Looking for manufacturer of huge inductor - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: October 30, 2014, 10:43:02 pm
content: Hi,I'm working on a high power adjustable DC-DC converter. Because of low frequency (30-60-ish kHz) I have calculated that the inductor I need would be about 150-200uH rated at 30A (ya, i know, it will be large). Do you know a manufacturer that has stuff like that available off-the-shelf? I have shecked typical suspects like Murata, Epcos/TDK, Vishay, Coilcraft, Bourns, TayoYuden and a few others, but they usually don't have anything remotely close to what I need. I'd prefer the manufacturer to be based somewhere in EU, but it's not a hard condition.I know I can buy the core and some bigass magnet wire (or rather litz wire) and wind that myself or have such inductor suctom made by a specialized company. First solution is time consuming and the second one is expensive and involves long wait time, in addition, the final demand for the component will be about 100pcs/year tops.

author: GK
author_rank: Super Contributor
timestamp: October 30, 2014, 11:02:07 pm
content: Quote200uH at 30A is an LI2product of 180 millijoules. A hefty ETD59 core gapped down to an ALvalue of 200 will support 55 mJ. Although it's not recommended to go much lower you could gap it to support 60 mJ and put three of them wound for 67uH in series.I am currently winding an ETD59 for 100uH at 20A.

author: GK
author_rank: Super Contributor
timestamp: October 30, 2014, 11:24:27 pm
content: I've done several ETD59's now to 50 mJ (That's still within the operating range specified by the manufacturer) and coil heating is mostly due to resistive losses.Admittedly three ETD59's are far from practical, but the OP's options are really limited here unless he rewrites his specification.I do not know of any off-the-shelf ferrite core that can support a whopping 180 mJ.

author: GK
author_rank: Super Contributor
timestamp: October 30, 2014, 11:53:04 pm
content: Quote from: AcHmed99 on October 30, 2014, 11:44:13 pmI didn't say otherwise and I even posted the ETD selection curves provided by the manufacturer and gave an worked example within the specified limitations.I thought it would have been obvious that I gave the ETD59 example not as a practical solution, but to give the OP some perspective for what he is asking for.If you're worried about going all the way to 60mJ with an ETD59 you can just play it safe and put, say, five 40uH ETD59 inductors in series instead

author: poorchava
author_rank: Super Contributor
timestamp: October 31, 2014, 12:23:55 am
content: A bit more on the spec:Vin: 15-35VVout: 10-30VIout: up tp 25AFootprint: 100mm x 150mm (more or less)So we're looking at target output powers of up to 750W. In current hardware revision, the inductor is about 400uH wound with 2.5mm^2 magnet wire on about 50mm diameter toroidal core, presumably Material 26. Figured out it's T184-26 with Al=169nH. How this inductor was calculated - no idea, i'm redesigning a circuit made by someone else. To get this inductance with this core it's in the ballpark of 50turns, which checks out.Anyway - the converter is purely software-controlled (PWM from uC driving the bridge). FET driver is IR2110S, which would do fine at somewhat higher frequencies. What bothers me is that I'd like to sample 4 values with ADC at precisely controlled moments to avoid switching transients, and I can hardly do that at frequencies higher than 30-40k, I'd need to average and digitally filter the results. Admiteddly I have not tested the current design at anything close to full-scale power, only about 80W (and it worked fine).Multiphase converter is kinda out of question, as the control board is already done and there is no pins left on the microcontroller (well, at fixed frequency I could theoretically phase-shift the signals with some 7400/4000 series logic or something). It seems that increasing frequency is the only way to go.

author: GK
author_rank: Super Contributor
timestamp: October 31, 2014, 12:47:57 am
content: Quote from: poorchava on October 31, 2014, 12:23:55 amYou should have mentioned that you are using powered iron toroids. They're a pain to wind and I couldn't find a local distributor for small quantities, but they are much more compact than the alternatives.However for 400uH at 25A a 2" diameter toroid seems a bit small. 3" would be better:Go here:http://www.micrometals.com/software.htmlYour 2" toroid would likely be fine if you only need 150-200uH. I don't fancy your chances of finding an suitable pre-wound toroidial inductor off-the-shelf. I think you'll be stuck with winding your own.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2014, 01:49:55 am
content: Mind that #26 and #52 powdered irons are so lossy, they're basically only suitable for extremely low ripple applications: excessively large inductances on lowish frequency converters (30-100kHz, say, and under 10% ripple), or subsequent filtering (where the AC loss impedance doesn't hurt too much anyway).I've seen ferrites over 3" across (EE80 or thereabouts), should be able to store a good bit of energy in those. But really, you should consider redesigning the whole damn thing... there's a reason it's looking expensive this way..Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 31, 2014, 07:09:06 am
content: I agree with Tim. Micrometals 26 material (yellow-white) is only meant for 50-60Hz EMI filters. It's extremely lossy at high frequency. If you insist on using powdered iron then 90 material (yellow-red) is far better. I've used 90 material for output inductors at 100kHz many times and it's fine, as long as you limit the flux swing.However, my first choice would be a gapped ferrite such as an E65. The operating frequency of 30-60kHz is so low that you'll be saturation limited rather than loss limited. I haven't done the calculation but suspect that you could use a single E65 easily. If using current mode control then limit the peak flux density to 270mT. If you distribute the air gap between all three legs then it shouldn't be too big.If you don't have a lot of experiance with switchmode PSU's then I suggest you stick to the lower frequencies as it's far more forgiving.Dick

author: GK
author_rank: Super Contributor
timestamp: October 31, 2014, 09:24:58 am
content: Quote from: T3sl4co1l on October 31, 2014, 01:49:55 amThat appears to be what he has. Admittedly he has a toroid of less than suitable material (assuming it has been correctly determined), but a 2" or a 3" toroid of the correct material could do the job he wants, with significantly better shielding than, say, a big E core, which may or may not be an advantage.

