Reducing fringing flux problems in gapped ETD cores - Page 1
Forum section: electronics / projects

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 10:05:18 am
content: I make quite a lot of power magnetics components, one of the perennial problems is the effects of fringing flux on gapped ETD cores in particular it's effect on windings over the gap. I have tried various methods including a slit heavy copper tube (non-shorted turn) but that in particular is difficult to make.Recently I decided to try making the winding in two halves either side of a spacer spanning the gap this seems to work very well, easy to fabricate and easy to wind, the following picture shows the result.The spacer was made from 6mm ply, I would have used FR4 had I had any. A piece is cut to size to fit between the outer legs, then a hole drilled to suit the former. The result is cut in half then epoxied back onto the former over where the gap will be. After drying wind the inductor in two halves connected together externally.This same method could be used for leaky resonant transformers.

author: Yansi
author_rank: Super Contributor
timestamp: October 16, 2020, 10:23:43 am
content: Wouldn't a short circuit turn (a band) around the whole transformer help with this a bit? I never kind of understand any other purposes of this shorted turn, other than to reduce leakage and fringe flux. Or... ?

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 10:31:30 am
content: Sorry I should have explained, the fringing effect that has caused me trouble in the past is eddy current heating of the windings bridging the gap. A screen around the whole assembly I have used in the past to reduce EMI.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2020, 10:48:32 am
content: Have you compared a coil that is spaced from the centre leg such the windings are equidistant from the gaps (ie. padding out the former that the coil is wound on). That was a common technique with ferrite that just had ground centre legs for a specific AL rather than user provided gap spacers - or you could do the alternative where a ferrite disk inserted in to the centre leg is used to space the cores (and hence minimise fringing flux from the centre leg, and then gaps would be effectively further away from the winding).

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 11:06:27 am
content: Quote from: trobbins on October 16, 2020, 10:48:32 amI tried that before but still suffered winding heating, could be the pad was not thick enough (see picture) but I also found it a sticky mess to fabricate, I used epoxied fiberglass tube turns (see pic).I have problems with other solutions as I only build one or two at a time for prototypes so custom grinding etc is not available to me, same with Litz, I have to make my own so my frequency range is limited. But thanks for the suggestions

author: chris_leyson
author_rank: Super Contributor
timestamp: October 16, 2020, 11:13:42 am
content: Hi fourtytwo42, I've seen a few transformers with gapped cores where the innermost windings were wound on single layers but the winding pitch is increased in the centre, over the gap. Haven't done any comparisons with different winding methods though. EDIT: I have seen some transformers where the middle leg was ground to a chamfer but these were large E100 cores.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 11:21:48 am
content: Hi Chris,Well this time I decided to go belt n braces with no windings whatsoever crossing the gap BUT that's just me not wanting another hot inductorYup I can see the chamfer idea working, like Trobbins suggestion of ferrite disks to produce multiple gaps but I don't have access to any of that technology, in fact I have even had to do without manufacturer gapped cores as these have also just about dried up from my sources (RS & Farnell). I suppose I should be thankful I can still get cores at all!!

author: trobbins
author_rank: Frequent Contributor
timestamp: October 16, 2020, 11:46:34 am
content: Just to clarify, my comment about padding the coil away from the centre leg relates to padding the entire winding length away from the former (eg. using multiple layers of thick tape) - which of course is only practical if the winding is not taking up the entire winding area - and the amount of padding aims to make the coil equidistant from the centre leg gap and the outer leg gaps.I can also see that increasing the TPI near the gap may have a similar effect.Can you describe how you are determining the coil operating loss between different construction samples? Is it an inductor or transformer part?Is the loss causing any other performance limitation than just power loss in the coil? eg. is the temperature rise of wire, insulation or ferrite, getting higher than deemed acceptable?

