7kwt LLC 45 khz transformer design issues - Page 1
Forum section: electronics / projects

author: axizep
author_rank: Contributor
timestamp: April 15, 2019, 08:46:00 pm
content: Hello dear engineering friends and Dave himselfIam designing 7kwt dc-dc converter that converts 750v DC into a 48V DC (140A) using LLC topology.working frequency is 45khz when at full load and up to 200khz when idle.and the problem is with transformer design.i have tried 6 different transformer constructions and all of them heat up very fast and severe.All other components of the converter are at 50C,which is very fine as for me. transformer gets hot up to 70-80C on the surface of the winding just in 10 minutes under full load.My first transformer designs where so called integrated designs,where resonant inductor was incorporated in a transformer using leakage inductance between primary and secondary.that design showed rapid heat build up on the surface of the primary which was facing secondary (primary and secondary where separated in space,thus big leakage inductance present). then i have read about proximity effect and made a conclusion that i have to make separate external resonant inductor and transformer should have lowest value leakage inductance as possible,because if i got it right,leakage inductance makes eddy currents and thus heating nearest layers of the winding leaving all the rest layers cold.All the transformer designs had air gap at the ferrite core (1mm). Main core is EE85 big peace of ferrite (860mm2 Ae).So i have proceeded to a foil secondary winding and finer litz wire (600x0.1 instead of 0.46x32). secondary winding is center tapped (half bridge synchronous rectifier) 2+2 turns 0.2mm foil 53mm wide.10mm2 for a 140 and 50% duty cycleand primary winding is 16 turns (8+8 in series) 0.1x600 litz wire,half of primary is outside of secondary and half of it is inside in it.that kind of design i have looked up in the infineon evaluation board of a 3 kwt llc converter appnote (https://www.infineon.com/dgdl/Infineon-ApplicationNote_Evaluationboard_3kW_dual_phase_LLC-AN-v01_00-EN.pdf?fileId=5546d462580663ef01582eb629b70118) you can check it on the 25 page.infineon design has even more secondary turns with a foil (5+5), it also has air gaps on all three limbs of ferrite,it also has external resonant inductor. but somehow it work on a fantastic 100khz+ frequency,where all the skin effect and proximity effect are very limiting.i have used spacer to pull inner layer of a primary on a bobbin away from a fringing air gap field. but that doesnt helped alot.transformer runs very hot,isolation start to boil after some working time.iam so exhausted from all those failed experiements,that i gave up and cant go on without a help. i have been struggling with this project for 13 months by now,half of it with a transformer.If there was some power electronics engineer among you that is familiar with a magnetics and transformer design i would really appreciate to get some help.Thank you for your patience,there was lots of words here to read (and sorry for my english).P.S. here some photos of my beast and transformer itself

author: MagicSmoker
author_rank: Super Contributor
timestamp: April 15, 2019, 10:04:09 pm
content: Are you interleaving the primary and secondary? Even just splitting the primary into two halves with the secondary in between dramatically reduces proximity losses. Also, note that even just a few turns on a new layer counts as a complete layer with respect to proximity loss. It is often the case that dropping down 1-2 gauges in wire size to squeeze all the turns into fewer layers results in lower total losses, even though the DC resistance of the winding increased.

author: TimNJ
author_rank: Super Contributor
timestamp: April 15, 2019, 10:11:26 pm
content: I'm having a hard time determining...1.) What youthinkthe problem might be related to2.) What you've tried so far, and what exactly the results have beenAlso, if you have any spreadsheets or design work that shows how/why you selected the current the design, that might be helpful. Maybe a transformer drawing?For now, just some thoughts.1.) Peak flux density? Any way you are saturating it? Maybe need more turns and a bigger air gap? (I don't think saturating LLC transformers is usually much of an issue, but it is definitely possible)2.) Did you do any calculations to determine if 0.2mm foil was optimal? (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.916.5488&rep=rep1&type=pdf)