author: GK
author_rank: Super Contributor
timestamp: October 31, 2014, 09:29:42 am
content: Quote from: Richard Head on October 31, 2014, 07:09:06 amI can only find (have) selector charts for E cores up to 57mm. However the 57mm E core (45724) is significantly less capable than an ETD59:Extrapolating for that graph, I'm not sure that a 65mm E core would cut it, though they appear to be available up to 100mm. The OP wants an LI2product in the range of 135 to 180 millijoules and his current design (25A @ 400uH) is 250 millijoules!

author: nctnico
author_rank: Super Contributor
timestamp: October 31, 2014, 08:58:17 pm
content: Quote from: poorchava on October 30, 2014, 10:43:02 pmPut several in series. Wurth has inductors which can handle that kind of current. They have these in several values and current ratings:http://katalog.we-online.de/pbs/datasheet/7443641500.pdf

author: CM800
author_rank: Frequent Contributor
timestamp: October 31, 2014, 10:05:37 pm
content: I would suggest contacting this guy:www.spwoundcomponents.co.ukHe may be able to help you, hes a really nice folk and has helped me a few times with transformers i have needed making. he should be able to make such an inductor for you easily i would expect. Tell him that Thomas Williamson linked you to him (its not an paid endorsement thing, im just a friend of his) I know he does have ETD59 cores.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 01, 2014, 04:43:05 pm
content: I've had similar things wound by a good transformer company on a gapped laminated core. A decent transformer company will do the entire design based on your needs. Around here it would be Tabtronics-http://www.raftabtronics.com/I'm sure they also ship worldwide.

author: poorchava
author_rank: Super Contributor
timestamp: November 02, 2014, 12:16:53 pm
content: I'm curious what is the merit behind using a bobbin-type transformer rather than toroidal one (aside from obviously easier winding process)? Toroids seem to offer better overall parameters. Using the tool from Micrometals website I've come up with inductor that is about 35-40 turns on T249-52 cor. I think I'm gonna use 35*0.355mm litz wire for easier winding.Peak inductance at low current is then ~330uH falling to about 120uH at 30A. Core losses are under 0.5W, which is nice. Coppers loss will be rather high at around 12-16W. I could bring this down to ~8W using AWG 9 solid copper wire, but that's like 3mm diameter solid wire and it's gonna be a total bitch to wind. I also don't know if the Micrometals software takes skin effect into account.Since T249-52 doesn't seem to be available from my typical sources (farnell, rs, tme) I may go for bigger size for experimentation phase (eg. T300-52).I would like to leave engineering by external company as a last resort as this will most likely mean additional time which I don't really have.

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2014, 07:17:10 pm
content: If you cannot find a standard inductor which will work and do not want to wind your own or pay for custom ones, then I would consider a series and/or parallel arrangement of standard inductors that you can find.Quote from: poorchava on November 02, 2014, 12:16:53 pmE and pot cores which use a bobbin are both easier to wind and you can adjust the core gap. I often do this when prototyping by winding to fill the window and then adjusting the gap for the desired inductance or peak flux.

author: poorchava
author_rank: Super Contributor
timestamp: November 02, 2014, 08:23:02 pm
content: Isn't adding/subtracting windings on a toroid going to vary the inductance as well?Btw, from what I understand based on various papers I've read over last few days and some calculations it seems, like in this application core losses are very small in relation to copper losses. I think I may be forced to use a large core just to fit all the necessary windings.I'm limited to either buying an off-the-shelf inductor or winding one myself. My application is quite space-limited so I think that multiple cores in series are out of question.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 02, 2014, 08:33:06 pm
content: Quote from: poorchava on November 02, 2014, 12:16:53 pmThe answer to your first paragraph is contained in your second paragraph. IMO, 12-16W copper loss seems very high and winding big wire on a toroid is a big PITA. The advantage of external engineering, if they're any good, is that you'll probably do the job once, which is usually the fastest route.The other difference is that when you buy a powdered metal core, you're buying a built-in fixed distributed gap. With other designs you can adjust the gap, giving you a lot of design flexibility.

author: dannyf
author_rank: Super Contributor
timestamp: November 02, 2014, 08:34:51 pm
content: QuoteWouldn't a power transformer more than sufficient for that?

author: nctnico
author_rank: Super Contributor
timestamp: November 02, 2014, 08:42:36 pm
content: Quote from: poorchava on November 02, 2014, 12:16:53 pmIn my experience having a transformer or inductor wound still means you have to do all the math and testing.

author: David Hess
author_rank: Super Contributor
timestamp: November 03, 2014, 05:53:55 am
content: Quote from: poorchava on November 02, 2014, 08:23:02 pmAdding and subtracting from the windings on a toroid changes the inductance in a very predictable way but it is easier to adjust the air gap on a pot or E core if necessary. It is much easier if more than one winding or tapped windings are involved. This is more important during development or if production runs are small.If I am not sure where I am going to end up during developement, I find the largest E or pot core which will fit in the space required, fill the winding window with copper, and then adjust the gap for either the desired inductance or more likely to a point where the copper and core losses are balanced.QuoteIterative designs tend to balance the core and winding losses for the most economic solution.

author: GK
author_rank: Super Contributor
timestamp: November 03, 2014, 01:34:26 pm
content: Quote from: David Hess on November 03, 2014, 05:53:55 amTo support a given L-I-squared product, for a core of sufficient size, there is a minimum gap size and associated ALvalue. The latter determines the number of turns required for the desired inductance value. You can't just fill the window with a (presumably arbitrary) number of turns and then gap for the desired inductance. If you wind too few turns you'll then have to gap for an ALtoo high to support the LI2product and if you wind too many turns you'll have to gap for an ALvalue below the manufacturers recommended minimum for that particular core, which negatively effects efficiency in other ways.Also, could you point to a readily available pot core large enough to support the OP's predicted LI2range?