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 01:21:06 pm
content: Quote from: trobbins on October 16, 2020, 11:46:34 amOK I understand.Quote from: trobbins on October 16, 2020, 11:46:34 amIts an inductor, I determine loss from temperature rise so it includes ferrite loss, however this should be roughly the same given the same operating conditions.Quote from: trobbins on October 16, 2020, 11:46:34 amIt's hard to measure with any great preciseness the different material & location temperatures using a simple IR thermometer but given a very high (>70C) surface temperature that indicates to me things internally are probably not acceptable.I have built several designs using different methods and all I can say so far is that the one 1st pictured is the 1st of the divided coil approach and it has given no problems.

author: sandalcandal
author_rank: Unknown
timestamp: October 16, 2020, 02:16:56 pm
content: Hey 42I've been brushing up on high power, high efficiency magnetics recently for some upcoming designs we need done. I'll share some of my notes on the subject of dealing with core gaps and fringing flux effect. I'm interested to compare notes and hear what you've tried before.Fringing flux is indeed a serious problem in high power magnetics. Fringing flux induces eddy currents in nearby windings as well a current concentrating, skin effect/proximity effect type phenomenon (haven't seen an actual specific term) where current density to concentrates towards the gap rather than distributing evenly across the cross section of the conductor (both part of copper losses). Another mechanism for power loss is increased magnetic losses (iron loss) due to non-ideal alignment and distribution of flux at edges of the gap.Some suggestions for dealing with core gap fringing flux effects:*Don't have windings over the gap, use a non-conductive spacer over it (as you've done)*Distribute the gap as multiple smaller gaps*Use powdered coresL. H. DixonMagnetics Design Handbook. Unitrode, 2001, Texas Instruments, 2011, ch. 5, pp. 4 Available Online:https://www.ti.com/seclit/ml/slup132/slup132.pdf*Locate the gap not in the centre but at the end of the winding window i.e. use E-I cores (less economical than symmetric ETD cores)L. H. DixonDesigning Planar Magnetics. Texas Instruments, 2006, sec 6.B, Available Online:https://www.ti.com/download/trng/docs/seminar/Topic4LD.pdf*Common sense idea but: use a larger core that can use a smaller or no air gap without getting saturated.*Edit: gaps on outer legs are an option but trade off is more radiated EMII actually couldn't find any more practical advice for reducing losses related to fringing flux from air gaps in my textbooks (or other material):W. T. McLyman,Transformer and Inductor Design Handbook.2011;2017;.W. G. Hurley, W. H. WölfleTransformers and Inductors for Power Electronics: Theory, Design and Applications. (1. Aufl. ed.)2013. DOI: 10.1002/9781118544648.M. K. Kazimierczuk,High-Frequency Magnetic Components: Second Edition.2013.Mind you Lloyd Dixon's models and understanding of magnetics seems to be based on an obsolete engineering-based magnetics model that has become divergent to the understanding taught in modern physics (certainly from my own experience). So his models and advice can get a bit confusing to anyone with a modern physics background but seem mostly relevant in engineering practice albeit not 100% theoretically grounded. Example:https://e2e.ti.com/support/power-management/f/196/t/796191?keyMatch=PLANAR%20MAGNETICS&tisearch=Search-EN-everythingSpacing the windings away from the gap in the radial dimension (e.g. by bobbin thickness) seems would worsen fringing and overall losses since close windings help to confine the flux fringing [McLyman, ch 1 pp 27]. This is different to spacing windings away axially since in that case windings are "away" from the fringing flux rather than above/inside the fringing flux.The chamfering of the core edge around the gap mentioned by chris_leyson seems to be a way to help deal with fringing effects on non-ideal flux alignment.I haven't heard of the slit heavy copper tube (non-shorted turn) method before and it's not obvious to me how it's meant to help so I'd like to hear more about that one!P.S.On the topic of machining/modifying cores, in these conference presentation slides:J. W. Kolar et al.Google Little-Box Reloaded. CIPS 2018 - 10th International Conference on Integrated Power Electronics, pp. 55-57 Available Online:https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/__CIPS_18_Keynote_FINALFINAL_270318.pdfKolar and his team ran into efficiency limits due to imperfections caused by diamond saw cutting in the core fabrication process and so swapped to onecut using a Focused Ion Beam!

