HV Nixie supply design help - Page 1
Forum section: electronics / projects

author: Mysion
author_rank: Regular Contributor
timestamp: August 08, 2015, 07:52:17 pm
content: I've been working on getting 175v at 10mA from 12v. I needed a nixie power supply for my clock. However SMPS are pretty complex and it's giving me issues.Right now if I run the psu at 35 Khz 94.3% duty from my signal gen I can achieve 175 volts at around 75% efficiency. However strangely enough when I generate the same signal on the AVR chip I'm using it can only go up to around 120 volts.I've been reading a long Ti application doc about boost converters and boy is it intense.The parts I'm using areMosfethttp://www.dropbox.com/s/n10zmh3886eff38/91054.pdf?dl=0Diodehttps://www.dropbox.com/s/bz7jjsqmrsw4c8k/MBR40250-D.PDF?dl=0Inductor generic 1mHSet upI've tried it with some higher value inductors I have and 20mH is a no go. I asume most of of my problems lie with the inductor selection but I have no idea what to look for.Any one got any suggestions?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: August 08, 2015, 08:11:55 pm
content: Ah, it kind of looks like you're directly driving the gate of an IRF740 from a logic pin? The *threshold* for that MOSFET is 2-4v, which is about all the voltage you can expect to get out of an output pin. To fully turn it on you'll need more. Note that Rds(on) is quoted at Vgs=10v.Now if you hook up a signal generator, select 5v output, and don't put a 50ohm terminator on your circuit, you can expect quite a higher voltage, which is why I'm guessing it's working with the signal gen.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2015, 08:29:54 pm
content: Don't do it in a single stage. Use a tapped inductor, or step-up transformer, or multipliers. Example:Increasing the "wild end" turns ratio (say 150 instead of 75 turns) and adjusting the feedback voltage divider ratio will get 175V easily (or, say, 100-200V adjustable). Though this circuit isn't really suitable for higher power levels (you'll need about 5mA for each Nixie).If you can't wind one of these yourself, you can buy some off-the-shelf. The most common are low ratio, coupled inductors, intended for various SMPS applications where you need an oddball voltage alongside a main output. You need a rather more oddball voltage here, so you might settle for a multiplier of several stages to get there (downside being, it takes more and more peak current to drive the multiplier, which is unfriendly to the switching circuit).You can also get high ratio transformers straightaway, usually for offline converters: for example, 120VAC to 12VDC flyback supplies. These are somewhat bigger, and much more expensive (usually like $5 at Digikey), but won't need any multiplication, and the switching circuit will run happily.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 08, 2015, 08:51:09 pm
content: Quote from: T3sl4co1l on August 08, 2015, 08:29:54 pmBoth the circuit and the construction are very much in the late Jim Williams' tradition!A tip for the OP: you can change a plain, off-the-shelf toroid inductor into an autotransformer for use in T3sl4co1l's clever circuit by winding more turns on top of the existing winding in the same direction then tying the end of the new winding to the beginning of the existing winding. The new winding will be your primary (ie - the one that is left of the tap in above circuit) and the existing winding will be your secondary. Don't forget to count the number of turns on the existing winding first (note that this is a lot easier to do if you choose an inductor that only has one layer of turns on it [which generally means lower inductance, all other things being equal]) and set the turns ratio somewhere between 1:6 and 1:10.

author: Icarus
author_rank: Regular Contributor
timestamp: August 08, 2015, 09:13:48 pm
content: By the way; It's better to use the prototyping style of T3sl4co1l's rather than Breadboard.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2015, 09:30:05 pm
content: Yeah, it's possible to breadboard such circuits (I believe this, and many others like it, originated there), but you must pay careful attention to current flows and inductive loops. Especially for more than a few watts, which you will need when lighting several Nixies.Tim

author: uoficowboy
author_rank: Regular Contributor
timestamp: August 08, 2015, 09:42:08 pm
content: Quote from: Paul Moir on August 08, 2015, 08:11:55 pmI think Paul is dead on here regarding the gate drive. Your function generator probably has a higher voltage, lower impedance drive than your AVR. Adding a proper gate driver will probably fix things right up.I have a similar circuit running on a Nixie clock with 6 IN-16 nixies. I've never measured efficiency but no part of it is even noticeably warm - maybe 5-10C above ambient.For this low of power I don't see a lot of need for a fancier topology like a flyback or tapped inductor boost, though if you want to learn about power supplies that would definitely teach you some things.