author: GK
author_rank: Super Contributor
timestamp: November 03, 2014, 01:35:53 pm
content: Quote from: Conrad Hoffman on November 01, 2014, 04:43:05 pmIron laminate? for a SMPS operating at a few 10's of kHz?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 03, 2014, 02:31:06 pm
content: It was a very similar application, resonant booster, a couple kHz, several hundred watts. I can't remember exactly what the core design was, but it was steel and it wasn't a toroid. There may be some crossover frequency where ferrite is better, but I remember everything we did with ferrite wasn't as efficient. We also wound about 1 toroid and said the heck with that! The advantage of working with a winding house is they'll typically have a large stock of different cores, and will do all the calculations with a very high degree of certainty, assuming you can describe the exact operating conditions. They know their materials and do it every day. That part about knowing your operating conditions is critical.

author: poorchava
author_rank: Super Contributor
timestamp: November 03, 2014, 04:18:07 pm
content: After speaking to our company's SMPS guru, I think I'm gonna settle with large material 52 core or alternatively an MSS core (although I can't find a local source for a Sendust core of required size). I will most likely use material with high permeability to wind fewer turns and use 2 or 3 strands of thick lot wire which should bring the copper losses down significantly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2014, 01:12:23 am
content: Quote from: Conrad Hoffman on November 03, 2014, 02:31:06 pmOnce upon a time, I designed a 10kW inverter, 600-2000Hz. Got a 20kVA toroid, stripwound core (3 mil GOSS), some internal cooling (a copper water pipe wound around the core a few times). A foot across, 30 pounds, $700. Not bad considering. Bridgeport Magnetics, I think was the supplier. Funny, because we quoted half a dozen other companies, whose replies ranged from "no quote" to a monster two feet across (and over $3000)! Gives you some idea what companies actually know how to do design.If it were much higher (maybe >3kHz), ferrite would've been just barely preferred I think. There's also nanocrystalline material, but the price goes way up when you're talking that stuff.Tim

author: GK
author_rank: Super Contributor
timestamp: November 04, 2014, 02:58:48 am
content: Quote from: Conrad Hoffman on November 03, 2014, 02:31:06 pmFrom this threads opening post:"Because of low frequency (30-60-ish kHz)....."You're out with your suggestion by over an order of magnitude.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: November 04, 2014, 10:07:13 pm
content: want to knowthe difference in performance of ferrite EE core in a nice machine winded case as compared tohandwided with turns here and there .Consider frequency 30Khz

author: Bud
author_rank: Super Contributor
timestamp: November 05, 2014, 03:37:18 am
content: Coilcraft makes big current inductors.

author: David Hess
author_rank: Super Contributor
timestamp: November 05, 2014, 06:58:28 pm
content: Quote from: GK on November 03, 2014, 01:34:26 pmI think you missed the gist of what I was getting at but my description was brief and I do not think we disagree. There is an assumption that filling the winding window results in more turns than necessary maximizing the inductance for a given gap. If that does not allow the flux to be lowered enough to stay within the manufacturer's recommendations at the frequency of operation, then the core is too small unless smaller wire is used to get more turns which raises the copper losses even more.In practice during development with a larger than necessary core, the flux is lower than the manufacturer's recommendations so core losses are lower than they would be with a properly sized core and wire losses are usually higher although not always; you can always make a multifilar winding or use larger wire if the core was too large to start with. An optimized design would use a smaller core with a minimum of wire and be more economical.I just do this during prototyping when I am unlikely to have an optimally sized core available.QuoteA pot core? No, although I am sure they exist. I have a whole pile of big 3C8 E cores (and smaller pot cores) which probably meet the required frequency and power requirements but they are old (is 3C8 material even made anymore?) and I have no idea where to buy more simply because I have not needed to in 10+ years.I have always found purchasing bare cores and bobbins to be a frustrating exercise.

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 12:00:11 am
content: Quote from: David Hess on November 05, 2014, 06:58:28 pmNo. Your post suggested that so long as you pick an oversized core you are safe to wind an arbitrary number of turns and then gap for the desired inductance. I was just pointing out that, without any baseline calculations, this isn't necessarily true.I think that you are making what is a basic task of engineering by numbers out to be some kind of protracted empirical exercise.To minimise core losses you only gap a suitable core down to the ALvalue required to support your LI2product and no further. It isthenthat you compute the necessary number of turns for the desired inductance. Once you know the number of turns required you select the wire gauge so that the winding window is utilised. If at that point your copper losses turn out to be too high you simply move on to a larger core and do the calculations again.You start by establishing your flux density and the required gap, not your number of turns.In this context I don't really follow your advice for balancing core and copper losses. Suppose that I have just designed my inductor. I have gapped down to an ALvalue to support my computed LI2product.I cannot make the gap any smaller as then my inductor will not maintain the desired inductance value at the required DC current. So my only option for "tweaking" is to make the gap bigger. If I make the gap bigger my core losses increase. However a larger gap means a reduced ALvalue, which means I will require more turns of a lower gauge wire to both achieve the desired value of inductance and utilise the winding window. That means I get increased copper losses along with increased core losses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2014, 12:27:26 am
content: That's closer to the essence of it, but: since inductance is derived from the gap (independent of the core size and material*), whereas flux density is dependent upon the core size and number of turns (independent of gap), you want to calculate turns first, and then the gap.*So long as the core itself has permeability much higher than the average / equivalent permeability in the final design. Or, to put it more generally: the gap is what's required after subtracting the core's contribution, which is generally small (and obviously, not variable).A typical design process for me: I need an inductor of some capacity (inductance and peak current), and optionally, some amount of loss. I pick through cores that seem likely, and calculate N = Vpk / (4*Bmax*Ae*F) (for a square wave, or for a sine of V = Vrms, the coefficient is 4.44 instead of 4). Bmax is determined either from Bsat (~0.3T for most ferrite) or from the loss budget (p_c = P / v_e; requires core v_e; look up B on the loss vs. B, F curves -- not possible if absent). Finally, the gap is given by l_g = mu_0 * Ae * N^2 / L. (Or more precisely, that's the total air gap equivalent; to get the physical gap, subtract the core's air equivalent, l_e / mu_r.)Tim

