1-2mH 20-30A peak inductor - how would you make that? - Page 1
Forum section: electronics / beginners

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 23, 2019, 08:09:55 am
content: HiI'm playing with LTSPICE and simulating a linear PSU pre-regulator that approaches the efficiency of a switching one while keeping the noise at the level and frequency of an ordinary linear PSU.Unfortunately that would require an inductor in the range of the mH able to sustain tenths of Amps of peak current without saturating.Since I never saw such a beast I wonder if that would be just too expensive compared to other possible alternatives (ordinary switching preregulator or multitap transformer).Would a 50/60Hz toroidal transformer nucleus work for that? Any idea about size calculation?thanks

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2019, 08:39:39 am
content: Presumably this is a low frequency switched mode?What about audible noise?Yes, I expect a toroidal mains transformer core will work quite well.

author: Berni
author_rank: Super Contributor
timestamp: January 23, 2019, 08:46:09 am
content: By itself 1mH is not that difficult, but yes doing that at 30A is going to require a very big inductor. Im guessing such a torodial core inductor would be about 5 to 10cm in diameter.This is a pretty useful calculator for designing inductors and transformers:http://dicks-website.eu/coilcalculator/index.html(Note that you will likely have to go find a core you want at a manufacturer and enter the data from its datasheet)The type of core also determines how well the inductor works at high frequency and the kind of losses it creates there. Cores such as silicon steel is only good for low frequency 50Hz operation. Ferrite cores handle higher frequencies and come in many blends that have different properties, some handle high fields (higher saturation current) some have high permeability (less turns needed), some operate well in >MHz ranges, some are very temperature stable etc. Typically a core optimized for one parameter is bad at all the other parameters.

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 23, 2019, 09:43:01 am
content: Quote from: Zero999 on January 23, 2019, 08:39:39 amIt is a sort of hybrid between a classical SCR preregulator and a classical buck regulator.There is a MOSFET switch that goes on after 2-3ms from zero-cross and goes off as soon as the inductor has enough energy to re-charge the output capacitor for the worst case load. A 1000-2000µF capacitor, before the MOSFET, dumps the voltage spikes and store the energy released by the transformer secondary. Obviously on low current load it is the capacitor which provides most of the energy for the inductor, so in that case it is a real, albeit quite lazy, DCM buck converter.I've no idea about acoustical noise, in LTspice it is quite quiet. Anyway current value and frequency are about the same that go through an ordinary transformer secondary when charging the electrolytic capacitors, so I hope that would not be much more than usual.

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 23, 2019, 09:53:12 am
content: Quote from: Berni on January 23, 2019, 08:46:09 amThanks for the link. I'll read carefully that page later.In the simulator there are current pulses ranging from fractions of ms to several ms (according to the voltage and load) at a frequency of 100Hz (120Hz). So I think that an AC mains transformer core would be more appropriate.I'm not an engineer and just studied electronics by myself on schoolbooks lot of years ago. My knowledge on electromagnetics is... just appropriately for an iron core... quite rustedI hope I'll be able to digest all that informations...thanks

author: BravoV
author_rank: Super Contributor
timestamp: January 23, 2019, 09:54:25 am
content: I guess you should be looking for Siemens's stuff, as scored these recently at local junk yard (electronic recycler)Maybe in series ? If the biggest they've made isn't big enough.Measured details : DCR = 0.04 Ohm , inductance = 502uH@100Hz and 460uH@100KHz. A TO-220 body as reference size.