author: axizep
author_rank: Contributor
timestamp: April 15, 2019, 10:45:20 pm
content: Quote from: MagicSmoker on April 15, 2019, 10:04:09 pmto be precise,what i have done is: i have tried 2 different methods (dont know which one could be called interleaving,but i guess one of them could be interleaving)my primary turns number in both cases was 16 turns.in case 1 i have winded 16 turns of litz wire on a bobbin,then i have wounded 2+2turns (all the secondary foil turns) and ontop of the secondary another 16 turns of primary. both inner 16 turns and outer 16 turn where paralelled otside the transformer.in second case i have winded 8 turns (half of a primary) then a whole 2+2 turns of foil secondary then on top of a secondary the rest of a primary (last 8 turns).

author: axizep
author_rank: Contributor
timestamp: April 15, 2019, 10:54:06 pm
content: Quote from: TimNJ on April 15, 2019, 10:11:26 pm1)peak flux density is around 150mTl far away from saturation. ferrite is only slightly warm.2)about 0.2 foil thickness- i have tried 0,3mm also that didnt helped much. i wanted to try 0.2 as i thought it impedance could be smaller than thicker one because of proximity effect. but it didnt show any effect.a similiar transformer drawing could be seen on the infineon appnote of a 3kwt converter on a 25 page.i have tried many designs,but none of them where cold,so iam asking if somebody knows how to design it right. basically i need turns ratio=8 and center tapped secondary (for a half bridge rectifier).sorry your link says No document with DOI "10.1.1.916.5488"

author: jmelson
author_rank: Super Contributor
timestamp: April 16, 2019, 03:06:55 am
content: Does it run hot at no load, or only under near-full load? So, for 7 KW at 750 V, the primary current is about 10 A. Can you run 10 A DC through the primary for 10 minutes and check how hot it gets?So, you used Litz wire in the primary, but tape in the secondary. Can you tell if it is the primary or secondary getting hot?I'm wondering if the secondary is developing Eddy currents, with that massively wide foil (53 mm)? Possibly splitting the tape into several strips and then joining the ends would reduce the eddy current. You also should compute the DC loss in the secondary foils, 0.2 mm thick seems kind of thin for 140 A.Jon

author: jbb
author_rank: Super Contributor
timestamp: April 16, 2019, 05:57:57 am
content: So, you’ve spent a bit of time on this. A couple of diagnostic thoughts:- can you get a thermal camera to observe how it heats up?- if you’re really out of money, you could try sticking the tape from a label maker on the transformer. It (permanently) changes colour with heat and could be an option (suggest videoing it) and can be used as a poor engineer’s thermal camera.- if you’re doing a no-load test to check core losses, I suggest disconnecting the secondary and manually tweaking the switchong frequency to get the right operating frequency and peak flux density (because the no load frequency is higher which reduces flux density).Have you got access to a big DC supply? You could try pushing DC currents equal to the AC RMS current to check if it’s a simple resistance problem. (Note AC resistance will be higher than DC resistance.)Finally, if you have an intractable problem, you can split your one big 8:1 transformer into two identical smaller 4:1 ones. Connect the primaries in series so that they share current equally, and the secondaries in parallel so that they share voltage equally.

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 07:32:55 am
content: Quote from: jmelson on April 16, 2019, 03:06:55 amwith no load condition there is no heating in the coils at all!transformer runs cool. Ferrite runs cool also at all situation :with full load and no load.i have computed dc current density. at most it was 7.5A/mm2Quote from: jbb on April 16, 2019, 05:57:57 amI will try to get some termal imaging,but it would be a problem for me. maybe termal printer paper test will do,ill try that.About core losses-ferrite runs cool (45C) under full load,no problem with it.about a dc check i guess 7.5A/mm2 is around acceptable current density,so it has to be only warm.about deviding into two small transformers,that is problematic,i need to keep design's weight and simplicity down.