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 12:34:53 am
content: Quote from: T3sl4co1l on November 06, 2014, 12:27:26 amI should have written "LI2product" rather than "flux density". In any case it makes no sense to start by calculating the number of turns. You can't calculate the number of turns unless you know the freaking AL value and the AL value is determined by the gap size. Did ANYONE look at the AL vs LI2core selection charts that I posted earlier in this thread and bother to comprehend them?You start by determining your AL value based on the LI2product and you gap the core for that AL value. THEN you are free to compute your number of turns.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2014, 12:41:35 am
content: It is more sloppy when working with powder cores, yes. In which case, you're about as well off looking at empirical graphs. On the upside, you rarely deal with powder core shapes, and therefore don't have to worry about gaps; in that case, you have no choice but to use the core's characteristics alone to store energy.You can find gap by energy, too: the energy density of a magnetic field is simply B^2 / (2*mu_0), so you can calculate the gap length if you know the gap area (~= Ae). Again, subtract the core's equivalent air gap length to find the physical gap.You didn't seem to have a problem with my equation which calculates N without needing AL, though.Ed: I could also be more explicit by noting: IF you have a gap at all. That's your one degree of freedom between flux density and field strength. If that factor is strictly fixed by your choice of core (e.g., a powder toroid), you can only make your choice through core selection and turns.Tim

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 12:43:41 am
content: We were specifically discussing user-gapped cores. This place is just as bad as an audio forum!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2014, 12:47:12 am
content: Quote from: GK on November 06, 2014, 12:43:41 amThat was uncalled for.But I guess if you're not interested in discussing it rationally, that makes you no better either. Ooooh!Tim

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 12:56:30 am
content: Quote from: T3sl4co1l on November 06, 2014, 12:47:12 amNow you just trolling and giving evidence to my assertion. I think the points (to which your replies have simply been tangential) I have made about designing DC inductors with gapped cores are clear and rational. In fact I haven't deviated from basic text book stuff.

author: johansen
author_rank: Super Contributor
timestamp: November 06, 2014, 01:00:50 am
content: I think you can manage three three ETD59 cores side by side.. but preferably, find a core with a square cross section for the center core.just two of them is probably sufficient:basing design off "150-200uH rated at 30A"for two cores side by side and a custom bobbin.. or none at all..a 1.5mm physical air gap24 turns for 150uH inductance..3T flux =68mJoules energy stored80% copper fill factor makes 8.7 watts resistance loss--if the ferrite saturates at .4T then you can take it all the way to 40 amps, which is 121 milijoules.increasing the air gap to 2mm physical gap makes about16 watts copper loss@ 32 turns for 200uH @ 30 amps makes .3T40 amps makes .4T at 27 watts copper loss at 80% fill factor.

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 01:04:43 am
content: For anyone out there genuinely curious about designing gapped DC inductors along the lines I have detailed in this thread, here is a link to the application note the AL/LI2 charts were taken from:http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCMQFjAB&url=http%3A%2F%2Fwww.mag-inc.com%2FFile%2520Library%2FProducts%2FFerrite%2FPowerDesign.pdf&ei=cshaVJffJYOwmAWD04HgBw&usg=AFQjCNHMwjsI8gqj_10odVfGPG5dIidKwwGo straight to page 13 "Inductor design" for the technical description for core selection and gapping.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2014, 01:53:46 am
content: Quote from: GK on November 06, 2014, 12:00:11 amCould you explain this statement in more detail?

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 02:48:10 am
content: Quote from: David Hess on November 06, 2014, 01:53:46 amWell I don't have any of my textbooks handy right now, but:QuoteFrom here:http://www.mag-inc.com/products/ferrite-cores/learn-more-about-ferritesMore gap more fringing flux. The fringing flux can also dramatically increase electromagnetic emissions, BTW.

author: Smokey
author_rank: Super Contributor
timestamp: November 06, 2014, 04:28:06 am
content: Magnetics are the real deal.http://www.mag-inc.comTheir inductor sizing software saves a ton of time. Of course since high flux core are almost as much black magic as RF, they could just be making it up and I'd have to believe them.Really though, they make some killer inductors and materials.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2014, 05:35:39 am
content: Hmmm, gapping should increase losses in local spots (assuming constant V, F applied to the same winding), due to fringing causing partial saturation around the edges of the gaps. But, because reactive power is higher (proportionally, whereas the core losses are only around the fringing -- a low order effect), the overall Q factor is higher.Often more significant is the strongly divergent field around the gap, which totally cooks any wires near there.So, both Q and reactive power go up, but reactive power goes up faster, so you can indeed cook things if you aren't careful!Tim

author: GK
author_rank: Super Contributor
timestamp: November 06, 2014, 06:27:23 am
content: Ferrite core performance degrades with increasing temperature. A flux density concentrated around the gaps will contribute to core heating and increased overall core losses before the inductor design gets that bad that the wires actually melt.

