drive if too high. This could be done with an NPN transistor and a few resistors. If the pass FETs were put in series as I suggest above, I would put the current limiter on the upper FET and have it latch off when there was gross overcurrent. This could be done with a PNP-NPN latching circuit. A more precise variable current limit could be implemented around the lower FET if desired.For what it's worth,

author: 001
author_rank: Super Contributor
timestamp: January 27, 2019, 09:09:22 am
content: Quote from: duak on January 27, 2019, 12:46:26 amI need Kepco schematic to help YouI googled it but no resultsCan You post Yours circuit diagram here?

author: duak
author_rank: Super Contributor
timestamp: January 27, 2019, 04:53:58 pm
content: 001, I don't have a schematic for the Kepco supply. It is working fine so I don't need it. If the vacuum tube fails the circuit is simple enough to trace out. I mentioned it because I have tubes of IRFP360 MOSFETs (rated for 400 V) and I wondered if I could use them for something. I looked into using them for regulators and loads and found the research on Electro-thermal Instability. Thank you for your offer which made be realize that my post was confusing. I've edited it to hopefully make it clearer.As an aside, I was thinking about the size of a vacuum tube versus a transistor. The active volume of the tube is a few cc and the plate area is a few square cm. The transistor die is a few square mm in area with the actual active volume just the top tens of microns.Cheers,

author: 001
author_rank: Super Contributor
timestamp: January 27, 2019, 09:42:50 pm
content: It is pitfallCommon power MOSFET is actually a bunch of many small devices in parallelAny small transistor can be overloadedSo local damage is highly expected

author: BravoV
author_rank: Super Contributor
timestamp: January 28, 2019, 09:04:32 am
content: Not sure if this fits you, 0-500V at 10ma ->EDN : Regulate a 0 to 500V, 10-mA power supply in a different wayI guess you need to modify to accommodate higher current.

author: H713
author_rank: Regular Contributor
timestamp: February 06, 2019, 05:29:11 am
content: Again, I'm hoping that by paralleling multiple devices I can cut down on the chances of the pass elements failing. I'm well within the DC SOA for the devices mentioned in earlier posts, and so long as I can work out a decent current limiting function the pass elements should never be subjected to the stresses that would cause them to fail. At the present time, I'm extremely busy and haven't had a lot of time to sit down and design a current limit circuit- the negative feedback loop in my current project (a push-pull 1625 power amplifier with a solid-state phase splitter) is taking longer than I anticipated to fine-tune, so this project has been pushed back a little. That is combined with the rather heavy workload that engineering majors are subjected to.Also remember, as I mentioned, I will be cooling these devices VERY well, so it's not like they're going to be running at 60C or anything like that.Lastly, I have no illusions that this is better than doing it with tubes. This is an application where tubes excel, but in my situation space limitations come into play.The reason I want this to be a regulated supply, rather than just a transformer with a variac on the primary is that I sometimes use the bench supply when I want to take the power supply ripple and voltage sag out of the equation.

author: Wolfgang
author_rank: Super Contributor
timestamp: February 06, 2019, 09:36:56 am
content: Quote from: BravoV on January 28, 2019, 09:04:32 amHi,I have build this but as expected it is very slow. You could speed this up by replacing the photcell coupler by a small transformer-based supply plus a normal optocoupler. This can also drive more current then. Furthermore, the BU208A could be replaced by a linear MOSFET, now available up to 2.5kV.

author: BravoV
author_rank: Super Contributor
timestamp: February 06, 2019, 12:02:09 pm
content: Quote from: Wolfgang on February 06, 2019, 09:36:56 amMind share your circuit in schematic, as I have difficulty to visualize it.Btw, you said very slow, do you mean the regulation ? How slow ?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 06, 2019, 03:02:25 pm
content: With High voltages Power dissipation increases rapidly with a bit of current, but of course you already know that.I was also thinking about building some kind of high voltage supply.My first thoughts are around using an LM2587 Fly-back converter and a transformer of an old PC power supply in "reverse".For a higher output voltage you can put the 230Vac windings of 2 transformers in series.It should also be almost trivial to add a few tranistors (or optocoupler) and a Zener that throttles the LM2587 down as soon as the output voltage is 10V or so higher than the output of the lineair regulator at the end.

author: Wolfgang
author_rank: Super Contributor
timestamp: February 07, 2019, 09:38:23 am
content: Quote from: BravoV on February 06, 2019, 12:02:09 pmGive me some time, I'll post a schematicsWolfgang

author: H713
author_rank: Regular Contributor
timestamp: June 07, 2019, 01:35:17 am
content: Little update:The project is not dead, I've just been extremely busy with work, school, and many other projects. Attached is how I plan to do current limiting in some form or another. The clean and elegant way of doing this would be to connect R1 to the output of the LR8 and use the series pass transistors to do current limiting as well.The belt-and-suspenders approach is to stick this mess in series with the rest of the circuit. The downside, of course, is that I use at least two (probably three) additional (costly) transistors. It is probably a safer approach, however.The one other issue I'm working at is the load switch, and how to switch voltages in excess of 500V. Something tells me that you're not supposed to use a standard 250V toggle switch for this.