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 16, 2020, 02:39:53 pm
content: It is the radial H Field component of the fringing that does most of the unwanted heating.Using a slit copper inner sleeve with reduce the loss in the outer windings,but the sleeve will still have losses and heat up, even with a slit.Loss can be reduced in the windings if the wire diameter is reduced to be less than 2 times the penetration depth.penetration_depth = 500 *sqrt( rho/f) example 50 kHzpenetration_ depth = 500 * sqrt(22e-9/50000) = 0.331 mm @ 100 CelsiusSo wire can be approximately 0.6 to 0.8 mm, using parallel strands in hand.Design of the whole inductor is best done using electromagnetic FEM. I use QuickField in 2D.In megaWatt transformers, I have made the inner turns of water cooled copper sheet to get a 1 or 2 turn transformer,but that is with no core gaps.Also used the split copper cylinder on DC choke, but that was just to get a solid winding former.Another (perhaps better) way for the problematic AC inductor design,is to make the core out of lengths of square section ferrite, to form a square core, putting the gaps in the corners.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 03:15:50 pm
content: Quote from: sandalcandal on October 16, 2020, 02:16:56 pmhttps://digital-library.theiet.org/content/journals/10.1049/ip-epa_20045234So many great links from you many thanks. TI was indeed a great source of open information, certainly got one started!In my case I use experience combined with experiments as I find the whole subject a bit of a dark art where results rarely agree with calculations. One of the first things I made myself years ago was a pulse saturation current tester, that if you observe the linear portion of the slope also shows you the inductance, useful indeedIf I can think of anything else to add I will as i come across it.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 03:24:41 pm
content: Quote from: mag_therm on October 16, 2020, 02:39:53 pmIn my case at low frequencies (25Khz) I was able to use a thick copper sleeve reducing the losses and remaining fringing flux intensity BUT like you say whatever loss there is in the sleeve is buried in the center of the winding so really I found it didn't help an awful lot. Of course at higher frequencies due to skin affect the sleeve is even less effective.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 16, 2020, 03:40:47 pm
content: Just remembered, I've seen the remains of a failed 200W flyback transformer where the innermost windings had failed short circuit due to overheating and eventual insulation breakdown. Can't remember if the innermost winding was primary or 50/0/50 secondary but I think the problem was solved by distributing the gap between all three legs of the EE cores with brass shims. I've seen the variable winding pitch technique used in a CCFL driver transformer from a broken copier. Powdered iron seems like one way to go but I have a feeling it may be too lossy.Just had a look at a 1kW Panasonic inverter transformer. It uses a pair of C cores like half of an ETD core and there is a 1.5mm gap in the outer leg midway along the litz wound primary. HV secondary is two PI windings in series followed by a space and a third PI winding for the magnetron heater. It's a single ended resonant converter rather than the older half bridge design but just looking at the secondary rings proximity effect alarm bells, maybe it's a subtle part of the design, who knows.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 16, 2020, 03:58:57 pm
content: Hi Chris, nothing like burnt remains for learning lessons, thank you for sharingDistributing the gaps certainly helps in ETD's, if I get to much external field (from the outer leg gaps) I use a shorted external shield around the whole thing.I must admit to not attempting powdered iron at high power because the calculated losses are to high (that could always be a miscalculation) but also I just don't like the fag of winding toroids

author: chris_leyson
author_rank: Super Contributor
timestamp: October 16, 2020, 04:28:28 pm
content: Winding toroids is a pain if you're prototyping stuff. I've seen powdered iron EE cores used in a high current buck forward converter and I've also used a large powdered iron toroid in an off line boost PFC converter. I unplugged the load when it was powered up and there was semiconductor shrapnel everywhere, damn. Magnetics Inc do powdered iron in EE cores but it's been a while since I last looked.