author: megajocke
author_rank: Regular Contributor
timestamp: November 08, 2014, 12:13:27 am
content: It sounds a little like it's not fully clear what parameters are held constant and which are varied in this discussion.For a fixed core size and flux density but a variable gap, the largest gap length (lowest Al value) gives the highest energy storage capability at saturation. However, for a fixed inductance a larger gap would require more turns, which means you'll have higher copper losses at a certain current.For this reason, there is a limit to the longest useful gap, although if the peak current is much higher than the RMS current, things will be different. Put in a huge gap and a certain core will store lots of energy, but it won't work for continuous duty.So you'd usually not want to gap the core more than neccessary because copper loss would be higher if you did. In the case of the LI² - Al curves, the manufacturer has presented this information as the Al you should use for a certain energy storage to get the lowest copper loss.If you don't have those curves and want to determine what is needed you could calculate it from scratch. I'd start with determining the amount of linked flux (flux times turns, often denoted by capital greek Psi) which needs to be supported, and this is given by inductance times desired saturation current:Psi = N * Phi = L * IwherePhi = B * A (flux equals flux density times cross sectional area)Theminimumnumber of turns for a certain saturation current on a fixed core, fixed saturation flux density, fixed inductance and variable gap length (or variable Al value equivalently) can then be calculated. Yes, it is actually aminimumnumber when parameters are fixed and varied in this manner. This can be seen by rearranging the equation above:B = Phi / Ae = (L * I) / (N * A)As L, I and A are fixed, B increases with decreased N so you'd want to make sure thatN > (L * I) / (Amin * Bsat)Actually, given that everything is given except the number of turns and the gap length, specifying the minimum number of turns at this point is the same as specifying the maximum allowable Al value. (minimum required gap length)Now you can determine if the turns will fit given a realistic current density, or alternatively make them fit by choosing thin enough wire and see if it gets too hot. (either by experiment or calculation) This is the lower Al limit (maximum gap limit) in the energy curves.If you don't require an optimal design, the previously presented method of winding as many turns as you can on a variable-gap core works to get maximum inductance for a certain current by selecting the wire to give allowable temperature rise (or current density by rule of thumb, for example).If you have considerable AC losses things get more difficult of course, but the bulk core losses will be lower for a fixed core size and inductance if you choose a large gap and a large number of turns. The same AC voltage over higher number of turns gives a lower AC flux density.One thing that can be useful to keep in mind I think is that if you have a fixed AC voltage then core loss is only dependent on the number of turns for a fixed core and not the inductance and gap length, at least as a first order approximation. Increasing the number of turns decreases the core loss in this case, typically at the expense of increased copper loss. This also applies to cores where you can't change the gap, including powder toroids. Sometimes optimizing inductor losses and size can be more important than having a certain inductance.Copper losses (eddy currents) from the gap fringing field I find more difficult to make general statements about as all dependencies are interlinked and scaling exponents are unintuitive. It also makes all the difference exactly which parameters you hold constant and which you vary. These losses can sadly be significant in many cases, especially if there are many layers of thick wire...

author: GK
author_rank: Super Contributor
timestamp: November 08, 2014, 12:49:02 am
content: Quote from: megajocke on November 08, 2014, 12:13:27 amThat's pretty much what I said in reply #34.Quote from: megajocke on November 08, 2014, 12:13:27 amWell, I've always approached DC-carrying inductor design with the idea that core gaping is a necessary evil to prevent saturation. If you are concerned with efficiency, then you only gap the necessary amount and no more. The suggestion that (for a desired inductance value and DC current) the core gap (and presumably the number of turns) can be adjusted for a happy or optimal medium between "core" and copper losses doesn't make any general sense to me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2014, 02:38:37 am
content: We're all talking about the same thing here, just from different directions (so, I don't get the "audio forum" epithet). The graphs are everything tabulated against core type, so you don't have to run through iterations manually. Energy is stored in the gap, but you're limited by how much you can supply from copper wire, under limits of power dissipation, or Q for a given rating or physical size. More turns than necessary for desired core performance will only increase copper losses (which requires more gap for the same inductance).Which reminds me, I want to crank some numbers and find out where the ideal permeability comes from. Something to do with copper resistivity and core geometry (you'll need some rough inputs on Ae/l_e), and it seems to lie in the 10-60 range (which is what most powdered iron materials come to).Tim

author: johansen
author_rank: Super Contributor
timestamp: November 09, 2014, 03:16:24 am
content: Quote from: T3sl4co1l on November 08, 2014, 02:38:37 ammy spreadsheet might help.http://johansense.com/bulk/spreadsheets/chokecalculator.odsideal permeability isn't really a thing... without lots of other somewhat arbitrary limitations.for example, the larger the core, the higher the eddy current.. there is an old as dirt application note i can't recall right now, you've probably read it, and i've seen it posted in this forum i believe, that covers the maximum Q as a function of frequency (assuming sufficiently ideal litz wire)--basically the larger the core, the lower the frequency for maximum Q--this is primarily a function of eddy current partly follows core dimensions squared, and skin effect as a function of frequency sets the core utilization.. as of yet, i'm not aware of laminated powdered iron cores... but you can stack smaller cores, the geometry just becomes not very ideal..but a stack of microwave oven transformer cores 2 feet long still gets you better power density Watts/kilogram, than a stack 1 foot long.. but only by about 3%.but the same kilograms of iron as a toroidal core, filled with its weight in copper, would deliver probably deliver 2-3 times as much power.. while also being intrinsically more efficient due to fact that steel is better when grain aligned.given that neigher ferrite nor powered iron suffer from manufacturing limitations.. it would not surprise me to find out that the ideal inductor topology is the pot core.. assuming that copper costs more than powered iron or ferrite.. which doesn't seem to be the case.when copper is cheap compared to ferrite the ideal core is probably a donut shaped toroid..btw, i have not yet begun to calculate the gain from O shaped toroid cores versus square ones, there must be some improvement you'd think, but my initial work has shown that the larger the core the better, because power density scales faster than volume.given the assumption that best efficiency is when eddy and hysteresis core loss equals copper loss, then the ideal permeability can be calculated rather simply.. however it is still frequency dependent.if the eddy current loss is also a function of permeability, then of course the slope of that curve dictates the ideal copper loss...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2014, 04:22:28 am
content: Not sure, I've seen Micrometals' curves of Q versus core selection and windings though.Skin effect is funny, in that it affects anything with loss, not just if it's bulk conductive. So you still get the same effect in ferrite and powdered iron; the limitation is around an inch or so width at 100kHz. This limits the construction of high power transformers (which are built from ferrite bricks instead) and high frequency structures.I was reading a paper on loaded transmission lines recently; they calculated the dimensions required of ferrite and dielectric for a given loading (impedance) and frequency response. The ideal coax structure is an alternating stack of dielectric and ferrite washers: the thin widths and gaps between ferrites allow magnetic field to penetrate the ferrite, while the dielectric has some loading effect itself and should be chosen for a low dielectric constant.Another data point from experience: if you observe the impedance vs. frequency plots of various ferrite products, in particular ferrite beads and chips, you notice something interesting: the curves rarely bear any relation to the published material property. There's a geometric aspect due to the fields propagating through the ferrite (which, because it has high permeability, occurs at a sizable fraction of the speed of light), and the radial and longitudinal standing waves contribute to peaks and dips in the impedance curve.Tim