author: jbb
author_rank: Super Contributor
timestamp: April 16, 2019, 08:09:33 am
content: Sounds like your core is OK then.The thermal label printer stuff is a polymer and has adhesive on the back. I think standard thermal paper wouldn’t work because it wouldn’t make good thermal contact.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2019, 08:49:59 am
content: Do you have a drawing describing the transformer windup?What is the foil strip picture? For that matter, what are all the pictures -- could you put some description on them? I can't really make sense of the overall picture, could you provide close-ups of relevant areas? Or a block diagram showing what's going on?Tim

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 09:36:49 am
content: heres detailed pictures of a transformer and diagrahm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2019, 09:46:18 am
content: Ok, so you have several turns of the secondary on top of itself? That shields the inner turns from getting the image currents from the primary. Instead, current crowds to the edges.I assume the copper cross section is adequate for the DC requirements?Try using four sections of 1 turn each. That would be below the first primary section and above the last, I guess. Though that still leaves two overlapping turns in the middle. Well, that'll help at least. You could add one more primary section to get four single-turn secondaries.With three primary sections, you won't need as many turns per section, so you might then also consider slightly thinner Litz cable, either wiring the sections in series-parallel (well, in 3 sections, a combination wouldn't work, they'd have to be all series, or all parallel, wouldn't they..), or using pairs of cable in parallel, laying flat (bifilar), basically to give a rectangular cross section to the cable so you maintain a high fill factor while reducing the number of turns in each section.Tim

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 09:47:55 am
content: sorry for the photoes,because i didnt took a 8+8 turns of primary on camera (i have disassembled transformer right now for the next design to try)i can show how 16+16 primary looked (in parallel) but it might confuse you.

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 10:30:51 am
content: Quote from: T3sl4co1l on April 16, 2019, 09:46:18 amThank you Tim for your attention!Dc current density for my secondary is 7.5A/mm2 for primary around 5A/mm2I have secondary 4 turns in total (copper foil strip 0.2 thickness and 53mm height. it has around 70cm length) near the center of that copper strip i have center tap for a syncronus rectifier.secondary coil consists of that 4 turns (2 turn-center tap and another 2 turn) and was allways like that.meanwhile i tried different primary configurations.all of them had 16 turns in total.so transformer=8.I really wonder how infineon design works with 5+5 turns secondary and 100khz + freq.I have doubts that i will fit so many sections as you propose.transformer window is very narrow and allmoust tightly packed by now.also i have to have good insulation because of 750v.so i lay down some insulation sheets of some kind of fibeglass tape.Tim have you tried similiar designs of transformer and similiar power levels?have you encountered those heating problems?what best solution did you found?Did you had problems with a fringing field from an air gap?