author: sandalcandal
author_rank: Unknown
timestamp: October 16, 2020, 04:53:28 pm
content: Quote from: fourtytwo42 on October 16, 2020, 03:15:50 pmThanks 42. Looks like a great paper. It's surprising to me that such a device has a net positive effect on efficiency. I certainly hear echoes that calculations often do not match results.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 16, 2020, 05:33:55 pm
content: Thanks 42, I had wondered about using a copper screen on the gapped leg of a transformer, it makes sense. For powdered iron cores I meant Micrometals and not Magnetics Inc sorry about the typo but I think Magnetics might also do powdered iron.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2020, 06:26:31 pm
content: Once upon a time, there was actually an online calculator for ideal bobbin geometry -- which could be 3D printed, keeping optimum distance away from the core and air gap. Haven't seen it in some years though, might've succumbed to the inevitable decay of internet content.Related, I've wound inductors on gapped UR (TV flyback) cores. For a gap around 3mm, the 16AWG wire got stinking hot in the middle. Even in a single layer, even with a big jog to avoid most of the fringe area -- there was enough induction heating just in the little piece passing by.But right now, I happen to have enough powdered iron cores (#8 and #2 are reasonably low loss) not to worry about fringing flux, and lots of salvaged litz (10AWG and other sizes) to deal with it otherwise...(Here's a combination of both: for the about 100W capacity ofthis boost converter, the choke basically doesn't get even perceptibly hot. Super overkill.)Tim

author: trobbins
author_rank: Frequent Contributor
timestamp: October 17, 2020, 12:04:03 am
content: 42, how close to an optimum are you with the winding itself (strand # and strand diameter), given the winding window with the core you want to use, and the operating frequency you are using ?Is the operating current sinusoidal or switchmode of some shape?What do you estimate is your loss budget for contributors like ac and dc coil resistance, core loss, and additional losses from gap fringing?Is IT thermometer temp differential the only method you have available to measure total inductor part loss? Are you operating the core such that you can measure a discernible temp rise on the core region closest to centre leg (eg. perhaps with a thermocouple fitted to the outer E surface at the middle leg and under the cable tie, so it is likely not disturbing the inductors operation). Measuring the dc and ac resistances of the coil could test your experimental efforts, as there are a few methods that could be tried - perhaps the easiest could be to run a dc current to reach the same temperature rise as a total loss benchmark.

author: jonpaul
author_rank: Super Contributor
timestamp: October 17, 2020, 06:38:40 am
content: Hello: Have done power magentics designs since 1970s, up to 12 kW and 55 kV.A few notes please :Toroids and powdered iron are a best for chokes and certain applications eg low external field.EI cores avoid the center gap, but need square bobbins.EER and ER, ETD with round center are easier to wind but more costlyAny machining of cores, gaps requires diamond tools and is very costlyEvaluation of losses eg copper/core/gap fringue eddy is complex and needs several steps for accuracy, eg open ckt, short ckt, etc.All SMPS have frequencies high enough that skin effect is important and conductor geometry must be considered eg litz, multiwire and foil to reduce effective resistance.Modern core materials are optimized for use at higher flux densities and temperatures.TDK in Japan remain a fine supplier of Ferrites.The texts mentioned are very old, and some have misleading info. For basic texts with good discussion of these topics recommandé Magnetic Components: Design and Applications by Steve SmithHope these notes are useful!Jon

author: sandalcandal
author_rank: Unknown
timestamp: October 17, 2020, 07:10:35 am
content: Thanks Jon! Some great notes to have from an experienced designer.Quote from: jonpaul on October 17, 2020, 06:38:40 amCould you specify what texts have the misleading info or what to be wary of?Magnetic Components: Design and Applicationsby Steve Smith looks like an excellent source. Much more practical than the textbooks I've seen so far.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 17, 2020, 10:57:14 am
content: Quote from: T3sl4co1l on October 16, 2020, 06:26:31 pmExactly what I got Tim, hence the completely split winding idea with the connection external to the bobbin.Quote from: T3sl4co1l on October 16, 2020, 06:26:31 pmLooks great, I am all for overkill, nothing worse than being held up by hot inductor problems.....

