High-Voltage (~500V, Low-Current (<50mA) DC Power Supply Design - Page 1
Forum section: electronics / projects

author: minime72706
author_rank: Frequent Contributor
timestamp: October 07, 2013, 07:08:39 pm
content: I haven't gotten very far in this one yet, but I figured some of the more experienced folks on here could offer some useful advice without having to think too hard.I want to make a high-voltage power supply that is adjustable from maybe 100VDC to 500VDC, for example. I don't necessarily need it to go down to 0V. I have a real bench PSU for that. My plan is to use a Cockroft-Walton voltage multiplier that is most likely driven by a square wave at around 50kHz and 20Vpp. I also want overvoltage protection as well as current-setting capability. My vague plan for those two functions include some sort of shunt regulator. I was initially thinking I'd have two circuits, one for voltage and one for current, but I can probably have one circuit handle both. I expect to use a high-voltage MOSFET (800V? or do you think I can get away with 600V?) to shunt current away as the circuit demands. The circuit itself might consist of two simple regulator circuits where one monitors a fraction of the output voltage and another monitors the current through a sense resistor. I would have both of these circuits do their business if either parameter goes out of whack.I may just use a preset voltage clamp that engages a SCR or something when the output exceeds 500V and just control the current. Drawing any meaningful current out of this thing should keep it's voltage down anyway. Right now I'm experimenting with some of the capacitors I have on hand to make a multiplier that won't get loaded too heavily by a few milliamps of current draw.This is partly a learning project and also partly so I can do some basic binning of some varistors I have on hand. There's no real point in putting them in parallel if their breakdown voltages vary wildly. I plan to measure what voltage appears across them when 1mA or so flows. I might find other uses for the supply, but that's the motivation.Cheers.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 08, 2013, 02:55:12 am
content: I'm waiting for an order of 47uF 100V 'lytic caps to come in since I don't have enough of any one type of appropriate capacitor to get as many stages as I need/want.Right now I'm using 1N5819 Schottky diodes, but I'll likely ditch those due to the low safety margin. I'd use some classy film capacitors but they're quite expensive for capacitance values that are worth a damn. I might include some circuitry/connectors for picking out which tap to use. Not sure how I'm going to do that in a safe manner, however.

author: BravoV
author_rank: Super Contributor
timestamp: October 08, 2013, 03:15:52 am
content: Not sure if this helps, just for reference another design for a 0 to 500V 10mA adjustable psu.Source ->EDN: Regulate a 0 to 500V, 10-mA power supply in a different wayMade a thread while ago asking experienced people around here about this design and you can read their comments and suggestions here ->Please rate this circuit, a DC 0 to 500V 10-mA power supply, and no, I don't have a time to build it yet.

author: Psi
author_rank: Super Contributor
timestamp: October 08, 2013, 03:23:43 am
content: Might be quicker/easier to make a 555 produce a 50/60Hz waveform and shove that into a generic 12:230V transformers you have laying around. Then have a diode+cap multiplier on the output and you will get ~500V DCAdjusting the 555 voltage within its range (4.5-18V) would make the output variable between ~100 and ~600VDC more or less.Could use pretty much any low voltage transformer 9,12,15,18V etc.You might have an issue getting the output under 100V with 4.5V input though, especially when using a transformer with low input voltage.However the losses in the switching would be quite high at low voltage so it may work out fine.

author: BravoV
author_rank: Super Contributor
timestamp: October 08, 2013, 03:33:46 am
content: Quote from: Psi on October 08, 2013, 03:23:43 amPsi, curious on above circuit description, I just fail to imagine how is the output voltage regulation loop works ? Sorry for such noob question, my knowledge and skill for this is still beyond me.Appreciate if you could draw a draft circuit, cause I'm also interested in building a HV dc psu.