author: H713
author_rank: Regular Contributor
timestamp: June 16, 2019, 06:54:21 am
content: Alright, the design has seen a revision. The LR8 is a great little device, and would be fine for most applications, but I want more than 450V. In addition to that, I think I can learn a lot more by going without it.Here is what I came up with. Not a complicated circuit, and I'm sure by no means unique. Sorry the schematic is such a mess, it is still a work in progress.Transformer tap switching will be accomplished using a 120:200 transformer with a voltage doubler. A relay will be used to switch between the full and 50% voltage points (convenient aspect of a doubler). With a beefy transformer, likely a .5 kVA "general purpose" control transformer (Tek transformer was commandeered for another project) and large caps, I can get decent regulation out of this. The switching of "taps" will be done on another PCB that is the control board, which will monitor temperature of the heatsink and adjust fan speed accordingly, handle "tap" switching based on output voltage and deal with "fault" conditions (pass element short, over-temperature, insect infestation, etc).One thing that I personally like about this is that if the pot wiper were to go open, the power supply would drop to its lowest output setting, rather than dumping the full 500V into the load. With this design, I plan to have the power supply adjustable from about 15 to 525 volts- good enough for most tubes.Adjustable current limiting is a nice feature, but I don't feel the need for it to be very precise. As such, I'm just going to use a 5 ohm rheostat for R11.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2019, 10:46:43 am
content: Source, not drain resistors!Mind that 47k pullup into huge gate capacitance will give quite low slew rate. So low you may have problems with ripple rejection.I don't get the 24V supply just for the opamp. Why does it need to be regulated? Why does it need to be so high? It could be anywhere from 3.3V (use precision 1.24-2.5V reference; downside, Vbe of level shifter is a larger fraction of Vout) to 9V (pretty much everything as shown, resistors adjusted as needed). Supply can be an unregulated DC wallwart, or a tiny SMPS, or maybe even something obscure since so little current is needed (e.g., coupling caps from mains, into a rectifier and shunt regulator, load side grounded -- a few mA won't trip an RCD/GFCI).Not that 7824s are expensive or anything, but on that note, a shunt regulator with a pair of 6.3V diodes in series would get you a nice pre-reference for low noise and ripple, without the bulk of a regulator and associated caps.Tim

author: duak
author_rank: Super Contributor
timestamp: June 16, 2019, 07:20:01 pm
content: 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.

author: H713
author_rank: Regular Contributor
timestamp: June 16, 2019, 11:59:14 pm
content: Quote from: duak on June 16, 2019, 07:20:01 pmOp-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.

author: peteb2
author_rank: Regular Contributor
timestamp: June 17, 2019, 01:07:12 am
content: 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...

author: H713
author_rank: Regular Contributor
timestamp: June 17, 2019, 01:47:53 am
content: 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.

author: duak
author_rank: Super Contributor
timestamp: June 17, 2019, 06:16:36 am
content: 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

author: H713
author_rank: Regular Contributor
timestamp: June 17, 2019, 06:38:38 am
content: 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.

author: duak
author_rank: Super Contributor
timestamp: June 17, 2019, 04:44:08 pm
content: 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/

author: H713
author_rank: Regular Contributor
timestamp: June 18, 2019, 03:01:41 am
content: 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.

author: splin
author_rank: Frequent Contributor
timestamp: June 18, 2019, 03:00:39 pm
content: 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_38The 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.

author: duak
author_rank: Super Contributor
timestamp: June 18, 2019, 05:37:15 pm
content: 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.

author: H713
author_rank: Regular Contributor
timestamp: June 19, 2019, 06:17:21 am
content: Quote from: duak on June 18, 2019, 05:37:15 pmYep, it was oscillating all right. Looking at the output on the scope, it probably a few hundred kilohertz. I should have guessed this by how twitchy the pot was- justtouchingthe pot shaft caused the output to drift.I added a 470 pF from the output of the op amp (pin 6) to the inverting input (pin 2), and that killed the oscillation- mostly.This thing doesnotlike capacitive loads. With a .082uF on the output, it draws almost 50 mA and the capacitor itself hums at about 3 kHz. As it turns out, poly film caps can become surprisingly effective transducers!I replaced the 470pF cap with a 100nF cap, and it considerably improved the stability, but there is still a bit of oscillation into capacitive loads. As a side note, I can put a big electrolytic cap across the output and it doesn't care- it's the smaller film caps with very low ESR that make it go ballistic.I tried switching the 5534 out for an OPA604 and then a TL071, and none of them were any better. I tried adding a negative rail, and it didn't seem to help much either.If I can get it stable into capacitive loads, then I think I've got a solid design here. Load regulation isexcellent, at least for a simple design like this. Connecting and disconnecting a 250 mA load (at 250V) causes the output voltage to vary no more than .1V. Quite honestly, I had a hard time really measuring much as the most precise meter I have is 4.5 digits, and even that isn't really enough to see what's going on here. I'll call that passable. It is still worlds better than the old Eico 1030. I took apart and cleaned all the pots in the Eico, and replaced the two main filter caps (most of the smaller electrolytics are original and should still be replaced). It is now a fairly usable power supply, but its output varies as much as +/- 2V with a 150mA load.So what about the FETs? The proof-of-concept has eased my concerns considerably. I'm testing with an IRFP450, simply because I have them in stock. I ruled out the IRFP450 from consideration early on due to its 500V limitation and the fact that it doesn't have a DC SOA in the datasheet, however even with some pretty awful things done during testing (intentionally trying to kill it), I was unable to do so. With transformer tap switching, I think the FETs will be running quite conservatively. As mentioned I will probably use the FQA8N90C-F109, though I may try a Fuji 2SK3675 as well. There is no SOA in the datasheet (), but if I do some worst-case-scenario testing and they hold up I would feel comfortable using them. Regardless of what FET I use, this testing will be necessary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2019, 12:53:07 pm
content: A lot of the classic HEXFETs don't seem to have DC SOA today, but if you look for historic datasheets you'll usually find a curve. Don't know why they dropped it, would guess it's just one more way to optimize cost on an old jellybean design, not having to do sampled SOA testing.Last time I cracked open an IRF740PbF, the die size was huge, probably not much changed from the original. I tested it to something like 140% the datasheet rating, at 300V (which is possible because they give RthJC(max), not typ). Would expect similar from IRFPxxx's.The capacitance of those old designs is massive though. You may need a driver just to get enough speed to reject mains ripple.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: June 19, 2019, 02:03:56 pm
content: Quote from: T3sl4co1l on June 19, 2019, 12:53:07 pmThe reason for no DC SOAR shown on new devices is that is so small. The old, large dies are not prone to hotspotting, the new ones are, and it getting worse with every generation. For linear apps, you need DC-qualified *linear* MOSFETs, e.g., from IXYS. For some theory, try "Spirito Effect".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2019, 02:28:24 pm
content: Quote from: Wolfgang on June 19, 2019, 02:03:56 pmThis is not true in general, as has been discussed many times before. Latest generation SuperJunction transistors apparently have full DC SOA once again, despite havinghigher power density than anything ever before!This only seems to be true of earlier generation MOSFETs, which were higher power density than the classic generation (taking HEXFETs as "classic", say), and which are now going obsolete. (I think IXYS PolarHV was an example of that low-SOA generation that comes to mind?)Clearly, there's something about the SJ design that can be changed, with little impact on switching performance or what have you, and they've taken to optimizing it, giving the wide DC SOA. Whatever it is, it's nice that it's available!I've also seen IGBTs with DC SOA -- I wouldn't be so inclined to takethatat face value, I would definitely test it before releasing a product dependent on it -- but they are putting it out there as such!Tim