author: rvalente
author_rank: Frequent Contributor
timestamp: August 19, 2015, 01:30:23 am
content: The bad side of boost converter is for more than 5 times output voltage the duty cycle get so high that gets harder and harder to control, them the core saturation is certain to happen.Them I suggest to use a simple flyback topology, you can easily borrow the trafo from a 5V USB adapter, check its frequency before disassemble. The output voltage will be kind proportional to the dutycycle, you can even use the MC34063 to control it, very easy to get.http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDFFollow the page 9, flyback its the big brother of the boost converter.Remember to put some snubber in the primary, since this topology will give you some kick back in the primary which is very proportional do the secondary voltage (can damage your switching transistor), use some fast recovery diode in the output (schottky is the ideal, but..)

author: marshallh
author_rank: Unknown
timestamp: August 19, 2015, 02:50:40 am
content: If you are interested in using a transformer i can write some step-by-step instructions to rewind a donor SMPS transformer as I just did the process again yesterday. It's very simple, just tedious.

author: krivx
author_rank: Frequent Contributor
timestamp: August 24, 2015, 02:50:48 pm
content: Quote from: marshallh on August 19, 2015, 02:50:40 amThis sounds interesting...

author: marshallh
author_rank: Unknown
timestamp: August 25, 2015, 02:01:29 am
content: https://en.wikipedia.org/wiki/Magnetic_core#/media/File:ER_core_assembly_exploded.png1. Find transformer, for your purposes a smallish one (about 1.5cm square)2. Desolder and remove any tape around the core. You should now see the two halves of the ferrite core. They are 2 separate pieces each shaped like the letter E, and fit like this: E33. Heat the core with hairdryer, heat gun, etc. It will probably be stuck together with excess lacquer.4. Pry the cores apart by leveraging against the bobbin. The cores are very fragile and brittle. If they aren't moving much, try heating more.If you break a core, you can still use it, if you can get it to "fit" together still.Now you can unwind all the wire off the bobbin. There will probably be a couple primaries and several secondaries woven in layers. Stick a screwdriver thru the bobbin and pull. Desolder from the pins as necessary. Each layer has a round of insulating tape, save a few.Figure out how many turns you want on your primary and secondary. For that last one I did, it was 10 on primary and 180 secondary. There should be enough leftover wire you can use. If necessary you can join several runs of wire and solder to an empty pin on the bobbin.1. With empty bobbin, solder one end of the primary wire to a pin, and mark with sharpie/scratch. Wind onto the bobbin for the number of turns you want.2. Run tape around to cover it, solder the other wire end onto the bobbin, and run tape over that too. This is your p/s isolation.3. Start secondary, marking the start pin and winding in the opposite direction as before. Technically you can also wind the same way, but just wire it backwards in your circuit.4. Finish, wrap tape around everything to cover it up.Now you should be able to put the cores back on. Gap the cores by putting kapton tape in between the ends as a spacer.Wrap thin tape around the cores to hold everything together, and there you go.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2015, 12:59:27 pm
content: I've often said, superglue is an invaluable tool ofdisassembly, when it comes to ferrite cores.The break is clean (unless you've accidentally smashed it into a bunch of tiny pieces..) and can be glued back together. Epoxy is good also, but takes time to set, while superglue is very fast.Breaks add a little airgap, so that if your intent was to have a transformer or CMC with minimal magnetizing current, you should probably start over with a different core; but for power switching and filter inductor applications, it doesn't matter.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 25, 2015, 01:45:40 pm
content: Quote from: marshallh on August 25, 2015, 02:01:29 amNote that this only applies if you are making a flyback transformer (or you want a negative output with a forward converter).Quote from: T3sl4co1l on August 25, 2015, 12:59:27 pmPro tip: swab one face of the broken core with a damp Q-tip and put the drop of (cyanoacrylate) superglue on the the other face; water is needed for cyanoacrylate glues to cure.