author: Psi
author_rank: Super Contributor
timestamp: October 08, 2013, 03:39:03 am
content: Quote from: BravoV on October 08, 2013, 03:33:46 amYou cant see the regulation because there isn't anyJust a variable AC voltage input to a transformer which has a fixed primary:secondary ratio.Wouldn't be hard to add some regulation though, using a LM317 to control the 555 voltage based on the high voltage output.I don't want to sound negative but if your knowledge is limited perhaps a high voltage powersupply is not the best project.Your goal of 500V @ 50mA is perfectly capable of killing you.

author: toli
author_rank: Frequent Contributor
timestamp: October 08, 2013, 04:11:04 am
content: Quote from: Psi on October 08, 2013, 03:39:03 amThat's what I want to hear every time I build something, or else where's the fun in doing it?!BTW, this trick with the 555 inside the FB loop will have some serious stability issues that need addressing. The low frequency of the 555 (50/60Hz) is well withing the loop-BW. From what I remember (and I'm no expert on this), in switching regulators (which is exactly what this is), the switching frequencies is placed above the unity-gain-bw of the loop to keep it from oscillating.I'd say go the "regular" way. It's easier to design, and while 500V is a lot of voltage, the total power you are aiming for isn't that much so parts won't cost that much, a 600-700V MOSFET is cheap enough these days.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 08, 2013, 04:40:44 am
content: Quote from: BravoV on October 08, 2013, 03:15:52 amI did see this circuit.Quote from: Psi on October 08, 2013, 03:23:43 amI do plan to use a 555 or something similar, but if you're going to go that route, you're much better off using a higher frequency. I'd also prefer to regulate the output for accuracy and I imagine transient response. We're talking milliamps and I don't care much about efficiency, so I have no problem shunting current to ground to get what I want. I also would note that I want to control the output current accurately too.Quote from: BravoV on October 08, 2013, 03:33:46 amHe didn't mention any regulation loop - I don't think he realized what I needed/wanted out of this circuit. I want some control over voltage (accuracy would be nice), but moreso I want accurate control over current.Quote from: Psi on October 08, 2013, 03:39:03 amHigh voltage is a new area for me, but I'm not as limited as my questions might make you think. I just wanted to probe the forum before I go experiment and make some less-than-optimal design choices. I have faith I can get something working that is fairly safe.Quote from: toli on October 08, 2013, 04:11:04 amI don't plan to use a transformer, though I may change my mind. I was going to start with an isolated AC/DC supply and something like a 555 timer set to the 10's of kilohertz range basically driving a half H-bridge to tickle the fancy of the CW multiplier. I would regulate the output and if I can think of a safe way to do it, I may allow myself to select different taps of the CW multiplier to get better efficiency. I haven't put much thought into that part, but I don't want sparking contacts like in a switch or anything. I also don't have relays that will tolerate this kind of thing.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 08, 2013, 05:17:06 am
content: First of all, 500 V @ 50 mA is not high power, but it is not insignificant (25 W). I don't think a C-W multiplier is a good choice for this. That is more often used for generating several kV starting from a few hundred volts. The big advantage of the C-W is that no single semiconductor or capacitor has to hold off the entire supply voltage, so you can use ~600 V diodes and capacitors to generate 10 kV.I would second using a 230 V line transformer in reverse to generate ~600 VDC and regulate that down. It isn't too hard to make a high voltage regulator where only the pass transistor has to handle the full voltage, everything else sees < 20 V.If you want something more compact, more efficient, or cheaper you can use a switch-mode converter. A flyback converter would be the logical choice here. You would have to hand-wind the transformer, but that is pretty easy.

