How to attach ferrite cores together properly with minim. gap (SMPS transformer) - Page 1
Forum section: electronics / projects

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 23, 2023, 08:33:49 am
content: In the very begining of my smps journey i used just electrical tape to press cores firmly together. Howeever audible noise due magnetostriction occurs and unstable feedback loop was present.Then i was using super glue, it was way more stable without noise but when cores get hot it losses bond. In most cases it was ok. Biggest disadvantage was need for quick manufacturing. There is also epoxy glue which is stronger and can withstand higher temperatures. But, glues has one enormous disadvantage, it creates gap between cores. Without glue and firmly pressed together, inductance is 10mH. When glue is applied it drops to 6mH. Lower inductance, higher losses.So how do you fight this in home conditions? thanks for any suggestions

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2023, 08:54:19 am
content: Typically spring steel clips are sold that match the core pair.

author: CosteC
author_rank: Regular Contributor
timestamp: September 23, 2023, 10:40:33 am
content: For prototypes I use steel springs supplied by bobbin manufacturer. Those work however only for cores with gap on central leg (proper way) but if you experiment with gap on all legs, steel spring is not an option. Cyanoacrylic glue will work also for prototypes.For final product special acrylic glues are used + insulation tape around core. I also vacuum impregnate whole transformer.Glue example:https://www.permabond.com/resource-center/adhesives-bonding-transformer-ferrites/#:~:text=Adhesives%20join%20ferrite%20halves%20in%20place%20of%20mechanical,curable%20adhesives%20as%20well%20as%20two%20component%20epoxies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2023, 10:44:53 am
content: Quote from: MacIntoshCZ on September 23, 2023, 08:33:49 amCitation needed!Is this your assumption, observation, or determination? On what basis / with what assumptions?Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 23, 2023, 11:23:31 am
content: OK, sorry. Transformer with lower inductance has higher magnetization current.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 23, 2023, 11:28:49 am
content: Quote from: CosteC on September 23, 2023, 10:40:33 amThanks for comment! Can i ask which tools do you use for vacuum impregnating?

author: strawberry
author_rank: Super Contributor
timestamp: September 23, 2023, 01:18:26 pm
content: Quote from: MacIntoshCZ on September 23, 2023, 11:28:49 amto increase capacitance for HF transformer?used on 50Hz transformer/motor for heat dissipation and moisture resistance ...

author: TimNJ
author_rank: Super Contributor
timestamp: September 23, 2023, 02:07:37 pm
content: Try this:Epoxy only on the center post where it’s gapped.Epoxy beads on the *outside* of the core, across the seam formed by the two mating cores. Usually four beads total is enough.Probably not as good as a full VPI treatment of course.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 23, 2023, 02:19:08 pm
content: QuoteIf you see any gap in ferrite core transformers, that is by design, not because of sloppy fabrication processes.The gap is not meant to be squeezed out, and the distance in the ferrite gap is according to the design calculations. The entire SMPS relies on that gap being of a certain volume, and air filled. If you squeeze the gap away, or fill it with something else than air, than the entire SMPS goes out of its designed specs.

author: fourtytwo42
author_rank: Super Contributor
timestamp: September 23, 2023, 03:29:03 pm
content: I have used nylon ty-wraps of various sizes to suit the cores for years, never had any trouble.

author: coppercone2
author_rank: Super Contributor
timestamp: September 23, 2023, 03:54:28 pm
content: https://shop.boeing.com/aviation-supply/p/31231=6P

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 23, 2023, 05:13:19 pm
content: Quote from: RoGeorge on September 23, 2023, 02:19:08 pmWhen designing chokes and inductors, gap is handy but when i want to design transformer (excluding flyback) i dont want in gap between cores.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 23, 2023, 05:15:09 pm
content: Quote from: fourtytwo42 on September 23, 2023, 03:29:03 pmAlso tried but without success.