author: codeboy2k
author_rank: Super Contributor
timestamp: August 25, 2015, 06:33:03 pm
content: Quote from: MagicSmoker on August 25, 2015, 01:45:40 pmAre you serious? I have recently tried a few superglues and was unable to get any to stick, no matter what the material, it just stayed liquid and did not cure. I threw it out with the garbage and used epoxy instead, and I just thought that the superglue of today was super-crap compared to 10 years ago (a time when I *COULD* make it stick). Now I am thinking there are humidity differences, since I live in a different part of the country now, compared to 10 yrs ago.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 25, 2015, 08:45:39 pm
content: Quote from: codeboy2k on August 25, 2015, 06:33:03 pmYup. Well, technically speaking it is the presence of the hydroxyl group (-OH) that catalyzes the polymerization of cyanoacrylate molecules so weak bases, alcohols and glycols, etc., could also work, but water is the easiest/safest to use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2015, 10:54:53 pm
content: Yeah, the stuff used industrially for activation is some sort of hydroxyl-amine-pyridine-nastiness, which by the sound of it, has all sorts of chemical appendages to activate CA. It literally sets before your eyes.In normal circumstances, atmospheric water is indeed the active ingredient.I go the simple route and exhale on the joint before sticking it together.Although I've found that doesn't always work well, either. I have an old tube of CA that took several minutes to set, and that was during one of the hottest, most humid days this year. Go friggin' figure.For cases like that, try to just prop it up in position so it doesn't fall apart, and let it cure on its own time. (Or, you're just as well off if you use epoxy at this point.)Tim

author: codeboy2k
author_rank: Super Contributor
timestamp: August 26, 2015, 12:54:59 am
content: My stuff wouldn't set after several hours. I did set it aside and didn't touch it all day once. By that time it was thicker and tacky, but the parts were not bonded.

author: uoficowboy
author_rank: Regular Contributor
timestamp: August 26, 2015, 01:24:03 am
content: Quote from: rvalente on August 19, 2015, 01:30:23 amI dealt with that by just running in discontinuous mode. Sure, it means the system is a bit less efficient, but for a low power system like this it really is not a big deal IMHO.

author: rvalente
author_rank: Frequent Contributor
timestamp: August 26, 2015, 01:59:04 am
content: Quote from: uoficowboy on August 26, 2015, 01:24:03 amYeah, but how big is your duty cycle? 99.5%?Why not to do the right thing and use a flyback? Flyback and boost are just cousins, same family.

author: RichardD
author_rank: Newbie
timestamp: August 31, 2015, 10:20:03 pm
content: Hello, here is a simple switching PS using a 555 timer chip and an LMV431 that functions as the feedback element to regulate the output voltage. I've used this for my 6-digit Nixie clock (each tube draws 2ma). The frequency that this circuit operates isn't very high so isn't that sensitive to layout. That does mean that the inductor needs to handle more current (than a higher frequency circuit would). The gate drive current is limited on the 555 and that impacts the ability to switch the MOSFET gate quickly - so a small heatsink may be needed depending on how much current is drawn (an NPN/PNP driver pair would solve that issue). Don't touch the tab when it is operating to check the temp though - it will be at 200V! Other MOSFETs will work, look for ones with same (low) gate charge and sufficient voltage rating (250V or higher) - e.g. IRF740.The choice of the LMV version of the shunt regulator over the TL version is because the LMV version provides better voltage regulation because it requires less current than the TL version and works better in this circuit. Note however that it is 1.24v rather than 2.5v for the TL version - so the formula to work out the output voltage needs to consider that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2015, 12:32:43 am
content: Note that the lack of current mode control means this is a voltage mode controller, and will suffer from destructive startup or heavy-load operation. It shouldn't stay latched on (the worst way a boost can fail), but if it dips into CCM, current can overshoot massively with no limitation whatsoever.Tim