author: jbb
author_rank: Super Contributor
timestamp: April 16, 2019, 10:56:24 am
content: Quote from: axizep on April 16, 2019, 07:32:55 amI respect that. More parts often mean more problems. But you still might find it easier to finish the design with 2 smaller transformers. You'd need half the copper in the secondary windings which makes life easier, and the surface area to volume ratio is higher, which aids cooling.Quote from: T3sl4co1l on April 16, 2019, 09:46:18 amHmm. In general, yes. On the other hand, I think that axizep is using a centre tapped secondary to do half wave rectification. That would mean that on any given half cycle only 2 of the turns will have significant current flow. So not as bad for proximity effect as 4 turns... Fringing flux could still be a b*stard, though.One other thought: the centre termination of the secondary looks a bit messy. It looks like there might be big lumps of solder & Litz wire in the centre tap part. Is this a standard method (I've never done a foil winding...)? Does anyone have other recommendations for terminating foil windings?Ultimately it looks like unless someone with more experience (ahem, Blueskull?) can chip in, you're stuck guessing until you get some more insight into what exactly is generating the losses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2019, 01:49:27 pm
content: I haven't done exactly this. I've made foil-based transformers in other sizes, and made converters in comparable power levels. (In particular was a 5kW inverter for induction heater service; it ran at 20-400kHz depending on configuration, and used a T107/65/25-3F3 core with Litz for primary and secondary. The Litz cable was easy, no problems...)Last one I did with foil, was... this:(don't mind the notes about finding the right gap), for a little converter,120-800V DC output, about 50-100W capacity (12V input), dual output flyback topology. (The outputs are wired in series, so that doesn't actually amount to anything in the end; it does make the waveform far cleaner, though -- the secondary is effectively a balanced circuit.)The windup is in order: 1 turn primary, 15 secondary, 1 pri, 15 sec, 1 pri. Don't have any pics of the transformer under construction, but what I did was, I used copper foil tape for the turns themselves, and made connections by soldering 20AWG bare copper wire onto the ends. Secondary is 28AWG. This took up very little space on the bobbin, only needing ~1kV functional isolation (since the output is common ground). I happen to have a bunch of these core sets, so it's rather oversized, or, the windup is rather disproportionate. It doesn't seem to heat up much under normal load conditions though. Just a simple module for lab use, no big deal.The obvious downside to this construction, is the thin strip of wire joining together what's otherwise a beastly thick foil turn. Preferably, you'd keep the foil going, by folding it 90° so it goes up and out of the bobbin, so the next section can be wound, and so on.That would still concentrate current around the foil edges, because you're trying to make a 90 degree turn -- the current has to bunch up somewhere. In this case, it's bunching up under the primary, because of image currents, gathering on the fat outside corner of the bend, and getting carried up the terminal edge and out of the bobbin. But this at least happens only at the ends of the turn, not over its whole length.)It also doesn't help that, every time you start/end a section with foil turned sideways, the available winding area is reduced by the foil thickness. Preferably, you have a wide winding area. But that implies you need a lot of room to turn the foil through 90°, so the core cross section should be rectangular as well, so the turn can be made on a wide face of the bobbin.The next-last foil winding I made, I did actually stack two foil layers, but I haven't tested that one yet at full load, so I'm not sure how good its losses are. (That was a necessary step, because the two turns are the ends of a CT winding -- as with your case. Overlapping foil gives low leakage between ends of the CT winding, necessary to minimize snubbing of those switches. Which in your case is the synchronous rectifier; in mine, it's a PP primary, same thing.)Arguably, this should still be acceptable, as the two halves of the CT winding draw current alternately, so there shouldn't be proximity effect where both turns (on top of each other) are active at the same time -- except only for the currents due to leakage and snubbing (if applicable), which are a small part of the total.Actually, that may be a much better example; unfortunately, I don't have any photos or drawings handy. Perhaps I can describe it. The windup was thus:- Start with T87 toroid- At 90° intervals around the toroid, apply single turns of copper foil tape. Insulate with polyimide tape, leaving the ends of each turn exposed for connection. The connections shall be exposed on one flat face of the toroid. (So, not the inner or outer cylindrical surface, the face between them.)- Apply single turns of copper foil tape on top of the existing turns. Cut the ends a little bit shorter, so they do not overlap the previous layer's connections.- Using copper tape, solder and polyimide tape, construct a 90 degree turn, so that the four connections from each pair of turns becomes a 4-layer laminated (flexible) connection, standing up perpendicular from the toroid plane, and aligned radially.- Wind 24AWG over top the foil windings, and secure with tape.- Unfold the laminated connections, and connect the foil turns in series-parallel to make a 2+2CT primary winding. Each half of the CT winding is made of the two stacked foil turns in series; two of these are further connected in series to make a 2+2CT primary half. Two of these are connected in parallel to give the full primary winding.- The primary wiring connections are made in laminated foil and tape construction again; primary leads is made with a 3-layer flexible connection.This is...probably a nightmare of a description if you have aphantasia.Ah well...Anyway, the important part, applicable to your case, is this: to get low leakage between ends of the secondary, you should really use two foil strips stacked (instead of just one), and wrap that around for two turns. Join the end of one with the start of the other, and that's your CT. Except, because this would have about as much current crowding as it is now: instead of making 2+2 turns in a single section, make a single 1+1 turn pair for each section, and use two sections, with primary sections around them (PSPSP if possible, but as you said, you don't have much room to do that; SPS may actually be better here, then?). Then wire the individual turns in series, to construct a 1+1:1+1 CT winding, where the 1's from opposite sides are closely coupled. That is, if we number the turns a, b, c and d, with a+b being one section and c+d being the other, connect them as a+c:b+d.So the full windup would be:[a b] [16 turns pri] [c d]I would also be tempted to suggest a planar transformer. This should be quite reasonable at the low impedance here, but cooling may be a challenge. I've seen designs advertised in this power range before, so it's definitely possible. Downside, lots of NRE and fab time, so if it turns out it sucks anyway, or you end up needing to change the ratio or something...Tim