author: fourtytwo42
author_rank: Super Contributor
timestamp: September 23, 2023, 06:33:58 pm
content: Quote from: MacIntoshCZ on September 23, 2023, 05:15:09 pmThat doesn't make sense, used properly and of sufficient size ty-wraps can exert an enormous amount of pressure especially if wedges are added to the slack sides after tightening (although I don't usually have to bother).One other thing are you sure your ty-wraps are not soft plastic ones that will stretch over time and also if warmed up a bit ?Perhaps your problem is elsewhere such as insufficient tension during winding or not using cores and bobbins designed for each other ?What manufacturer and what size of core and bobbin are you using (I typically use TDK ETD series such as 39/44/54/59) ?

author: Benta
author_rank: Super Contributor
timestamp: September 23, 2023, 07:49:57 pm
content: Epoxy is excellent for this job and if done correctly will have a miniscule thin joint that's magnetically about as good as no epoxy.But it's not done by just glueing like in kindergarten.The secret is heat and pressure.First you need to build a fixture that will put the core halves under mechanical pressure (like metal glue clamps), and at the same time fixing them so they can't skid sideways.This is a task in itself, spring loading is necessary to temperature compensate when heating the assembly.For epoxy, choose a "24-hr" type. My personal favourite is UHU Plus Endfest 300, very easy to work with.For your transformer, check the temperature ratings of the different parts (core, bobbin, wire lacquer etc.). If you can heat the assembly to 150 C, that's optimal, but less can do it.Clean the glue surfaces, acetone is good.Stir your epoxy, add to surfaces (a small drop on each is enough).Put in fixture and apply pressure to the assembly. Wipe off excess epoxy (ethanol works best for cleaning).During all this, and while your wife is out of the house, you should have preheated the kitchen oven to 150 C (or less, according to the part specifications).Put the whole thing in the oven.You'll see that the epoxy viscosity drops dramatically, to almost like water. This guarantees the extremely thin glue joint.At 150 C, the epoxy will cure in appr. 10 min, at 100 C perhaps 30 min. There should be a table on the epoxy package.Have Fun!PS: don't forget baking paper under your assembly, epoxy stains are pretty impossible to remove from the oven.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2023, 03:01:35 am
content: Having a toaster oven handy is good not just for reflow projects, but assembly (as above) and disassembly (heat to soften varnish/epoxy) of magnetic cores... without the odd kitchen smells.Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 24, 2023, 08:20:47 am
content: Quote from: fourtytwo42 on September 23, 2023, 06:33:58 pmIn most of times i am using EE, or ETD cores. At the moment my 4kW transformer is done using this corehttps://www.semic.info/lj-ec9030-cf297-en/

author: fourtytwo42
author_rank: Super Contributor
timestamp: September 24, 2023, 08:41:24 am
content: Quote from: MacIntoshCZ on September 24, 2023, 08:20:47 amAhh so about half as big again as the largest I have used. If the key is disassembly for prototypes then a clamp might be better than ty-wraps, two strips of U-channel aluminum with either bolts or studs at the ends to align with the slots in the ferrite. If you think circulating eddy currents could be a problem one or both of the U channels could be replaced with several thicknesses of scrap FR4 with copper removed around bolt holes. OR it could be the bobbin loose upon the cores, you could try some wedges around the center pole ?Could you say a little more about your application like operating frequency, hard/soft switching and primary/secondary voltages ?I am still intrigued by your audible noise, 4Kw is quite a bit of power so some noise is quite likely IMOP especially if it is suffering sub-harmonic oscillation that might be your true problem!An ETD59 at a mere 1Kw squeals loudly when suffering sub-harmonic oscillation I can assure you even with clamped cores and very tight windings

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 24, 2023, 09:48:48 am
content: Quote from: fourtytwo42 on September 24, 2023, 08:41:24 amIts 100V/40A lab PSU, fullbridge, tl494 -> hard switching. In average (without AC line 10% tolerance) maximum primary voltage is 325V, it should drop by 10% on caps until next halfwave (8x 680uF). primary/secondary ratio is 2.22.I need to buy that mini oven. I have just tried it again (no oven, 20C degrees room temp). I spread a really small amount of glue on the cores with a toothpick and pulled the core with a puller. I did also calibration of RLC meter.So without glue 9,5mH, With glue 5,5mH, with glue and hardly pressed together with puller 6,1mH at maximum. So it drops by aprox 36% on inductance. You can see how epoxy spread under preasure at one of the photos.Quote from: Benta on September 23, 2023, 07:49:57 pmCan i ask how big drop is in yours experience if done correctly? I assume it will be just few %.