author: H713
author_rank: Regular Contributor
timestamp: June 19, 2019, 03:03:42 pm
content: Any suggestions on improving stability into capacitive loads?Also, is anyone here familiar with the 2SK3675? Would it be decent in this application? Allied Electronics has them on clearance for less than $2 each, so if they're tough enough, that would be great.

author: Wolfgang
author_rank: Super Contributor
timestamp: June 19, 2019, 03:34:15 pm
content: Never seen an IGBT with DC soar. Interesting ! Any types recommendable ?

author: duak
author_rank: Super Contributor
timestamp: June 19, 2019, 05:42:48 pm
content: H713, the pass transistor stage may be oscillating by itself. Bipolar transistor emitter follower circuits can and do oscillate with capacitive loads. The solution there is to add damping resistance in series with the base. I expect that FET source followers could do the same. You already have series gate resistors here of a reasonable value so I would try increasing the values of the source resistors R7 - R9 to a few ohms and see what happens. As I mentioned before, larger resistors here will help balance the currents in the pass transistors. You can also try a series RC damping network between the output terminals - try 10R and 100n and see what happens. You've already found that electrolytic capacitors with their higher losses do not cause the trouble that the film caps do.You've got a lot of voltage gain in this circuit, and it's distributed among a number of stages each of which introduces a delay - this is a recipe for an oscillator. I would analyze the circuit to determine its frequency and phase response and do what I could to improve the phase margin. Do you know about Bode plots or Root Locus analysis as applied to feedback systems? (I used to know more at one time - if you don't use it, you lose it)

author: H713
author_rank: Regular Contributor
timestamp: June 20, 2019, 06:55:30 am
content: A little further testing revealed that smaller values of electrolytics do cause a small amount of oscillation, something like 30-40 millivolts. A series RC network did not cure the problem, nor did a gate resistor on Q1. I currently don't have a source resistor in the test sample, as there is only one pass element being used. I can try that, but I doubt it will cure the issue.What did cure the issue is the brute-force solution of putting a 100uF electrolytic on the output. This sort of defeats the current limit protecting the DUT, though it still protects the pass transistors. While I personally would not have any problem using a supply like this (it's still less scary than a lot of the power supplies I work with), It's really a brute-force solution to the problem and makes this design considerably less useful for others.Duak,Bode plots and Root Locus analysis is a bit over my head, or at least the terminology is. With audio amplifiers, I occasionally do test the frequency response and phase shift to get an idea of where instabilities are coming from, though normally with well-designed tube amps it comes back to the output transformer, which there isn't a ton you can do about. That said, I'm really not sure of the best way to test a circuit like this. It's not like I can feed an AC signal to the input and look at it on the output.

author: duak
author_rank: Super Contributor
timestamp: June 20, 2019, 05:18:49 pm
content: H713, Many linear power supplies do indeed use an output capacitor for stability and it sure does render the current limiting moot for transient loads. A sure fire way to get around the current transient is to design a supply that inherently current limits, but the voltage is varied by clamping it to the desired value. The hp 6186C supply works this way.Back to your circuit, consider it to be the positive going side of a Class-A audio amplifier. By injecting an AC coupled square wave, say 100 Hz, into U1 pin 2 and then following the signal path you can see how the various stages respond and try various things. Because the feedback loop is delaying the signal, you can try paralleling R13 with some small capacitance, say 1n0 with some series resistance. This is called lead compensation. Also, because you have gain to burn and the problem is that there is too much at high frequencies, reducing gain without adding delay can be helpful. Try increasing the value of R3.