author: Psi
author_rank: Super Contributor
timestamp: October 08, 2013, 06:01:58 am
content: Yeah, hand-winding a custom high frequency transformer is an interesting experience.It can be quite fun trying out different windings, switching frequencies and measuring the efficiency.I would recommend a bobbin style toroid with separate primary/secondary winding areas.However, it adds more danger to the projectSafest way to go is to keep it all low voltage until right at the end and use a generic 50hz transformer

author: minime72706
author_rank: Frequent Contributor
timestamp: October 08, 2013, 01:09:57 pm
content: I do have transformers I could switch around, but I just don't like the idea of misusing them like that. I have 12.6VCT and ~24VAC transformers and probably a few others. The 12.6VCT transformer can effectively be used as a 1:10. I still don't want to do it that way, though.Learning how to really understand inductors, especially in the purview of switch-mode power supplies, is on my short-term bucket list, however. I have a few books but haven't had the motivation to really get into it. I also have some cores lying around as well. Still need some magnet wire.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 08, 2013, 03:59:39 pm
content: My local Radioshack(s) claim to have magnet wire based on what the website stays about their stock situation, but I've never been able to find it. Maybe this time I will ask one of the daft man-children that work there.One of the two Radioshacks I visited had a single package of magnet wire. Lucky me. I plan to get more legit stuff later from electronix express (fairly local site that does electronic sales to vocational schools).

author: minime72706
author_rank: Frequent Contributor
timestamp: October 08, 2013, 09:07:52 pm
content: Interesting little thing to note: Right now the CW multiplier I have plugged into my funky gen is a combined effort of electrolytic and film capacitors. I just wanted to see a high voltage appear. I put the 330uF 50V 'lytics first (6 stages) and then followed that with 9 stages of 0.33uF film caps.This makes quite a bit of sense anyway, but what I noticed was multiple peaks in output voltage while scanning frequency. I didn't record any numbers or anything, but it makes sense to me. At some frequency the impedance of the electrolytic caps will be favorable and at another frequency the impedance of the film caps will be favorable - and maybe a third point where both types of capacitors are relatively low impedance.I really want to play with it some more but I can't build a meaningful CW circuit until I have the capacitors. I have plenty, but not enough of any suitable kinds. I may throw together the input pulse generator (555 with a couple of transistors or a legit half bridge) so I can test with something other than my awful Hameg function generator. (It goes up to 5MHz, but you certainly don't get a square wave at anywhere near 5MHz if you ask for one)Again I have some 47uF 100V lytics as well as some 0.56uF 100V film caps coming. I also took my chance with their $20 "Box of Capacitors" (at least 1000 pieces they say). I like surprises, though usually I'm not surprised in a good way when I get this kind of thing from Electronics Goldmine. I still seem to try again and again, though ...If I'm severely disappointed this time, I'm just going to add their site to my router's block list to discourage me from being dumb again. Last time I got like 300 aluminum TO-220 heatsinks in one of their "surprise boxes". I will note that the capacitors I'm playing with now came from their surprise boxes, so it's not like you don't get SOMETHING useful.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 09, 2013, 03:55:34 am
content: TIL (more like "today I was reminded") that the LM555 is not the best choice if what you're after is 50% duty cycle with variable frequency. I could probably just make an opamp-based square-wave oscillator. Right now I'm just going to go with the LM555 set for roughly 50kHz. I even thought of being lazy and just using a watch crystal as the oscillator (32.768kHz).Anyone have any better ideas? I'd like to keep the part count down while also having the most flexible setup that I can. I could follow the 555 with a flip-flop but that's a serious waste of a chip.EDIT: Yeah I know I could use a microcontroller but I'm not really in the mindset for programming right now. Maybe down the line - it'd allow me to add features like displaying voltage and current among other things. I have a large stock of basic ICs - may as well use them.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 09, 2013, 04:13:31 am
content: Quote from: minime72706 on October 08, 2013, 01:09:57 pmAs long as you are using them at their rated voltage it isn't misusing them. i.e., if you use a 230 VAC primary / 12 VAC secondary and you use it to generate 230 VAC from 12 VAC that is perfectly acceptable, and a standard way to make a power inverter. You only have a problem if you try to use a transformer at a voltage higher than its rating (for instance, use a 115 V transformer to generate 230 VAC). You can't think of a power transformer as a generic 1:N device: they have been optimized for a specific voltage and and frequency. They do not however care which direction power flows.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 09, 2013, 04:21:03 am
content: Yeah as the end-user you have no idea (usually) how thick the wire in the primary and secondary is nor do you know anything about the insulation breakdown voltage, etc.One can't be sure (at least without having more intimate knowledge of common practices) if the wire in the low voltage secondary is really safe at the primary's voltage (230v versus 12v, for instance).EDIT: I might assume that the primary and secondary wire, while likely different thicknesses, based on current requirements, would have the same quality of insulation, but I'd rather not unless someone shows me something to convince me otherwise.EDIT #2: Would YOU make assumptions when radioshack transformers are involved?