author: Benta
author_rank: Super Contributor
timestamp: September 24, 2023, 10:57:53 am
content: Just big enough that a little epoxy will emerge from the joint when put under pressure.That clamp won't work, it'll loosen as soon as you heat the assembly. I said spring loaded.Like this (remove the plastic parts):

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2023, 12:38:32 pm
content: I mean, there's plenty of spring in those, especially if they're cheaply made. The core will shift a couple mils as it settles, a low spring rate is not required. More to the point, force will not be distributed evenly over the core, and indeed you might even crack it this way -- use broad, flat plates to distribute the load over the core. Just clamping in the middle, allows flex to be taken up by the lateral parts.Such strict assembly isn't meaningful anyway, as the surface flatness is typically within maybe a hundred microns or so, and you can't deliver enough clamping force to level that out further (read:crushthe materials flatter together). You must have lapped/polished faces to do better. At which point, the glue line will matter very much to the fitness of such a joint; but any thin, unfilled CA glue, for example, will still yield less air gap than the equivalent airgap of the material (l_e / mu_r, ballpark 10s of microns), so it's very much in the "do not care" range, and simply fitting and clamping will suffice.I certainly don't measure values outside of tolerance from just taping cores together; I have yellow (Scotch #56) and polyimide (Scotch #92) on hand, which can be tensioned reasonably well in say two layersAnyway, the required inductance hasn't been established. Ordinary forward converters are often said to benefit from magnetizing inductance, as it affords a shorter startup transient, that is, when charging the coupling capacitor from initial to the operating value (which even if intended to be 50% of supply, is never perfectly balanced).Even half-wave forward (passive clamp) may benefit from slight airgap, as inductance past some point simply won't develop enough current to charge the switch node to the clamping voltage in reasonable time; maximum duty cycle then suffers. Core losses will be higher in high-mu materials as well (i.e. mu_r >= 10k if one were optimizing for minimum Imag).The answer to both, more or less, is the two-switch forward, where magnetizing inductance is stirred directly into the supply and therefore doesn't matter -- to a first degree, that is; its only impact then is the slightly increased conduction loss, and conduction and maybe recovery loss of the catch diodes. (Or resonant such as LLC converters, where Imag is part of normal operation, a sizable fraction (5-20%?) of load current, and permits ZVS switching over the full load range.)All in all, typical power materials (mu_r ~ 1000-3000), with stock finish (ground but not lapped faces), and little (fractional mm?) or no airgap, is typical for this family of converters.Tim

author: CosteC
author_rank: Regular Contributor
timestamp: September 24, 2023, 06:14:13 pm
content: Quote from: MacIntoshCZ on September 23, 2023, 11:28:49 amI order samples from supplier... They have tools for this. At maybe 10 USD for small transformer it is not prohibitive.

author: Benta
author_rank: Super Contributor
timestamp: September 24, 2023, 08:53:25 pm
content: Quote from: T3sl4co1l on September 24, 2023, 12:38:32 pmThe cores are plenty rigid to accept pressure on the laterals and the middle. I can't imagine which material would "distribute even pressure" on the core. Also considering that the outside surfaces of the core are not specified for flatness. And your "flat plates" will flex more than the core.Concerning the glue surfaces: on quality ferrite cores, they are ground to maximum flatness and surface quality. Not quite "Johansson/123 block" surface, but close to it.In connection with that: for international understandability, please avoid the term "micron". The unambiguous SI term is micrometer or um (unfortunately this forum does not support Greek characters). "Micron" was phased out over 50 years ago. Likewise "mil" is not the same as millimeter, but 1/1000th of an inch, which is a pure US unit.To the OP: looking at the data sheet of the core, I'd simply press in two pieces of Ø6 mm nylon/brass/delrin rods into the slots on the sides of the core. That should ensure alignment while glueing. They'll probably also be glued to the core. Is that a problem?