author: H713
author_rank: Regular Contributor
timestamp: June 21, 2019, 06:21:11 am
content: I didn't have a chance to do the frequency/phase analysis today, however I did experiment with a few of your suggestions. I was able to increase the value of R3 to about 2.5K if it is connected to the -18V supply rather than GND. This makes things better but doesn't kill the issue cold. Unsurprisingly, the power supply is less stable at lower output voltages. With this configuration, it is good down to about 75 volts with a 33uF output capacitor, but below that threshold and the 33uF capacitor causes an oscillation in it of itself. With a 100uF capacitor it is fine. If I have time tomorrow I'll try the 1nF cap across R13 and see what that brings about. As I understand it, this is essentially to roll off the frequency response (and lower the overshoot on square waves).Quite honestly, I don't have a big issue with a big output capacitor in the 100uF range. It will take up considerable board space and add cost, but this isn't a production unit so that is not a big deal, especially when I have the parts in stock. I also don't have an issue with its impact on current limiting. The point of the current limit in this design is not to make this thing idiot-proof (no 500V power supply will ever be), but rather to avoid literally melting the DUT and/or blowing the pass elements. The main thing I don't like about it is the fact that I feel like it is a band-aid fix to avoid proper debugging and circuit modifications. With that said, it seems to be a fairly common solution to this problem. I'd like to get this circuit as stable as possible before adding the output capacitor, however.

author: H713
author_rank: Regular Contributor
timestamp: June 24, 2019, 06:41:55 am
content: Alright, the 1nF cap in parallel with R13 seems to have done the trick. I would do the square wave testing, but I successfully managed to properly nuke my proof-of-concept/prototype.I'm fairly confident that this is not a design issue, and more of a sketchy parts from who-knows-where issue. And no, the pass transistor did not short. I have yet to try this with the actual driver and pass transistors, so there likely will be some tweaking required. Square wave testing will be done then as well.Meanwhile, I have an updated schematic and a version 1 PCB layout. Gerber files are attached and can be viewed in an online viewer. Feedback greatly appreciated, as my layout and routing skills are something I'd like to improve. I did try to keep the PCB a somewhat reasonable size whilst limiting myself to 2 layers for cost reasons.

author: blackdog
author_rank: Frequent Contributor
timestamp: June 24, 2019, 05:09:14 pm
content: Hi,I have some remarksSplit R2 in 15K than 4.7uF to ground en 33K to the drain of Q1, this wil help the hum suppression this wil be a small 30dB.Is the SOA enough for Q1?Place a 8.2V or a 10V Zener over R14 to protect the opamp in case of short circuit, anode to ground.If you want to spend a little money, replease the diode D5 with a 10V reference.The minimum output voltage will be a bit higher, but the power supply will become more stable and will get less noise.Adjust the value of R14 to get the right ratio again.If the current value of C5 just keeps the power supply stable, then something's wrong...The 10nF of the pin-2 and the 100nF over the compensation connection of the NE5534 is really ridiculous...TipIf you come up with values for components that are far above the normal values during testing, then you know that something is not going right.C5 is for normal use with gains lower than 3x about22pF!Above gains of 3x a capacitor in not necessary for the NE5534.But...This is not a normal circuit, you have extra gain in the loop, so compensation for a stable regulator wil be far more difficult.Yes C7 helps :-) but that is nog good enough, and no do not make this capacitor higher in value!You have to solve the problem where it arises and that is the gain or Q1.Place a resistor and a capapcitor in series from the Drain van Q1 to ground, this wil lower the gain in the loop.Try to find out for yourself which value's ar needed.As a starting point you can take the value of R2 which is the dominant part of the impedance of the drain of Q2.Q5 need a small base resistor, 100-Ohm or so.You need a Zener to protect the Gate-Source voltage of the power MOSFets.Place direct on the gate of Q1 en resistor, say 150-Ohm.Kind regads,Bram

author: H713
author_rank: Regular Contributor
timestamp: June 24, 2019, 05:34:17 pm
content: If I remember right, the 100nF on the compensation pins was not really needed but I put it there anyways just so it made it onto the pcb in case I needed it later. 100nF is definitely ridiculous, same with the 10nF, I want to say that 1nF made it stable. I think I put the really large caps in just to see if it made it stable, and unsurprisingly it did not.Good call on The gate protection zener, I totally forgot to draw that in.

author: duak
author_rank: Super Contributor
timestamp: June 24, 2019, 06:18:05 pm
content: H713, since you're going to layout may I suggest splitting R13 into two or three parts? I calculate that with a 500 V output, R13 would have to dissipate 2.5 W. It'll be easier to find smaller precise and stable resistors than one larger one.When the voltage crowbar circuit trips, triac D10 will have to dissipate the 7.2 Joules of energy stored in C14 and C15. I doubt it will survive and will most likely short out. If the pass transistors weren't initially damaged, they'll probably overheat and short out too. A fuse in the +Vout line between C14 & D9 to limit the current should minimize damage. Note that the fuse should be rated for breaking the circuit voltage - these parts are usually sand filled to extinguish the arc that occurs at a higher voltage.