author: Berni
author_rank: Super Contributor
timestamp: January 23, 2019, 10:31:39 am
content: Yeah that does sound like low frequency operation so things designed for 50/60 Hz mains would likely work. Perhaps you can find a big enough balast inductor for a powerful mercury vapor lamp that would fit the specs. Perhaps stacking a few microwave oven transformer cores together would also get you there.But in terms of being a preregulator fed by mains its easier to get a transformer with secondary taps and then use relays(or tiacs if you prefer solid state) to switch those taps into the rectifier as needed. It requires no extra heavy iron core device , is run by very simple control electronics and adds no efficiency loss.The size of your required inductor is the very reason why switch mode supplies operate at higher frequencies, it makes there inductors and capacitors much smaller for the same power output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2019, 10:59:06 am
content: How?Well, 2mH at 30A is 0.9J(!!), and assuming the frequency is low enough for laminated iron (mains frequency + harmonics?), that means Bmax ~ 1.2T is reasonable. 0.9J / (1.2T)^2 * 2*(1.26uH/m) = 1.57u m^3 or 1.57 cm^3 of air gap. At a typical aspect ratio of about 25:25:1, you'd need a core of 1.36mm gap and 34 x 34mm cross section. Which isn't terrifyingly big, about 200VA transformer size I think.That'll be a couple hundred bucks to get made custom, or maybe $50-100 if Hammond has one, stock?Whereas the filtering for a proper SMPS is just a couple bucks -- you can see clearly the economy heavily favors one over the other!Making a good SMPS is the hard part. Shielding and filtering one, easy!Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 23, 2019, 04:10:07 pm
content: Quote from: not1xor1 on January 23, 2019, 09:43:01 amIn what way would this be a lineair regulator instead of a switching one, as you suggested in your original postSize, wheight and costs go down with higher frequencies SMPS, and that is why higher frequencies are being used. Upper limit is mainly set by the quality of the semiconducter swich, because switching losses increase at higher frequencies, and switching frequencies for an SMPS of 1MHz or higher are no exception anymore.Several years ago I built an "emcie" / "elmcie" compatible LC meter with a LM393 comparator and a uC. primary coils of E-core transformers measured as a few Henries, and Toroidal transformers measured in the tens of Henries, but the measurement errors were probably very high."Wide bandwith" is sometimes quoted as a disadvantage of toroidal transformers, but good for youSo if I assume a 10H primary inductance for a 230Vac toroidal transformer and you need 1mH then the secundary winding would be around 230/sqrt(10000:1) = 2,3Vac, and you want it to be able to handle 30A, so the size would be for a 70VA transformer.So if you have a 70VA or larger toroidal transformer in the closet and you put a handfull of windings from thick enough wire around it you have an inductor of the size you want.(I may have made some gross errors / assumptuions here, so check for yourself)If you want to do some measurements / experiments with power inductors, and if you have an scope, I can highly recommend:http://elm-chan.org/works/lchk/report.htmlhttp://www.pa4tim.nl/?p=1859With a pwm signal, and a handfull of cheap components you can easily measure inductances and saturation effects of inductors.

author: BravoV
author_rank: Super Contributor
timestamp: January 23, 2019, 05:33:58 pm
content: Quote from: Doctorandus_P on January 23, 2019, 04:10:07 pmIn this forum, we have similar inductor saturation tester circuit with "energy recycling", so the energy used to test the inductor will be recycle again rather than wasted, designed by respected member here Jay_Diddy_B ->Inductor Saturation TesterAnd successfully built by other member here too

author: mvs
author_rank: Frequent Contributor
timestamp: January 23, 2019, 06:04:27 pm
content: Quote from: not1xor1 on January 23, 2019, 08:09:55 amAgilent U8002A power supply has preregulator with 900µH inductor. It is small "transformer" screwed to the case at photo below.http://caxapa.ru/thumbs/324496/TopView.JPG

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 24, 2019, 06:36:25 am
content: thanks everybody for all your repliesBravoV:QuoteProvided that it would work, that would make hard to replicate a possible project.BTW Yes, I already know (and will probably build) Jay_Diddy_B's Inductor Saturation Tester.Berni:QuoteI know about multitap transformer solution, but, apart the fact that they are more difficult to source (unless you stack multiple transformer with increase of cost and space), they are much less efficient when the output voltage is close to the lower limit before tap switch.T3sl4co1l:QuoteThe peak current would be inversely proportional to the inductance, so unless one needs 10A DC output, for 3-5A DC and 2mH 20A peak should be enough. Besides that we are not dealing with DC current, 20-30A would be the peak of few ms pulses...Anyway considering the energy of the load, e.g. 20V*3A*10ms = 0.6J, we are in that order of magnitude.Doctorandus_PQuoteYour calculations agree with what I got applying the formula in thepage linked by Berni:Imax =Φmax*N/Lto a 40VA transformerI got about 1.4mH from 26 turns and calculated 0.6mWb as Φ and so the result was about 11A of max current.BTW I already know about those inductor saturation tester projects and the advantages of a real switching PSU, but was just wondering about the feasibility of this different solution.mvs:QuoteI found the schematic in a Chinese forum, but I think it is likely wrong as there are many parts which do not make any sense.