author: Benta
author_rank: Super Contributor
timestamp: September 24, 2023, 09:05:43 pm
content: Quote from: T3sl4co1l on September 24, 2023, 03:01:35 amActually epoxy doesn't smell... but when SWMBO sees something like that in the oven, the WAF is far below zero.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2023, 10:37:35 pm
content: Quote from: Benta on September 24, 2023, 08:53:25 pmThe top and bottom are often ground parallel, though it isn't specified as such. This is less common on E cores and related families, and more common on PQ, RM and others.Ferrite has a pretty reasonable modulus, as it happens (comparable to steel), but the fact that a plate can be arbitrarily thick, while the limbs are given size, pretty well covers the last point.QuoteNot sure what "quality" means here; I've only ever seen polished faces on exactly one pack of P22/13 cores I have. And those might've been special order, I don't know (they were inherited). They do fit very nicely together thoughIt's as I said. I don't have a roughness gauge here, or much of anything capable of resolving such small details, but this is my guess based on handling and grinding them.There is a standard ground finish that you get on 99%+ of parts from common distributors, whether western or otherwise. It's fine, but not extraordinarily so. Tool marks are just evident by eye, but roughness is imperceptible by the touch. That's at least under 25 micron or so; I mean, feel depends on steepness, and a sharp step for example is much easier to feel than a gentle slope. It could be as much as, oh, 50 micron p-p across the whole surface; but more than that would be readily visible against a straight edge and backlight, so I strongly doubt it's more than that.I've personally lapped parts to a finer (though still not mirror) finish, just to see what it does. The visible swish marks on the original face get worn down unevenly, evidencing ridges; sides or middles also show up more prominently, as expected for a randomly-slightly-unflat object. (Or that manifests from grinding pressure, which is definitely not uniform by hand. On the micron scale, it doesn't take much force (finger pressure) to deform something, even something as stiff as this. Or the spot heating from finger warmth, versus the cooling of the lapping plate, or the evaporating lubricating water. Anyway, I don't really have a good way to tell what's truly flat at this scale, or if my lapping plate even is.)I've also done that with a thin (EMI shield ferrite) plate (which is now about 2.0mm thick), which, using a micrometer as a thickness / ersatz flatness gauge, I can definitely tell it grinds differently based on where I'm pressing on the thing. That's a more extreme example of course, but the point stands, everything is rubbery if you look at it closely enough / press on it hard enough.QuoteAh good, no confusion has been made then. If I had wrote "1 mils", there was a risk of misinterpreting that asmillimeters (though less often in this domain). And I didn't feel like typing out µm all the time. As far as I'm aware, "micron" only means µm, or only ever meant, as the case may be.

author: Benta
author_rank: Super Contributor
timestamp: September 24, 2023, 10:47:01 pm
content: One question, how did you do this: µm ?thx.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2023, 11:20:28 pm
content: Forum's had Unicode support for a while nownot sure if you noticed:https://www.eevblog.com/forum/chat/unicode/&mu; unfortunately(?) doesn't get parsed, but ALT+0181 (on Windows) does the trick, or other combinations, or you can copy-paste from other sources. Or use MathJax \$\mu\$.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: September 25, 2023, 07:42:42 am
content: Bonjour a tous...I Just spot this fine topic, a few moments please ....1/ Ferrite cores for power applications are manufactired in gapped in stock Al, and ungapped, for max Al.The specs depend on a clamp or spring pressure to hold the halves togater.2/ Neither glue, tape or other mentioned are reliable assembley over time/tempreature/magnetostriction.3/ Any material between the haves faces eg impregnant or adhesive will increase the gap and reduce the Al ina gapped or ungapped coreset.4/ The gapped cores are more costly and less avail than ungapped,in decades of design and mfg, we normally used ungapped cores and added a fiberglass (FR4, G10) or fishpaper gap if needed.5/ The core and bobbin mfg eg TDK, Siemens, Ferroxcube, EPCOS had certailn core shapes with matching bobbins and clamps.By specifiying and purchasing the coreset, bobbin and clamp from ONE manufacturere or disty, we could return to the vendor if any issues developed in aopplication, QU , logistics.This is not the case if parts from differnt vendoes or some DIY is used in assembly eg tape, adhesive, etc.6/ Vacuum/impregnation with Varnish is for environmental protection but cannot be depemded on to maintain proper Al over time/temp.7/ The use of gapped parts is mainly for resonant inductors and flyback SMPS where a specific L is needed. Transformers will spec a min Lpri mag but ungapped cores give maximium Al thus minimize magnetizing current.8/ For large cut cores and some ferrites we used Band-It clamps amd metal packing straps....Just the ramblings of an OLD retired EE.....HAVE AN ABSOLUTELY FANTASTIC DAY!From Jon in Paris 6