author: megajocke
author_rank: Regular Contributor
timestamp: November 09, 2014, 09:36:06 pm
content: Quote from: GK on November 08, 2014, 12:49:02 amYes, if the current is pure DC enough, core losses will be negligible even in a minimum gap/minimum turns design like you say. In that case minimum gap is certainly the configuration which will give minimum total loss. Many authors call this a "saturation limited desgin" or something to that matter. If you have for example +-20% of ripple current and design for a peak (AC+DC) flux density of 300 mT you'll only have +-50 mT of AC flux density. (300 mT / 1.20 gives 250 mT of average DC flux density) Even the crappiest power ferrites have quite low loss up to 200 kHz or so for this ripple level.With high enough AC current amplitude and frequency however, core losses would dominate over winding losses if the core is used all the way to its saturation flux density. In this case it is likely possible to achieve lower total loss by increasing the gap length and number of turns (keeping the same core and inductance) as this would decrease the AC flux density. This is likely to be needed in for example converters operating in discontinuous mode from 100 kHz or so and up (for ferrite cores) and the resonant inductors in quasi-resonant converters.A bit off topic-maybe, but toroidal mains transformers using grain oriented steel also tend to fall into the saturation-limited category. Even if driven very close to saturation the frequency is so low that core losses are still almost negligible. Here you would also go for the least amount of turns that won't saturate at the expected highest input voltage and lowest frequency.

author: johansen
author_rank: Super Contributor
timestamp: November 09, 2014, 09:46:59 pm
content: Quote from: GK on November 08, 2014, 11:49:02 AMQuotethis is the commonly taught and fundamentaly backwards way of looking at the matter.given that core losses follow flux squared and frequency squared, inductor energy storage per kilogram of inductor must decrease with frequency to keep the same Qalso, energy storage follows flux density squared.the gap length, or alternatively, the core permeability, sets the copper losses required to achieve a certain flux density.you can later come to the conclusion that energy storage is proportional to the inverse of permeability keeping flux constant.a common mode choke core with a permeability (datasheet says it, it must be true!) of 10,000! can only store litterally something like single digit microjoules per cubic cc of core before it is saturated.regardless of core material, energy storage is proportional to the square root of copper losses keeping flux density constant.likewise copper loss squared equals energy storage when keeping flux constant

author: GK
author_rank: Super Contributor
timestamp: November 09, 2014, 10:45:21 pm
content: Quote from: megajocke on November 09, 2014, 09:36:06 pmI'd like to see a worked example of this, backed up by measurement. Assessing / quantifying and actually measuring the effects of core / gap losses isn't as trivial as some make out with their simplified formula and theory, and especially so if these phenomenon are to be accurately juggled for optimal efficiency.Incidentally, when designing an inductor for typical buck/boost SMPS I select my gap on the DC current plus the peak ripple current. That generally works out to be a good "rule of thumb" compromise between minimising copper losses and having some safety factor to keep away from saturation.

author: johansen
author_rank: Super Contributor
timestamp: November 09, 2014, 11:09:43 pm
content: Quote from: GK on November 09, 2014, 10:45:21 pmmegajocke.. what if instead of using a lower flux density to lower core losses... what if you just... used a smaller core?what you said is true, core losses dominate over winding losses when the core is used at high flux levels.. but you can always decrease the permeability of the core to increase copper losses to arbitrarily high values --this is a big difference between inductors and transformers, transformers ideally store no energy.Somewhere on the internet is account and photos of someone's 12kw boost converter using 12 awg solid wires and he didn't even put the I on the E core, left the E core out on its own... sure you can't saturate the core without melting the wires.. but with the right sized gap he could have cut the volume of the inductor at least in half, and it would probably be twice the "Q"theoretical examples such as the above thought experiment are hard to accurately determine because they cover too many variables.What it really comes down to is acceptable power density, why spend twice as much if its only 10% lower losses...where does that stop.Some people spend 3$ per watt for a 99% efficient dc-dc widget to manage power from 1$ per watt solar panels. this is illogical unless you can't buy more solar panels right?. same analogy goes for the inductor at the beginning of this thread, just different variablesi also suggest reading these papers.http://power.thayer.dartmouth.edu/shapeopt_intro.htmlthe second paper linked there, and here:http://thayer.dartmouth.edu/inductor/papers/cooscost.pdfmentions a Q of 300 for a 19mH 100khz half amp inductor.. which is fantastic.--the available window area decreases quickly with frequency.

author: megajocke
author_rank: Regular Contributor
timestamp: November 09, 2014, 11:42:18 pm
content: If you design for continuous mode with typical ripple current amounts (say below +-20 % of the DC current) and typical frequencies (say below 200 kHz), you're unlikely to see much core loss with ferrites so just keeping out of saturation in the way you do by following the energy storage diagrams is certainly enough.Bulk core losses are nothing strange, and those are the ones I am talking about. If you try to build something like a discontinuous mode flyback converter operating at 200 kHz with flux density swinging between zero and 300 mT each cycle you have almost certainly blown more than all of your loss budget just on core loss. It would overheat even without any help from copper losses. On for example a 3C85 (not very high performance) material ETD49 core, that would be like 25 W of core loss, which is way too high for any reasonable cooling scheme.Extra copper losses from gap fringing fields and other "strange" losses are not so trivial to reason about, but in a case such as the one I just described, a gap just large enough to satisfy the energy storage need wouldn't do.