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 04:11:28 pm
content: Tim! its very interesting what you do! please tell me have you been experiencing problems with an air gap fringing field? how can one estimate its influence on the winding impedance?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2019, 05:32:48 pm
content: Heh, I've managed to avoid fringing effects most of the time...Estimate... how would you estimate it, anyway? The gold standard would of course be simulation. This is perfectly doable, but requires specialized tools.I think I would... model the gap by transforming the corner of the core into a pair of complementary current loops carrying about as many amp-turns as the windings. These currents are positioned at the corners of the core piece, pointing in opposite directions around the core, so that they cancel at a distance, but the field is quite intense up close. Then, say we put a sheet of metal near the gap: the induced eddy current is equivalent to a differential microstrip over ground plane situation, so we can think in terms of transmission lines and couplings, which are easy to calculate (say for arbitrary cases, use ATLC2). When the plane is closer than the distance between wires (the gap length), coupling is stronger to the plane, and therefore most of the current is induced there. Say you have 100 amp-turns on the core: at a distance equal to the gap separation, that might be around 50At induced in the plane (or maybe it's half that distance, I don't know), which implies you need a pretty stout piece of metal to have low losses (in particular, something which magnetic fields can permeate: Litz), and if not, you'll get that much current flowing on its surface, probably having big losses as a result.So we can, at this point, also consider it as an application of induction heating, noting that the induced current is a shorted turn against some fraction of the total amp-turns, and we can calculate the L and R of this based on the geometry and material properties.Hmm, now... amp-turns isn't necessarily theactualwinding total amp-turns. You'll get intense fringe heating in a transformer that has net ~0 amp-turns (i.e., primary N1*I1 = secondary N2*I2). The trick is to also find the amount of flux that is fringing, and convert that to amp-turns in free space. So this depends on the gap width to height ratio, and flux density is converted to amp-turns per length with the magnetic constant µ0.This is a very rough and off-the-cuff approach but I think it could be developed to give intuitive results within a useful ballpark, say 20% accuracy on eddy current losses. I'm not sure how practical it is, in terms of "intuitive", while needing so many steps. You'd want to do a bit of analysis and see if it can be simplified down to a transformer equivalent model, and what variables are needed to fill that in (namely, a transformer with an effective shorted turn, and what coupling factor and resistance that turn has), and how to derive those from geometry.In the mean time, if you can keep the gap thinner (bigger core, fewer turns -- also helps with windup if you can cut the turns in half, say), or keep the windings more than, say, 2 gap lengths away from the gap, that will help greatly.Increasing frequency also helps with volts/turn and core size. Downside, you might have to use SiC MOSFETs instead of IGBTs (I assume you're using IGBTs), which is a much bigger change than the transformer alone. No idea if this might be prohibitive for development or cost reasons.Tim

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 06:31:34 pm
content: Hey,Tim,Thank you for your thoughts!i will comment starting from the bottom thesis.yeah,i have started thinking about Sic fets and rising frequency.that would,of course,lead to designing completely new gate drivers,with optoisolators instead of gate transformes,and dedicated isolated power supply with +15v -5v outputs. but i thought that if iam experiencing such overheat presumably due to the fringing/proximity effect and other hf effects which maybe iam not aware of - problems on higher frequencies will worsen all those problems and make overheat problem more significant.yeah,i guess its possible for me to avoid fringing field from a gap,if its enough 2 lengths of air gap to avoid those effects.Also i wanted to ask you about a connections to the copper foil. i was thinking about a cross section of transistors for example. they have 0.5mm2 leads,but datasheet claims 150A drain current.i guess its possible to handle this kind of current density,maybe its not that bad idea to have thin copper stripes soldered to the foil on the edge, 90 degrees angle.alowing for a connection to leave the transformer,and outside litz will be soldered to it right on exit. do you think its acceptable if on few mm length there will be reduced copper cross section?iam asking,because i have doubts about my method of connection to foil,as it could raise impedance of the whole secondary,because of thickness of those connections (field will not penetrate such thick layers with those solder tabs and will build up at those areas and lead for heating)?Also wanted to ask you if you can say something on paralleling primary's inside and outside of secondary.can eddy currents circulate on such contour of closed loop of wire and build up excessive heat? or is it preferable to divide primary into series connected portions?