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: October 01, 2023, 11:12:32 am
content: I did not find any spring loaded Clamp, so in my first try, i went to the stone age =).1) Cleaned surfaces with aceton2) Made thin epoxy layer with toothpick3) weighed down with a stone4) bake at 150C degrees on 30 min timer (i dont have thermometer inside, so ive just set 150C and turn it on for 30mins)and now 14h waiting till it hardens

author: coppercone2
author_rank: Super Contributor
timestamp: October 01, 2023, 04:11:47 pm
content: jon paul said on the forum that basically all methods that don't use some kind of clamping system are dodgy and gonna failand be careful weighing down epoxy, its not wood glue. There is a minimum thickness level. Some epoxy have glass beads in them so that you are guaranteed a minimum thickness for clamping. Over clamping epoxy leads to weaker joints. I don't know why, but its written all over the place. Google epoxy clamping. Its one of those backwards things

author: Benta
author_rank: Super Contributor
timestamp: October 01, 2023, 04:19:16 pm
content: Quote from: MacIntoshCZ on October 01, 2023, 11:12:32 amPerhaps I explained badly: the heat makes it cure (=harden) in 10...30 min. If it's not cured in that time, your epoxy is bad.

author: jonpaul
author_rank: Super Contributor
timestamp: October 01, 2023, 04:28:57 pm
content: Rebonjour: Can you reveal the full design, SMPS spec, freq, topology, etc?1/ Pix shows a standard EE , ETD or EER core,TDK, Siemens. EPCOS provides the appropriate spring clip or screw clamp hwd.examples: ETD 49https://www.farnell.com/datasheets/1468409.pdf2/ The epoxy will cause a gap, even 0.1 mm is noticeable especially as it affects the center and both legs of the core.You should first measure Lmag on UNGLUED cores clamped and then after the epoxy you will see a substantial reduced Lmag due to the gap you have created.Depending on the epoxy thickness and perp, entrained air bubbles will affect the joint and gap. Only a vacuum oven can ensure no air bubbles.That gap and Lmag will not be stable with time or temp.At least the finished piece needs to be annealed.BEWARE: Epoxy curing agents are very powerful chemicals (aniline) and emit noxious fumes when heated. Suggest that youDO NOT USE THIS OVEN FOR FOOD IN THE FUTURE.Depending on the SMPS topology, KVA throughput and design, eval effect of reduced lmag .3/ My first impression is your winding design and wire type need review, depending on the current, freq, and design eg P-S safety insulation.The bunched thick wires seem a poor choice to me.4/ catalog sheets for the core shape and material used, should show the matching bobbins and clamp hdw.5/ We have Used Intusoft magnetics Designer SW for core size, gap and bobbin/wdg design.Highly recommendedhttp://www.intusoft.com/mag.htmBon Chance.Jon in Paris

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: October 04, 2023, 09:01:05 pm
content: Transformer is for 50khz, fullbridge, 100V40A secondary output, two secondary windings (20A+20A).1) I have tried those types of clamps in ETD cores, but seems i am unhandy. Bobbins cracks quite often.2) Yes, my first attempt was unsucessful. Resulting inductance is 5mH. Without epoxy 9mH, With epoxy and firmly press together with wood clamps 7mH. After oven as in picture above 5mH... =/3) Its not ideal, i am using 5x1mm wires for 20A, twisted using electric drill. Quite time demanding process. I am unable to find alternative on market in CZ.It will be probably costly to order finished transformer. This is just small scale production. For some hobby folks arround. I need to find out best way possible at home.I use 2 layers of capton tape as insulation between windings on same side (primary / cesondary). And 8 layers of capton tape between P/S.