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 24, 2019, 06:44:16 am
content: I noticed there are various relatively cheap common mode choke rated for tenths of Amps.e.g.:on mouserHow can that be possible? Is that due to the fact that current flows in opposite directions and so the total magnetic flux is just the difference (i.e. close to null)?They also sell a 1mH 100A inductor. That is on E-I core and is really cheap... just about 264€BTW I have a few large common mode choke I probably salvaged from old broken ATX PSUs.Does anybody know if they can be re-winded and used for a switching PSU? Is the core suitable for that?

author: Berni
author_rank: Super Contributor
timestamp: January 24, 2019, 07:26:02 am
content: Common mode chokes indeed use the opposing current trick to get to the impressive current ratings.They always have two identical coils around the same core, but when it is used as a common mode choke all the current flowing forwards trough one coil is returning in reverse on the other coil. This causes the coils to produce the same field strength but in opposite directions so they cancel out and create near zero field in the actual core. As a result the core can be smaller or made out of a high permeability material to get more inductance for a given form factor.The cores might also be a ferrite blend that is designed to have high core losses so that the differential mode current that it is meant to stop is swallowed by the losses rather than being fed back out from the stored energy in the core.Yeah ATX PSU power factor correction inductors are a common cheap way to improve the power factor of them. Id say go ahead and try measuring one to see what you get from it.

author: Psi
author_rank: Super Contributor
timestamp: January 24, 2019, 07:34:03 am
content: i made 6mH 10A dummy load for 12VDC so i could exactly simulate and test a product that was to drive a 10A automotive solenoid.Used four 'Apple' sized electromagnets wired in parallel. This came out to around 6.3mH total or something close enough to 6mH for my application.However i didn't need 10A continuous. Just short bursts of a minute or so.Even my large dummy load can't handle continuous use at 10A which is 120W of heat.I once left it on for 10min by accident and I started to smell burning enameled wireAfter that experience I thermal epoxied a 20A 60degC thermostat on to it for safety.It's been going good for 4 years now

author: mvs
author_rank: Frequent Contributor
timestamp: January 24, 2019, 08:24:29 am
content: Quote from: not1xor1 on January 24, 2019, 06:36:25 amYes, it is wrong, since its not easy to reverse engineer multilayer board. Butlast versionis pretty decent result and might help to troubleshoot problems or get some ideas for your own design.PS They have placed inductor before bridge rectifier, so it will not saturate that much.

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 24, 2019, 08:28:01 am
content: Quote from: Berni on January 24, 2019, 07:26:02 amI salvaged common mode choke.PFC correction inductors have usually higher inductance and are rated for much lower currents as they are connected to AC mains.

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 24, 2019, 08:52:30 am
content: Quote from: Psi on January 24, 2019, 07:34:03 amyes, but in my case the pulses would last just fewsecondsmilliseconds.Anyway I think that at the end I'll build a switching preregulator.Probably it would still be useful to sync it to zero-cross so with low voltage and low current load I could use short pulse trains and so the low frequency noise would be synch-ed with AC mains.

author: Berni
author_rank: Super Contributor
timestamp: January 24, 2019, 09:06:51 am
content: Id just put plenty of passive filtering around it so that it doesn't make any high frequency noise, the low frequency stuff is hard to filter but any properly designed linear regulator should have >60dB of noise rejection at those so they wouldn't make it to the output. If that's not enough then add a capacitance multiplier circuit in front to pre-pre-regulate the noise out (They work up to 1 MHz with lots of noise rejection).Keep in mind that the mains has plenty of other noise than 50Hz. There are harmonics at multiples of 50 and there are other devices on the mains network spewing a broad range of noise into it. Anything with a brushed motor spews harmonics of its RPM back out and that can be in the KHz easily and none of it is in sync with the mains.We have a 4kW heat pump heating our house. It uses a 400 Hz three phase compressor that is driven by a single phase input VFD. Two levels up the building and all the outlets in that level running on a different mains phase i could tell if the RPM of the compressor is ramping up or down from the noise switching i was picking up from it