author: jmelson
author_rank: Super Contributor
timestamp: April 16, 2019, 07:01:38 pm
content: Quote from: axizep on April 16, 2019, 07:32:55 amOK, that helps a lot. So, it seems that eddy current is NOT the issue. Now, I think the obvious problem is just primary resistance. Note that Litz wire has a lot less copper area than plain wire, so you have to increase the wire size a lot. I think that is what you are running into, here.Possibly, since the ferrite stays cool, you have too many turns. So, you could also try reducing the turns of both primary and secondary by a factor of two.That will make the core run hotter, but will cut primary resistance by about half.Jon

author: jbb
author_rank: Super Contributor
timestamp: April 16, 2019, 07:19:48 pm
content: Had a thought about fringing flux. If it turns out air gap fringing is the cuplrit, and you can’t design around it, you could look into a distributed gap core matrial.

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 08:21:23 pm
content: Quote from: jmelson on April 16, 2019, 07:01:38 pmHey,Jon!my primary has around 5A/mm2. and my first designs was with reduced turn count as you said. ive made a transformer with 8 turns primary. ferrite got 95C hot just in 4 minutes at 55khz freq. i have calculated flux density,it was around 250mTl. i was worried about how hot it can get,so i decided to increase turn count by a factor of two to a 16 turns.iam thinking this idea also right now and before to make 8 turns once again and to raise frequency. maybe halfbridge igbts will be replaced by mosfets because acceptable flux density might be at 100khz with my 860mm2 ferrite cross section and 750v voltage on halfbridge.so that option i will definately use,if i find no solution on low frequency setup (45khz).it makes sense,because my first setups with 8 turns didnt had foil as secondary winding.so its worth trying a new way.Also i have idea to wound a primary in between a secondary foil. to split primary in 4 parts,each 4 turns as 4 turns are the secondary turn count.and connect all those 4 parts of primary in series to get 16 turns. what do you think about that?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2019, 10:07:27 pm
content: Quote from: axizep on April 16, 2019, 06:31:34 pmYeah, with the same gap and foil geometry, higher frequency wouldn't help any; aside from the possibility that fewer turns may yield an opportunity to get better core-to-foil distance.So the other big opportunity for fringing, is changing the core geometry to something more favorable. Don't know how many options you have in this size, or without significantly changing your mechanicals...QuoteNote that they say "pulsed" or "die limited", when they do this! It's ridiculous, and so are power ratings (they're measured in a bath of boiling Freon), but that's what we've got.You may have to dig around in appnotes to find these admissions. It's not always obvious...So, the prepared engineer understands that this is bullshit marketing, and wisely uses a few devices in parallel instead.QuoteI'm not too worried about it, honestly. You can't do any better, without going to something much harder to fabricate -- say, litz cable that starts out in cable format, then widens out into a ribbon format for the winding, then back again.Such things exist -- offhand, West Coast Magnetics makes a braided flat construction like that (patented? I forget). Probably not as available as you'd like, though...You can't avoid solid connections, sooner or later. Just as long as they aren't much of the total length, is what really matters.Or if you can make the total length super short (like, a single turn), there's not much metal to generate heat in the first place! The power density might be vastly higher (say you ran it at 1MHz and crammed it into a 5cm cube?), in which case you'd need water cooling, but, heck, it might very well prove more efficient. (Or, just the transformer might, if not the inverter as well.)QuoteYes, this is a possible current path, as is currents between strands of a Litz cable. As long as the EMF difference is statistically insignificant, it's no problem -- or at least better than the alternative (a single section winding, or solid cable, etc.).Quote from: jbb on April 16, 2019, 07:19:48 pmYes, this is an option. Another option might be to use 'I' pieces and build your own C or E core with them, placing the gaps wherever is suitable. For example, using an EI core might be interesting, as that puts the fringe field off to the corner, where it can't intersect much (if any) foil. An EI core but made out of all 'I's (of a few sizes), would have two gaps around the bobbin, each being thinner as well.That also potentially gives more freedom, compared to the relatively few shapes available in these large sizes.Yet another option would be simply winding the whole thing out of Litz cable on a toroid, in which case you don't have to worry about overlapping foil turns crowding themselves out. This would be a really good application of wide braid, instead of round cable, since with 2+2 turns, it's not like you're going to get turns evenly distributed around the ring.Tim