author: jonpaul
author_rank: Super Contributor
timestamp: October 05, 2023, 06:56:30 am
content: Rebonjour: A 4 kW SMPS/inverter transformer needs better design. expect Pcu+Pfe losses range ~ 1..3% = 40..120W dissipated.Thus thermal management and hotspot temp is an issue.We use Intusoft magnetic Designer SW.http://www.intusoft.com/mag.htmETD is costly and may not be the best core shape,ETD bobbins and clips need careful assy. especially if you have low quality Chinese parts.We used only genuine TDK and EPCOS( Philips/Siemens/Ferroxcube) cores, bobbins, clips.Epoxy/Gap: Retest L pri after annealing and after some weeks, it will change. Your ckt design will guide you the required mon Lmag pri for the application.Insted of ETD, we used EE, EI or EER, as well as PQ.At 20A and 50 kHz you may need MUCH finer bunched wire than 1 mm perhaps Litz to avoid skin/prox effect losses. Litz wire readily avail from MWS, Rubidue, and others, even in CZ. Check losses of wdg at max I , max F.We often used Cu foil for high current and low lkg apps.Dupont Kapton (TM) tape is easy to tear and hard to work with.A rated electrical tape such as 3m #56 2 mil Mylar (yellow) is an industry std.If insulation or safety is an issue you may need vacuum varnish impregnation.Suggest to find an experienced transformer manufacturer, design a winding sheet and get and test protos.Bon ChanceJonTDK PQ50/50https://product.tdk.com/system/files/dam/doc/product/ferrite/ferrite/ferrite-core/data_sheet/80/db/fer/pq_50_50_dg.pdf

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: October 05, 2023, 07:20:04 pm
content: OK, thanks. My aprroach when selecting wire for winding:1) I picked accetable wire losses in resistance of 1 meter same as losses in 1mm2 wire loaded with 10A. Which is aprox 1,73W/m5*1mm wire has crossection are of 3,96mm2 thus resistance 4,32mOhm in 1meter. Loaded with 20A resulting in 1,73W/m aproxx.Primary winding is 5,3m, secondary 3,2m and 3,6m. so 12,1 x 1,73 = 21W in resistance.losses for core:By looking in datasheets for losses at core CF297 with volume of 138000mm3 per set. Losses aprox at 50khz are 111kW/m3.then aprox Pfe is 16W. so we are at 37W now.There will also be some eddy current losses, no idea how estimated them, what else i forget?I though i calculeted winding correctly =/

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 06, 2023, 07:01:15 pm
content: Quote from: MacIntoshCZ on October 05, 2023, 07:20:04 pmEver heard of skin effect ?If you are running 50Khz then each one of your 1mm wires only has an effective diameter of half that making a serious mess of your calculations!Like has been suggested above you might be better to sub out the transformer design and construction as 4Kw for a first design is a very big ask.EDIT Good point about proximity effect below, I had not looked at the picture but your winding window is very full meaning this is liable to be significant problem also

author: uer166
author_rank: Super Contributor
timestamp: October 06, 2023, 07:05:15 pm
content: It's hard to tell from photo, but presumably you have multiple winding layers. If true your losses are much worse than predicted by skin effect alone. Look up proximity effect. They're both one and the same really, it's just by convention one is caused by a magnetic field from the wire itself and the other by external magnetic fields as well.

author: jonpaul
author_rank: Super Contributor
timestamp: October 07, 2023, 05:25:19 am
content: OP needs to read , study SMPS magnetics and trsf design.Create a good winding sheet that specifies P/S wire/turns/insulation/wire specs/bobbin/core.Your Pcu loss is ONLY the resistive, and fails to account for skin/prox effect.Pfe is core matl/temp/flux dependent see spcs of cores, choose appropriate power matl.Finally Trise at hotspot will cause core losses to change (increase) and the 0.3%/deg c pos tempco of copper will cause the Rdc to increase with temp.Depending on losses and cooling you may get thermal runaway.Enjoy,Jon