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 24, 2019, 10:02:00 am
content: Quote from: not1xor1 on January 24, 2019, 06:36:25 amYes, I took the worst case figures given. You certainly don't want it saturating around the peak, that would cause the current to increase suddenly.QuoteYes, that implies we are somewhere around continuous current mode.You might use even more then, to get more stable output voltage with respect to load current; or use less, so it's closer to a peak voltage switch thingy (that is, the output voltage is about equal to the peak voltage at the moment the switch turns on), with an inductor to help absorb the peak current surges (which would then be higher, maybe say 40A or something, but not unlimited destructive peaks; of course, at the expense of even worse power factor).What changes is how much capacitance you need for filtering, and the response rate of the control loop (gain and cutoff frequency).Incidentally, filter capacitance tends to rise with inductance, because the impedance of the filter Zo = sqrt(L/C) you don't want too high -- this is equivalent to the output impedance at its resonant frequency, when the resonance is critically damped. In other words, a step load change will see a peak voltage change of about Zo * delta I. In still other words: suppose the load suddenly stops drawing current, so the inductor was charged to some energy (say it's over 1J in this high-L case), and the load stops so that energy is discharged into the filter cap, so its voltage overshoots by that amount of energy.So, to put numbers to that, we can use an energy argument, and say the capacitor's voltage was 10V, and its value 20mF (just guesses). Its initial energy was E = 0.5 C V^2, or 1J also. Say we add 1J to it. Oh good, that's an easy ratio, it's doubled so it's obvious V has sqrt(2)'d, or 14.1V. A 40% overshoot is pretty generous, and probably not desirable, so you'd probably want a lot more capacitance.For the general case, invert the equation: Vfinal = sqrt(Etotal * 2 / C). If you continue substituting intermediate formulas until you get back to just the intial voltage and current, and inductance and capacitance, you'll have proven the same peak voltage change as calculated by Zo.This implies a smaller L is desirable, which is more or less true, until modulator nonlinearity causes instability, or PF gets so bad it's just a mess.In short, a middling value, where the inductor current goes nearly to zero every (half)cycle, at full load, is probably not horrible.The same mechanics apply to SMPS design -- you don't want to use too big of an inductor relative to the filter capacitors, or if you have to (usually because inductor losses are just too high to run at high ripple), you need to use more caps to keep the output impedance sensible. That's the gist of most ATX PSUs, and why they responded so damn slowly (cutoff frequency of just a few kHz!).QuoteIt's absolutely feasible, a whole generation of "brick shithouse" boat anchors were designed this way! But there's really only one reason why they were: they had to use SCRs to get the reliability and efficiency up, and the huge pile of iron is basically cleanup for that.Once good-enough transistors came along -- even just mere BJTs -- these designs were quickly migrated to more economical, and better performing, ones!Which, on a related note, generated more than a few tales of caution: the original RCA 2N3055 was a diffused mesa process or something like that I think -- almost as slow as a germanium power transistor, fT in the 10s kHz. Laughably useless for these sorts of things. When Motorola offered their epitaxial version (with fT ~ 2MHz, but they can't put that on the spec sheet because 2N3055 is a JEDEC spec -- not a description of the part you've ordered!), it was capable of switching comfortably in the ultrasonic range. Until a process change occurred, or someone in purchasing bought the wrong brand, or old stock or something, and KABOOM, product failures left and right... Fortunately, we don't have to worry about this much today (but it does remain a good reason to avoid overly general part numbers like those).Anyway, the phase controller -- it's still a neat design exercise, especially if you run across an occasion that really isperfectlysuited to phase control, or SCRs (or TRIACs) -- these are ever-fewer, but luck favors the prepared, as they say. But beyond a design exercise, actually building it -- if you learn better through doing, yeah, go ahead and do it.Here's a tangentially similar project I once did,Schematic:https://www.seventransistorlabs.com/Images/SCR_Inverter.pngOverview:https://www.seventransistorlabs.com/Images/SCR_Inv1.jpgUnloaded, low frequency waveform:https://www.seventransistorlabs.com/Images/SCR_Inv2.jpgLoaded, showing... commutation current I think? as well as supply voltage (fat trace is 120Hz supply ripple):https://www.seventransistorlabs.com/Images/SCR_Inv3.jpgZoom on commutation event, it's mostly smooth but notice the very slight, but very sharp, kinks at turn-on and snubber diode recovery:https://www.seventransistorlabs.com/Images/SCR_Inv4.jpgThose tiny (fractional volt) but fast (sub-microsecond edge) events are what you need to filter, to get a low noise output from an SCR inverter circuit. That can be difficult, too.And also, of course -- all of this notwithstanding the Agilent mentioned above. It's not clear to me that that's being used the same way. Similar applications include passive power factor correction (which you also see in later ATX PSUs from time to time, before active PFC and 80 Plus took over). But whatever it's doing, clearly that's how they chose to do it!Cheers,Tim