author: H713
author_rank: Regular Contributor
timestamp: June 24, 2019, 09:37:56 pm
content: You aren't wrong with the crowbar, and I wasn't really expecting the triac to survive. It is a last resort to stop the full DC bus from being dumped into the load if a pass element shorts. I was going to have a small fuse in series with the output of the capacitor bank (separate to this board), so if the triac shorts I don't expect the pass transistors to blow. I already found a suitable 600v rated fuse holder and fuses on mouser.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 25, 2019, 02:56:39 am
content: This makes me wonder...Is there:-a reliable, robust method to parallel 100 TO247 or TO220 fets-a fet which is cheap that could be paralleled using that method, to essentially create some giant combined fet that could handle linear mode dissipating 5KW or more no problem into a water cooled heatsink or what not?

author: H713
author_rank: Regular Contributor
timestamp: June 25, 2019, 06:29:21 am
content: Sure you could. The FQA8N90C-F109 that I linked earlier in the thread is probably one of your better options in terms of SOA/cost ratio. At least if you're going for high voltage. Where I work we have some power supplies that each have over 250 pass transistors (I believe they're BJTs) in parallel. Each one has a fuse and current sharing resistor. They are liquid cooled. So I can say that it's possible because I've seen it done. But the cost is... excessive. I mean, if you want to use this for a power supply I'd really look at designing a switcher. If you want to use it for a load... I mean, nothing is stopping you...As for this power supply, I managed to track down the problem with my prototype. Dodgy breadboard connections. I also discovered that the 5534 doesn't really seem to work with this circuit. While dealing with the breadboard issues I realized that the TL071 that I had subbed in for troubleshooting purposes was still in. I tried switching it out for a 5534, and discovered that it doesn't really work. The circuit works perfectly fine with TL071 and OPA604 op-amps. My strong suspicion is that the relatively low input impedance of the 5534 is loading down the voltage divider. There are a few other updates as well that I will add to the next schematic revision.Also... at this point I do not believe that the gain of Q1 is too high, as if I lower it much more I start to lose output voltage range. It was certainly too high in the first schematic versions, but I do believe that this has been corrected. In addition to that, things are really fairly stable. Not surprisingly, with the faster OPA604 it is a little less so, but with a 20uF output cap it seemed fine with both resistive and capacitive loads throughout the entire voltage range. I still need to do some more testing to make sure it is stable into any output load, but I'm going to wait until I get some boards and the actual parts in so that the testing is more relevant.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 25, 2019, 04:19:26 pm
content: Quote from: H713 on June 25, 2019, 06:29:21 amI'll keep this in mind for a rainy day!Also what about changing things to whereMAINS 175VAC to (low voltage)VAC (24? 50?)low voltage VAC gets rectified then linear regulation then switched into transformer which steps it up to 500V, feedback loop connectes to 500V but mosfets never see high voltage

author: H713
author_rank: Regular Contributor
timestamp: July 08, 2019, 05:03:39 am
content: Okay time for an update: Boards and parts from Mouser have arrived, I've done a bit of testing, and I have some not-so-good news. The good news is that the boards work. The bad news is that the FQA8N90C-F109 MOSFETs are not proving to be nearly as robust in linear operation than their SOA ratings would suggest. I did some lazy testing by setting my Eico 1030 to 410V and using it to power the test board. I then connected the output of the board to a 150R resistor and slowly brought up the output voltage over 15 minutes.The FQA8N90C was good to maybe 170mA with about 350V across it, while the IRFP450 was good to about 210mA, but its 500V rating is a problem for a power supply that I want to output up to 500V (which necessitates a much higher input power than 500V). Not going to lie, the fact that the FQA8N90C transistors couldn't even handle half the current at 350V as their datasheet suggested was a bit frustrating and discouraging, especially since they were clamped to an oversize heatsink with a 10k RPM server fan blasting on it the whole time- they were gently warm when they blew. And no, they were not oscillating. I knew this could be a problem- but I'm still confident that this can be a good and functional design with a little work.I ordered a some Fuji 2SK3675 parts to try, but we shall see how they perform. They are older, so that might bode well. Aside from the FETs not being nearly as tough as I was hoping, the circuit is working quite well. Attached is an updated schematic. I'll post pics of the unpopulated boards tomorrow, as of right now I can't be bothered.I'm going to be a tad flexible on the max output voltage and current, but I'm wondering if anyone here is aware of any MOSFETs (at least 600V rated- more is better) that might actually take linear operation (aside from the very expensive IXYS linear-rated fets). Perhaps one of the older HEXFETs? It'd be great if someone smarter and more experienced with these parts than myself could give a list of parts to look at. The IRFPG50, IRFPE50 and IRFPF50 are interesting, but I've never used any of them and the datasheets do not have a DC SOA.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 08, 2019, 10:11:47 am
content: Quote from: H713 on July 08, 2019, 05:03:39 amHmm... you aren't going to get an SOA of >200mA at 500V of drop in a single MOSFET (except, perhaps, if it's in a SOT-227b package, as that's 100W of Pd right there). You could, however, make a pass transistor capable of up to 500V and 300mA by wiring several MOSFETs in series.Attached is an LTSpice file of a circuit ripped straight from an EDN article a few years ago that is a constant current source for charging capacitors but which can be modified to act as a voltage regulator easily enough.