author: GK
author_rank: Super Contributor
timestamp: November 09, 2014, 11:54:19 pm
content: Quote from: megajocke on November 09, 2014, 11:42:18 pmWell that's a pretty extreme example that I would say is a fair departure from inductor design problem that started this thread.But considering your example, why would you even choose to make such a thing with a "not very high performance" ferrite core with a (possibly) bloody huge gap and who knows what kind of EMR (and associated issues) due to all that fringing flux? To me that problem just screams out for a high (frequency) performance powered iron toroid.

author: megajocke
author_rank: Regular Contributor
timestamp: November 10, 2014, 12:03:24 am
content: Quote from: johansen on November 09, 2014, 11:09:43 pmWell, I was assuming the inductor was limited by heat dissipation rather than some arbitrary loss goal. Using a smaller core if you can should be done, and because the surface area grows slower with increased size than volume does, smaller cores can support higher loss densities and therefore higher flux densities in a core loss limited design. I was assuming you had already found the smaller core to be unworkable, or that you would evaluate the design as the last step and change core size if it seemed feasible. If you start with some core and optimize the design and end up with something that has much lower temperature rise (or loss) than you were aiming for (or what is safe) the core could be made smaller.And thanks for the links! I have read some of those papers before, but I don't think I had seen that 19 mH inductor. Neat.

author: GK
author_rank: Super Contributor
timestamp: November 10, 2014, 12:18:24 am
content: Quote from: johansen on November 09, 2014, 09:46:59 pmUmm, it's an entirely practical one given the constraints that I either loosely defined or thought were obvious. So now in this thread we have leaped from the specific topic of "DC-carrying inductor design" to flyback transformer design.............

author: megajocke
author_rank: Regular Contributor
timestamp: November 10, 2014, 01:32:50 am
content: Quote from: GK on November 09, 2014, 11:54:19 pmYes, quite different. Now I forgot what my point was.Maybe it was that both your method, the other one seen in this thread, and the one I like to use really are the same thing but starting from different ends of the problem. And that the application discussed here is going to be saturation limited, in which case the manufacturers LI^2 graphs do apply, as opposed to my somewhat contrived example. Though, where is the dividing line between DC-carrying inductors and high-ripple stuff?At 60 kHz an iron powder based inductor may very well be core-loss limited which means the number of turns and inductance (for a certain core) will by necessity be high. If using a ferrite core, it might be beneficial to use a much lower inductance if the higher inductor ripple current is acceptable for the other parts than what would have been practical for an iron-powder design. I've once used a ferrite based 10 µH inductor for a 60 V input 30 V 20 A output buck converter at 100 kHz (the output has lots of low-ESR capacitors) which works quite nicely, and I actually needed the high ripple and low inductance to achieve high current slew rate.For my discontinuous mode example, about 0 - 120 mT flux density swing would probably be about right (with about 100 mW/cm³ of core loss) if using 3C85. More modern materials could take more flux swing for the same loss. Whether the gap would have to be huge or not depends on what output power one is aiming for, which wasn't a fixed parameter.Iron powder toroids are nice in that there is no gap with fringing fields that could cause excessive winding losses, but on the other hand even the lowest-loss iron powder materials have higher core loss than quite high-loss ferrites at typical SMPS frequencies at the same flux density. So you'd need more turns for the same cross-sectional area for the same core loss density. Tradeoffs everywhere!If you are operating at low frequency and/or low ripple current (which is quite typical) the high saturation flux density of iron powder cores is a very nice feature.With low ripple (unlike my example) you don't need good core loss performance anyways. For the application discussed at the start of the thread, an inductor based on a powdered iron core or similar is probably a good fit.

author: johansen
author_rank: Super Contributor
timestamp: November 10, 2014, 01:38:54 am
content: Quote from: megajocke on November 10, 2014, 01:32:50 amI have shoved 250 watts through a single T106 green/blue powdered iron toroid pulled from a mystery atx supply. --at 60Khz.it got very warm.. but if i recall correctly, it was a 50v in, 24 volt out buck converter at about 10 amps out. losing half a volt in the switches, and about the same in the inductor.. so that's 5 watts for both the inductor and the switches. epoxying the inductor to a heatsink would have worked.

author: johansen
author_rank: Super Contributor
timestamp: November 10, 2014, 01:52:27 am
content: Quote from: GK on November 10, 2014, 12:18:24 amI mentioned transformers because they do not store energy.flyback transformers are coupled inductors, they make crappy transformers.the fundamental difference between myself and your perspective is looking at the gap as a necessary evil.the gap or permeability sets the maximum energy storage.energy storage is proportional to copper losses squared.the number of turns is unimportant, and the last matter to worry about.Quote from: megajocke on November 10, 2014, 12:03:24 amwell no offense but all those hidden assumptions are why so many people find inductor optimization to be black magic.consider this theoretical thought experiment.given a T106 mix 26 corefind the optimum frequency and ripple current % to shove 300 watts through it with the least losses. assume a buck or boost converter.assume that you are limited to 70% fill factor with 26 awg wire.edit:forgot to mention, assume 50% duty cycle for worst case senario.

author: GK
author_rank: Super Contributor
timestamp: November 10, 2014, 02:00:32 am
content: Quote from: johansen on November 10, 2014, 01:52:27 amIn a typical ferrite design of which this thread topic started core losses are generally negligible, maximum efficiency is had by minimising copper losses and and core gapping therefore IS a "necessary evil" from a practical perspective. Have you ever built a SMPS with a 50+ mJ (let alone the 100+ mJ the OP wants) gapped ferrite inductor next to any sensitive control circuitry or tried to test it for electromagnetic interference? Huh?