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: October 07, 2023, 05:36:06 pm
content: Nice, most of the time i am suing 0.5mm wire for winding. Now i know why =D. Becouse i checked skin depth. Now i completely forgot that and fuck up... Though 1mm is ok...Thanks for pointing that out.Jonpaul, can you suggest yours favorite books in yours bookshelf? Thanks very much

author: jonpaul
author_rank: Super Contributor
timestamp: October 08, 2023, 05:35:19 am
content: MacCZ: Am overseas, away from my large electronic library and lab,from memory:Static electromagnetic devices Hunt & Stein 1963(used in CCNY courses 1965)https://www.amazon.com/Static-Electromagnetic-Devices-William-Thomas/dp/1014357209Magnetic Components: Design and Applications , 1984Steve Smith (old friend since 1980s)https://www.amazon.com/Magnetic-Components-Applications-Steve-Smith/dp/0442203977Litz wire: See Magnet Wires Supply (MWS) and Rubidue or excelletn catalogs and wire tables, app notes.Enjoy,Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2023, 06:48:16 am
content: Note that skin effect is only just barely sufficient when the current cancels out between adjacent strands.The derivation of skin effect, in fact assumes an infinite wire in space, absolutely nothing nearby. This gives an exponential decay where current flows mostly on the surface, and less and less at depth. (Actually actually, it's a Bessel function, for diameter comparable to depth; exponential is merely the limiting value as diameter --> infty. Or for a plane wave reflecting off a half-plane of solid metal. Bessel means there can be zeroes, or reversal entirely, for some sections within the wire, for D ~ δ. But it's a small difference from the exponential case, a few percent I think, maybe 10-ish % worst case, so it's okay to ignore this complication.)For neighboring strands carrying the same current, their fields pinch each other, and skin depth is reducedsharply. This is called proximity effect. This matters for multilayer windings (i.e. where one winding has multiple layers on top / below the opposing winding), and for litz cable.One consequence of proximity effect is that, as you increase strand count, the strand diameter must decrease much faster than proportional; more like quadratic. That is, if you need a wire of say 3δ diameter to get the required ampacity, it's not sufficient to use 10 strands of δ diameter, but you must use 100 strands of 0.3δ diameter. This is why litz is generally very fine, even for modest frequencies; for example, at about 100kHz, for ~10AWG equivalent, 36AWG stranding is about optimal, despite δ ~ 0.2mm, or 26-28 AWG being about optimal by itself.Conversely, if we need say 1.5, maybe even 2δ diameter wire, it might be better to simply bump it to 3δ (solid), or use very few strands in parallel, instead of full litz -- which will end up taking up as much space anyway, because of the wasted space due to strand packing plus enamel coating.Or at very high frequencies (for typical sizes of litz, some MHz), fine enough stranding simply isn't manufacturable, and round wire or sheet is more or less the only way to go. This is fine for inductors, as the Q factor generally goes as sqrt(F) so good performance is available, but it's a bit rough for transformers. On the other hand, shorter windings are needed, so although the wire might have much higher maximum current density (i.e. in the much thinner skin), good efficiency can still be targeted.Note that inductors have no opposing winding, so the same logic applies to them, with the entire winding counting against itself; the innermost layers get absolutely cooked if the inductor carries majority AC and fine stranding is not used.Which is a good reason to prefer even mild CCM operation for filter inductors. But obviously we have no choice when it comes to transformers; the only mitigation there is to interleave primary and secondary as frequently as is reasonable.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: October 08, 2023, 03:51:16 pm
content: To T3sl4co1l, very fine notes, agree.to MacIntoshCZ,The Hunt et al is a genral text for EE undergrad and postgrad.Basic magnetics, transformer and inductor theory. writen in 1960s, little on design or SMPS.Steve Smith is a physisist and EE. Consultant in materials, HV, radar, pulse, X ray PS and magnetics.Book has great chapters not seen elsewhere, eg HV pulse, foild wdg, lkg L, insualtion design.RE: Litz: design Fine 2020 paper at PELS/PESC/https://www.psma.com/sites/default/files/uploads/files/Litz%20Wire%20Practical%20Design%20Considerations%20for%20Todays%20HF%20Applications%20Jensen%2C%20Rubadue.pdfEnjoyJonPhoto: Magnes Stangenes with custom HV pulse transformer for defense, nuclear research.