author: Wolfgang
author_rank: Super Contributor
timestamp: July 08, 2019, 03:38:12 pm
content: Quote from: cur8xgo on June 25, 2019, 02:56:39 amlets see ...- even the linear rated IXYS FETs have a max. dissipation of ca. 100W per chip in TO247 case in linear mode- that makes 50 FETs a 10$ each, if your need 5kW,.- and, you have to make sure that they are equally loaded. Meaning the balancing resistor drop isseveral time the variation in threshold voltage.- Then, your total gate input capacitance is in the range of a microfarad, and it must be driven from a really low impedance.Plan B: Make 5 driver modules a 10Fets, each with a separate driver. You could stack this scheme until you have the current capability you need.Another thing you could consider at 5kW is a tube. The circuit will be a lot simpler, the mechanics not so much.have fun, play safeWolfgang-

author: duak
author_rank: Super Contributor
timestamp: July 08, 2019, 04:54:19 pm
content: I've posted a reply in this thread earlier on. To re-iterate, MOSFETs have a problem called "electro-thermal instability" when used as pass transistors when they have to support a high drain to source voltage at low currents. (see 1st attachment). The other papers talk about it in more detail. Basically, MOSFETs have multiple cells and each one has a slightly different threshold and transfer function. This means that they don't share the current equally and then localized hot spots develop. At some point, a cell will fail, short out and the device is pretty much useless.What can one do about it?- IXYS linear FETs are better, but they still exhibit the problem. I found that out when testing a high voltage electronic load.- older, high voltage FETs are better than new ones that are optimized for switching.- the FQA8N90C-F109 shouldn't be too bad because of their 900 V Vds rating. However, it uses a newer cell design so it likely was optimized for switching.- use larger source resistors to better distribute the current among the FETs. I would say something that develops 5-10 V at full current.- use a foldback current limiter to reduce output current at lower output voltages. This can be done with a high valued resistor from the drains of the FETs to the base of current limiter transistor. It will need a resistor between the base of the transistor and the current sense resistor. However, the extra current from this resistor may increase the minimum output voltage slightly because it is bypassing current around the pass transistors.Food for thought: I've been thinking that dithering the drive to the FETs might be a solution. This is where a high frequency signal is applied to a system that has a granular response and the output is filtered to remove extraneous noise. An example of this is the 50 - 100 KHz bias used in analog magnetic recording to reduce distortion.

author: Wolfgang
author_rank: Super Contributor
timestamp: July 08, 2019, 05:14:29 pm
content: Hi,I personally have never seen an IXYS MOSFET fail in linear mode. I run them at max. 70% or their max. dissipation spec in linear mode, and with 80% of rated voltage, so no extraordinary precautions.Dithering does not help much, if you do that you could resort to a switching regulator right from a start. The ripple removal effort will be similar, and then normal switching MOSFETs could do the job.regardsWolfgang

author: H713
author_rank: Regular Contributor
timestamp: July 08, 2019, 06:04:52 pm
content: Where did the 5kW number come from? 500v * .5A is a maximum of 250W. If that .5A got reduced to 350mA I could live with that so long as I have a reasonable safety margin.If I could find a FET that could reliably handle 600V at 200mA that would be fine as there will be 4 in parallel. The other thing I can try is putting the FETs in series rather than in parallel, though I don't expect that the improvement will be drastic.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 08, 2019, 09:10:20 pm
content: Quote from: H713 on July 08, 2019, 06:04:52 pmParallel operation of MOSFETs in linear mode is perilous business because their gate threshold voltages have a negative tempco and they are also prone to vicious oscillations. I would expect to derate to no more than 25W Pd for a TO-247 package and they absolutely must be on a common heatsink for good thermal coupling. So, plan on requiring 8-12 in parallel, not 4.If you can get the Vds drop down to 100V or less via series wiring, however, each MOSFET will be able to handle the full 500mA so only 4-6 should be necessary. The MOSFETs are much less prone to oscillation when in series and the negative threshold voltage tempco is also not a problem (just results in unequal voltage sharing). It's a bit trickier to wrap your head around how a series cascade works, and there will be a larger minimum dropout voltage, but that is usually not a problem in a HV regulated power supply.

author: H713
author_rank: Regular Contributor
timestamp: July 09, 2019, 04:59:06 am
content: So I simulated and redrew the schematic for series operation (this what you had in mind for the configuration?)- seems to simulate well. In addition to stability, the other big advantage of this is that I can use slightly lower voltage FETs- which means that IRFP460s or other old MOSFETs will work for this design- even in a shorted output, no device will have to drop more than 170 volts. I think I will add one or two more FETs just to improve my safety factor. Tomorrow I'll try to test series operation to see how it behaves before redrawing the board. Not thrilled about needing to do so, but a bit of reading suggests that other people have had lots of trouble paralleling HV FETs. Oh well, the old boards will be useful for something or other.

author: duak
author_rank: Super Contributor
timestamp: July 09, 2019, 04:25:57 pm
content: H713, may I suggest adding a 10K resistor between the junction of U4-2 & the 1n capacitor and the wiper of RV1? This will present a more constant resistance to the RC network formed with the 1n capacitor and make the feedback more predictable.You may also find that Q1 may not completely shut off when U4-6 goes to maximum -Vout. The bottom end of R3 should probably go to GND.