author: ejeffrey
author_rank: Super Contributor
timestamp: October 09, 2013, 05:19:50 am
content: Quote from: minime72706 on October 09, 2013, 04:21:03 amI am not sure what you mean here. When you use a mains transformer as a setup-up transformer the role of the high- and low-voltage wiring stays the same, only the direction of power flow changes. There should be no 230 V on the low voltage wiring.QuoteI wouldn't assume that and you shouldn't have to.You seem set on building the C-W multiplier, so go ahead, but there is nothing wrong with using a power transformer in the way suggested here.

author: Psi
author_rank: Super Contributor
timestamp: October 09, 2013, 07:19:51 am
content: When you change the direction of a transformer the only thing to swap around is the definition of primary and secondary.Step downprimary = the high voltage coil on the transformer.secondary = the low voltage coil on the transformerStep upprimary = the low voltage coil on the transformersecondary = the high voltage coil on the transformer.The current ratings on each coil, insulation thickness, wire thickness, voltage on each coil... is all the same.You just think of the transformer from the other direction.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 09, 2013, 09:25:09 pm
content: Quote from: Psi on October 09, 2013, 07:19:51 amHow can you really be sure, though? You may be 100% right, but power transformers (wall transformers) were designed to be used in a specific way.It's also just not very important here since what I really want to do is have an excuse to use a CW multiplier, hah.Some parts searches on hobbytronics sites as well as a review of the section of "The Art of Electronics" on oscillators brought some better ideas to my attention.First of all, though the LM567 is a tone decoder, it can also be used as a general-purpose oscillator and/or PLL. I have a few of these on-hand for another project involving ultrasonic transducers.I also noticed, at Tayda Electronics, there is a self-oscillating half-bridge which basically fits my application perfectly. It's effectively a timer and a half-bridge in a single IC.There's also the venerable CD4046 (or 74HC4046) CMOS PLL.Plenty of other options as well, but there's a good chance I'll play with the LM567 now and grab some of those self-oscillating half-bridge drivers for later.EDIT: If only Hans Camenzind included ONE MORE flip-flop in his 555 timer design - an adjustable-frequency, 50% duty cycle oscillator could be easily realized!On a random note, in case someone who this applies to reads this by chance: The L293 is a nice chip, but I manage to cause what I assume was catastrophic latch-up in a L293D chip. This is the one with flyback diodes on the outputs built-in. In initial tests I saw feed-through between my logic and driver supplies (5V and 10V at the time). The symptom of this was "magically" changing voltages and currents on the front panel of my bench PSU. This was concerning. Later I managed to short out something on the driver side of the chip such that massive amounts of current would flow if you connected the driver power supply to anything. The logic side continued to function. I presumably caused this by removing and reattaching power to the driver side while the rest of my circuit was powered up. I wonder if anyone else has run into this. I was using two of the four half-bridge drivers in the chip to drive a little ultrasonic transducer in my single-supply application.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 10, 2013, 02:42:33 am
content: I purchased a few International Rectifier IR2153 self-oscillating half-bridge drivers. It's kind of perfect, at least for the lazy man. It's a half-bridge driven by a programmable oscillator very similar to that in a CMOS 555 timer. The kicker is that extra logic inside the chip enforces a 50% duty cycle. The only problem is that it needs external MOSFETs, but that's not really a problem for me.For now I'll employ the LM567 as an oscillator.