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 25, 2019, 09:25:41 am
content: Quote from: T3sl4co1l on January 24, 2019, 10:02:00 amAFAIK the old 2N30355 fTwas 800kHz.I have an old ST datasheet (1977 - it was SGS-Ates back then) listing both old mesa and new epitaxial models. The old (mesa) model is specified for a minimum fTof 0.8MHz while the new one is 2.5MHz.BTW I still have a few pieces from the early 70's ... SGS and Fairchild...QuoteThe circuit I'm simulating is not even a phase controller. The MOSFET is always switched on around pi/3, about the same time when a simple bridge+capacitor PSU would start charging the capacitor. I change just the on time according to the output voltage and load. There is a relatively small value cap after the bridge and before the MOSFET, just to dump the energy released by the transformer secondary when it is switched off.I do not think I'll go on with this project as I do not have a suitable core.I might rather change the phase of the switch-on (and remove the input capacitor). That might reduce the peak current (as there would be a much lower voltage delta when the inductor is switched-on), but would affect negatively the PF, the overall efficiency and stress more the transformer....or I might just check, before all that trouble, if I can get a reasonably low noise from a switching (pre)regulator.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 26, 2019, 08:23:33 am
content: I do not understand why Jay_Diddy_B designed such a complicated inductor saturation tester.My saturation tester will be used for about 10 minutes each month on average, so no need to re-circulate the current into the Elco's.I just used some scrap elco's from the spare parts box and a 15ct HCPL3120 integrated gate driver from Ali instead of a bucket full of chicken fodder components.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2019, 08:38:16 am
content: You surely won't understand why I built this diode recovery tester either:Same basic idea, though in this case it's TVS clamped rather than recycled energy. (Schematic.)As you can guess from the large (air cored) inductor attached, it's quite sensitive to saturation and serves nicely for that purpose as well.Incidentally, I used to test saturation more frequently, but as my design knowledge has advanced, I've bothered less and less often. I suppose, take this as inspiration that it's a quite knowable process.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 26, 2019, 09:47:37 am
content: That's right I do not understand.I do understand the learning experience in designing analog electronics, building it and the satisfaction of having a self designed circuit working.But why add a whole signal generator?Why go through the trouble of dropping a 9V supply to 5V and then pumping it up to 10V againI'm happy with a gate driver chip which receives it's input from my JDS6600, and my lab power supply for whatever voltage I fancy at that moment.Everybody his own I guess.I can understand the use of a discrete gate driver for the adjustment of the switching speed of the MOSfet in this case.This adjustment is also usefull for testing the MOSfet itself, and that is yet again one of the reasons that I've built my tester manhatten style.I can easily add / remove / adjust a series resistor between the driver and gate with a dab of heated liquid metal. Pot's don't work too wel here.