author: H713
author_rank: Regular Contributor
timestamp: July 13, 2019, 05:59:41 am
content: I spent some time today testing IRFP450s at the voltages they would see when used in series. Even with 175 volts across it I was able to draw about 850mA. I can only imagine that IRFP460s will perform even better. Interestingly, the 2SK3675 performed quite poorly. Good thing I only payed $1.50 each. I think that the 500V Hexfets are probably going to be the ticket for this, though I still have more testing to do. If I figure that the FETs are good to ~800mA, that gives me a safety factor of around 1.7. That goes up if I go from using four series devices to five or six. Even with four pass elements, the safety factor of 1.7 is a worst-case-scenario number for a power supply set to 500V with the output shorted and with perfect load regulation on the DC bus- in practice it will be better. I still have to do a bit more testing before I feel confident in the design, but so far things look good.Again, no FET will have to drop more than 150V across in in the worst case scenario with 4 devices. In fact, they will never see even that because with a shorted output the supply will current limit at 500mA (or whatever it ends up being), and at that current the DC bus will drop by about 100V- that's another issue I need to address is the load regulation, and that may involve trying to find a transformer with lower copper losses. I've dumped the idea of a voltage doubler (not necessary with the control transformer I have now), and I'm going to implement range switching on the primary side so as to avoid the need for expensive high-voltage relays.One other thing to consider- the only time the FETs shouldeversee 120V across them is in the case of a shorted output when the supply is set to its maximum voltage. This is (or should be) a rare occurrence. If the output is shorted while the supply is set to ~250V or so, then the FETs won't have to drop more than about 75 volts each.I may also move the crowbar circuit off this board and onto the control PCB to save some space.I sure would like to see the test setup where they came up with the SOA graph for some of these transistors.In the case of the FQA8N90C, the DC curves are optimistic by about 200%, and I had them clamped (with very good thermal compound) to a large block of aluminum- when they failed, they were gently warm. This is quite a shift from the tube world where many tubes can be pushed to 200% their absolute maximum ratings without much ill-effect.I do hope that I can come up with a viable solution, since I'm not the first one to want a solid-state high-voltage linear bench power supply. I've read through the few threads that actually got passed the brainstorming phase. In many cases, everything simulated well, but once testing ensued the optimistic SOA ratings, current sharing and oscillation issues rendered the design impractical. Hopefully the series operation will fix these issues.

author: duak
author_rank: Super Contributor
timestamp: July 13, 2019, 05:27:05 pm
content: Back in my design days, some parts would require 50% derating for reliability. The corollary is that the parts' datasheet promise 100% more than you'll ever get. eg., Motorola used to make the MJE802/4502 complementary power transistors for audio amplifiers and servos. They were in aluminum TO-3 packages and if the environment was quiet enough you could hear them sing at the input signal frequency - very much like "needle talk" with records. I was integrating systems using tape drives that used these transistors in the reel and capstan servos. These transistors were failing in the field after a couple of years even though they weren't anywhere near their limits. The common heat sink never got above 50 C. The tape drive manufacturer changed to different devices of similar ratings but in steel cases and the failure rate dropped to nothing. Turned out the transistor dice would partially detach from the aluminum case because of the different thermal expansion rates.I'd like to see your final schematic when done. I have an old Kepco DC supply with one vacuum tube in it for the pass device. If it expires, it'd be nice to replace it with semiconductors of some sort. I have tubes of IRFP360s (400 V Vdsmax) so putting them in series may work well.

author: H713
author_rank: Regular Contributor
timestamp: July 14, 2019, 04:12:02 am
content: Here's the current schematic. There are a few improvements I'd like to make and some more stability testing to do. I'd like to make the current limiting more adjustable (maybe without requiring an expensive 20 ohm rheostat?) and I have yet to run a test with 6 IRFP460s in series- the most I've done is 4 FQA8N90Cs in series, but I have no reason to believe it won't work. Oscillation is not a big issue with this circuit. It would be nice to improve the ripple rejection a little, though with a well-filtered DC bus it shouldn't be a huge issue.

author: duak
author_rank: Super Contributor
timestamp: July 14, 2019, 05:11:27 pm
content: Thx kindly for the schematic.You can likely improve the ripple rejection by adding a 1N4007 and another RC filter stage in series with R6. This will provide a cleaner and slightly higher DC voltage to the driver stage.For the current limit, how about using a 22R resistor and a pot connected as a voltage divider it to vary the voltage applied to Q5? If the voltage drop across the 22R resistor is too high, then how about two ranges? eg. 500 mA and 50 mA. If you have a built in meter for current, it could switch the meter's range as well.

author: H713
author_rank: Regular Contributor
timestamp: July 15, 2019, 05:18:05 am
content: Quote from: duak on July 14, 2019, 05:11:27 pmThat's kind of what I was thinking, the only thing I don't like about it is that it does require another capacitor on the board that has to deal with the full DC bus voltage. To those unaware, once you exceed 450V, electrolytic capacitor prices skyrocket and availability plummets. This means capacitors in series, with equalizing resistors, and this quickly starts to eat up board space. I managed to squeeze it in though, since the improvement was dramatic. I tested with just a 25uF filter capacitor on the DC bus (just a 660V oil-filled cap), with a 150mA load, and adding the cap right after D6 was about the same as adding 900uF of capacitance on the DC bus. Schematic is attached. I haven't had time to play with the new current limit idea, but if it behaves well then it too may be worth the complexity. IMO, adjustable current limit on a HV bench supply really doesn't need to be that accurate, but Rheostats are not the easiest thing to come by, and while I probably have something in stock, most people don't and I like to minimize the usage of needlessly expensive parts when I can.Speaking of expensive parts:The OPA604 is an rather pricey op-amp targeted at the audio market, and at $3.30, ridiculous for a power supply. The circuit works just as well with a TL071, the only reason I used an OPA604 is that 1) I have lots of them, and 2), they can handle very high supply rail voltages- convenient for testing. Most generic FET-input op-amps should work fine.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 16, 2019, 10:34:23 pm
content: I have a module from an industrial electronic load that is rated 600 VDC and fused for 20A. This shouldn’t be that much different than a pass module of a high voltage power supply and might give you some ideas on MOSFETs to use. There are 8 identical sections in parallel on this module, each using two 900 volt N-channel MOSFETs in TO-247AC high voltage insulated packages. Each of these sections uses a 0.82 ohm balancing resistor in series with the two MOSFETs, which would have to handle about 2.5 amps. Here are a couple of photos. The overall view only shows one side of the heatsink assembly so there is another identical half with more MOSFETs on the other side. The entire electronic load had several of these modules for very high current and heat was removed by three fast 5” square fans that sounded like a cyclone.You had mentioned the IRFPF50 earlier and while the specs don’t say specifically it is for linear use, that is what this electronic load module uses.