author: johansen
author_rank: Super Contributor
timestamp: November 10, 2014, 02:20:05 am
content: Quote from: GK on November 10, 2014, 02:00:32 amI still don't understand how you see it that way.Grind the gap in the center leg, 3-d print a custom bobbin to keep the wires away from the gap, those things are fantastic for that purpose, and wrap the core with copper foil.There is no other optimization to make. if OP can handle the heat from three or four cores side by side, then he could probably manage with 2 cores and copper strip wound on edge, with a slow stream of air flowing through the coil.in both cases the gap isn't evil.. its what makes it an inductor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2014, 05:42:11 am
content: I don't get why you're getting so frustrated...Did you learn to design inductors *one way only*, without understanding the underlying theory enough to form your own, critical yet open, opinion on the subject?As has been indicated time and time by the experts* in this thread, there are a great many variables which are inter-related, complex to analyze, poorly quantified, and generally not worth much concern, given that the resulting error is in the sub-10% "good enough" range.*For lack of a better word; meaning, those who have presented theoretical or empirical evidence (which includes you).This approximation of approximations means that, in the space of solutions which are all "about good enough", one can have very differing opinionswhich are nonetheless valid. This should be, not a cause for concern, but an understanding that, in all disciplines, technical or otherwise, there is no absolute certainty, indeed only absoluteuncertainty, and one must accept and understand that, in order to work best with it. This is not the hard logic of mathematics, this is the art of engineering!I'm not trying to be condescending, I would just rather have you understand, than be frustrated and angry..Tim

author: GK
author_rank: Super Contributor
timestamp: November 10, 2014, 06:04:27 am
content: Quote from: T3sl4co1l on November 10, 2014, 05:42:11 amThe goal posts keep getting shifted around and everyone, at best, keeps competing to say the same things in different words. That gets frustrating. I presented one very simple method to design the inductor because that method is directly applicable to the design problem at hand - and having an immediate, intuitive grasp of that fact doesn't betray a general, underlying lack of understanding.QuoteWhere did I say anything that contradicts this?Quote

author: GK
author_rank: Super Contributor
timestamp: November 10, 2014, 06:30:02 am
content: Quote from: johansen on November 10, 2014, 02:20:05 amThe gap makes an inductor that can maintain its inductance with a DC current flowing. The operating conditions, for example, of a flyback transformer are completely different from those of an inductor carrying a large DC current with a comparably small ac ripple component.Please explain to me how a general example of the latter case can benefit from a gap significantly larger than that necessary to avoid saturation.

author: johansen
author_rank: Super Contributor
timestamp: November 11, 2014, 06:40:16 am
content: When a smaller core + larger gap is cheaper than what you suggested; I made no such general rule apart.such.You keep adding unknown varariables, I explains from the beginning how to calculate watts per $ and provided a spreadsheet designed for that purpose.I covered flyback transformers in reply #63, that has no bearing here

author: GK
author_rank: Super Contributor
timestamp: November 11, 2014, 08:43:14 am
content: Quote from: johansen on November 11, 2014, 06:40:16 amThat's not even a logical answer to the question. If the inductor can be made with a cheaper, smaller core then question still begs - why should that core be given a gap larger than necessary to avoid saturation? And what exactly did I suggest? Right towards the start of this thread I simply gave a brief worked example to put some kind of perspective to the (physical) scale of the inductor the OP was asking for. I even said that it wasn't supposed to be a practical solution. Is that what you are talking about?... wait.... forget that I asked. I think that I am through with this thread. There are much better things I can do with my time. In fact I have a freshly gapped core for a 50mJ inductor ready to wind right here.....

author: johansen
author_rank: Super Contributor
timestamp: November 11, 2014, 11:25:00 am
content: Quote from: GK on November 11, 2014, 08:43:14 amfantastic..i heated the copper you see here to 500C at least twice to get it to fit.--90% fill factor btw.http://johansense.com/bulk/P1050464.JPGQuote from: GK on November 11, 2014, 08:43:14 amYou have had other hidden unknown variables since the beginning.answer my question.what is the optimal frequency for which to use a T106 mix 26 core for a 300 watt buck converter at 50% duty cycle.you should know that to find that answer requires a shit load of data... for which the manufacturers will not provide.

author: johansen
author_rank: Super Contributor
timestamp: November 11, 2014, 10:36:40 pm
content: I hardly ever use bobbins.It is not hard to wrap some slippery paper around a wood block carved out to the right size and then slip the coil off the block and onto the core.Concerning ferrite cores, you can easily grind them yourself with most any grinder.If you want to grind swing chokes, tapered gaps, step gaps or whatever, that is also very easy.find a fine grit diamond tile saw blade, 4 inch diameter, 1/16th inch thick, no slots.the ferrite is soft enough you don't need to bother with water, but you could easily get a cheap tile saw from harbor freight to do all the work.

author: GK
author_rank: Super Contributor
timestamp: November 12, 2014, 01:45:18 am
content: QuoteWTF? You can't answer the question (or rather admit that answering makes your original claim of contention moot) so you just completely change the topic?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2014, 04:22:53 am
content: His point is, the missing variables to answerbothquestions are unavailable...Tim

author: GK
author_rank: Super Contributor
timestamp: November 12, 2014, 04:33:46 am
content: Quote from: T3sl4co1l on November 12, 2014, 04:22:53 amBaloney! So far only one person posting on this topic has actually understood the point I have made:Quote from: megajocke on November 09, 2014, 09:36:06 pmGoing back to the specifications actually listed by the OP, the desired inductance was in the range of 150 - 200 uH and the switching frequency 30 - 60 kHz, a current output of 25A and Vout up to 15VDC with Vin up to 35VDC.At Vin = 30V and Vout = 15V for a 50% duty cycle and picking the lowest f and smallest L (30 kHz and 150 uH) the ripple current works out to 1.7A peak-to-peak. For a DC load current of 25A, that's well under +/-5 % ripple.