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 17, 2020, 11:11:32 am
content: Quote from: trobbins on October 17, 2020, 12:04:03 amMany thanks, the current is typical switchmode shape. I don't worry to much about optimization other than not using excessively expensive cores and ending up with a winding that will fit in the available window if that's what you mean.Of course the issue of flux density is material to power dissipation, for these cores saturation is about 320mT but I design way below that as saturation is usually fatal to lots of things like expensive mosfets and hard to replace pcb's.I design for the saturation current I want and inductance. From that I try different combinations of cores and gaps till I get a reasonably economic solution with a bit of headroom in it. Core loss is from the material data sheets (I use N87 at 25Khz so not to bad), skin effect determines maximum wire size and I aim for 5-7A/Cm2 for the Litz. Unfortunately I do not have any thermocouples so cannot get into the absolute detail of where the losses are but I have built many and you get a feel for it! One example is just using fingers on the core surface vs the bobbin surface asap after start before everything equalizes can tell you if your worst losses are copper or core. Like Tim and Chris have said the fringing flux can cause unexpected hot-spots in the winding and that's really what this thread is about.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 17, 2020, 11:44:03 am
content: The finger test may not allow a confident call on whether centre leg ferrite could get above 100C, which is likely the first design and operational risk, as other risks may not be significant until well above say 125C.If your design keeps ferrite temp well below 100C, even with a standard coil and gap with fringing flux, then imho it sounds like a total part loss assessment, with the aim of reducing the total loss, is the only figure of merit worth striving for. Just aiming to reduce the temp in one portion of a coil, by touch testing, does not directly mean a total part loss reduction has been achieved.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 17, 2020, 11:51:07 am
content: Quote from: trobbins on October 17, 2020, 11:44:03 amI totally agree with you but I am limited by my resources. If the copper hotspots it's usually possible to tell with a postmortem examination but I have never seen the ferrites discolored and I imagine a very high temperature would be required to do that so sadly I don't have a handle on that other than by overall heating by conduction from the center pole.

author: jonpaul
author_rank: Super Contributor
timestamp: October 17, 2020, 12:30:00 pm
content: Bonjour encore:To SandalCandle RE books: I cannot disparage any particular books,I note that Magnetic Core Selection for Transformers and Inductors: Col. McLyman was writen ~ 1975 and the Col.s experience was focused on mil and avaition, satellite PSU of that epoch, especially using tapewound and cut cores up to 20 kHz. This is far from most of our applications nowadays.In College ~ 1968, we had an excellent text Static Electromagnetic Devices Hunt Jr. and SteinMost of my experience was not from texts but "seat of the pants' and trial and error.To FourtyTwo42:In large system magnetics but even 35..50 mm coresets, we bury type K very fine guage Thermocouples with thin Kynar inulation in several places during prototype.Power distribution and transmission transformes always have internal TCs.To all: Modern soft switching toplolgies eg resonant mode, can operate at 1-20 MHz.Requirements for efficiency and low EMI as well as minimal size drive us to maximize the frequency.Modern power switches are GaN and SiC give the fastest low losss switching. All of this affects the magnetic designs.The transformers may be part of a PCB or planar designs and can have very few turns.More info: IEEE Power Electronics proceedings and conferences as well as APC, PESC etc have state of the art papers on topology, magentics, etc.IC firms app notes can be helpful but often they have not optimized the magnetics, and choose off the shelf parts for convienience.Just the ramblings of an old retired EE!Bon weekend from FranceJonPS: Did a fine optimization in 1983A 12 kW cinema arc lamp ballast at ~ 300 V ~ 150A, 50/60 Hz.~ 50 cm legnth, custom cut core, 1 cm gap and #6-8 square HPTZ wire, or rect (custom) in perfect layers.Ventilated design, very quiet fan cooling.Hotspot was ~ 80 deg C. (from memory so not exact)