author: duak
author_rank: Super Contributor
timestamp: July 17, 2019, 04:52:03 pm
content: AD, I see in the first picture that one device is an IRFPC40 and the other is an IRFPC50. Are they in series or in parallel? Would you have any schematic diagrams or other info to give us an idea about the design and maybe a way to better use power FETs in linear mode?As H713 is showing with connecting the FETs in series, it looks like using a high voltage device at lower voltages and higher currents puts the device in a more reliable combination in the SOA. It makes sense as electro-thermal instability occurs with high drain to source voltages.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 17, 2019, 10:15:38 pm
content: Like most unique industrial equipment there is no schematic and I haven't tried to totally figure it out because it was designed to work and I'll believe it does and I'm too lazy to go any further. Each of the eight sections on this module has two MOSFETs and a .82 ohm power resistor plus two 1/8 W 15 ohm resistors. Each of the two MOSFETs seems to be driven separately by one section of a dual JFET opamp. At first glance I thought the two MOSFETs were in parallel but that isn't the case, if I traced that part of the circuit out correctly. If you look at the schematic I have below, the IRFPF40 (rated 4.7A) has the two small 15 ohm resistors in between its source and the source of the IRFPF50 (rated 6.7A)., and the IRFPF50 has the .82 ohm power resistor basically in series with both MOSFETs. As a guess I think the lower rated MOSFET handles the load when the current is lower, perhaps up to 1.5A, with the IRFPF50 off and the higher rated MOSFET, with the power resistor, is used in addition to the lower rated MOSFET when the load increases from 1.5A to 20A. Maybe they get better control at the low end that way-I don't know.If you were only looking for 300ma and each of the IRFPF50 MOSFETs can handle 2.5A, I'd assume you might be able to just use one of these or a similar part on a proper heatsink with your own feedback and control circuit.

author: duak
author_rank: Super Contributor
timestamp: July 17, 2019, 10:44:37 pm
content: Fair enough AD. No reason comes to mind other than different current ballasting resistors for different ranges.I remembered a design idea in EDN for a wide range load where paralleled FETs were used and the gate drive switched in and out for high/low ranges.

author: H713
author_rank: Regular Contributor
timestamp: July 18, 2019, 06:19:02 am
content: I also found that there is a "sweat spot" range for FET voltage rating. Too low, (100V like the IRF510), and if obviously has minimal SOA, but too high (like the FQA8N90C and the 2SK3675), and they seem to perform rather poorly at 120V in the linear region compared to the IRFP450s. Some of this may be due to the age and product family of the transistors, but I think that 900V parts are not designed to handle as much current as parts in the 500V range, and I suspect that this is hurting their performance here.Really, I think parts like the IRFP460 and probably IRFP360 like Duak mentioned are probably the best option. A lower power version could be built using IRF640s, they performed reasonably well (they don't have DC specs, but as we have seen that means little), but the TO-220 package is a limitation. They were used by BSS Audio in the EPC 760 and EPC 780 power amplifiers from the early 90s as output devices, and they are a good part. For a 0-500V 0-100mA supply, they would be pretty comfortable with about 6 in parallel. The FQA8N90C performed considerably worse in this application, blowing up at or before the IRF640s did, and costing over twice as much.IMO, range switching drastically improves this situation, as a 200V @500mA output (perfectly reasonable for many applications) would result in a total dissipation of 200 watts. That's a lot of heat and comes out to about 32W per transistor. That doesn't seem bad, but that makes for a big heatsink, noisy fan and and efficiency of just 33%. Using something with a pair of 120V primaries would solve this issue, at least in North America. In the low range, the primaries would be wired in series (as if wired for 240 operation), while in the high range they would be wired in parallel. This transformer doesn't have to cost of a fortune as a result. In civilized parts of the world, you would want something with a pair of 240V windings on each side.While we're on the subject of the power transformer, I know there may be people who worry about the use of a control transformer wired backwards to get a pair of 120 windings on one side and a pair of 240 windings on the other. It is fine in most cases to use a transformer backwards (secondary as primary)so long asthe voltage is correct. That is, applying 240V to a 120V secondary will saturate the core. When this happens, the inductance drops like a rock and the winding will pull heavy current and burn up in short order. By contrast, applying 120 to a 240 winding is fine, though you need to keep your copper losses in mind.The really nice way of doing this is to use a variac driven by a servo motor for this. I've seen this done on some really big magnet power supplies from the 70s. With a smaller variac (rather than about 500 pounds worth of them), it could be pretty fast-tracking. Something for someone a little more ambitious than me. That could, in theory, drastically increase the possible output current and make this quite an efficient power supply.