author: axizep
author_rank: Contributor
timestamp: April 16, 2019, 10:31:49 pm
content: Tim,what do you think about idea to wound a primary in between a secondary foil. to split primary in 4 parts,each 4 turns as 4 turns are the secondary turn count.and connect all those 4 parts of primary in series to get 16 turns. it becomes some kind of bifilar windnng,but with a foil instead of two wires. this idea is worth trying?

author: jmelson
author_rank: Super Contributor
timestamp: April 16, 2019, 10:43:12 pm
content: Quote from: axizep on April 16, 2019, 08:21:23 pmOK, so the thing that limits you is the secondary, either 2 turns CT or 4 turns CT. And, so it looks like the 8 turn primary is too few for the core.QuoteWell, that may help, as the higher frequency would reduce the peak magnetization. But, it will also increase losses. So, depending on the loop characteristics of the core material, it MIGHT help, or might not.I still think that your primary doesn't have enough cross section. First, Litz wire has a lot more enamel and not a whole lot of copper that regular wire.Due to the complex magnetic interaction of the multiple strands, the wire doesn't exactly follow the skin effect rules for a single conductor, but can still have some channeling of current due to skin effect. Are you absolutely sure you know the correct cross section of this Litz wire? Just looking at your photos, I'm just a bit skeptical about the current densities that you give. And, of course, if some of the strands are not fully stripped of enamel at the ends, then some of the strands could have NO CURRENT in them, raising current in the remaining strands.Just a few of the things I can think of. I did some work with Litz-wound inductors some years ago, and had problems with getting them to run at proper temperatures.Jon

author: axizep
author_rank: Contributor
timestamp: April 17, 2019, 09:14:10 am
content: Quote from: blueskull on April 17, 2019, 02:40:56 amThank you,Blueskull,that you responded so quick!I feel pity,because i dont have thermal imaging,neither friends with to borrow it. but core is definitely only warm (45C or so).its the winding that is heating up like crazy.I have bought tektronix a6303 current porbe,bu its faulty (has problems with dc offset,guess its hall sensor fault). so i do current measurements with a current transformer or sometimes with LEM current sensor. ill attach waveform from a usuall current transformer,which i have integrated into the converter for controlling overcurrent situatiuons.yellow trace is gate drive,purple trace is one end of the secondary,and blue trace is current transformer.Also i have witnessed zvs,when i was watching for primary halfbridge node voltage in respect to a gate drive volltage.Resonant inductor was overheating few designs before,but i have installed bigger core for him,bigger gap increased turn count and used fine litz wire,and now i have no problem with him now,its fixed and working fine.About a pushpull,iam very surpirsed to know that pp topology could make a zvs (what stops current reflected from a secondary to charge a drain of a transistor at deadtime,which is about to open?) .but pp will not suite me because i have 750V voltage on my halfbridge ,so pp will be give 1.5kvon drains,which will be the problem. also iam very surprised to know that there is full bridge pp topology,i could not imagine it.

author: moffy
author_rank: Super Contributor
timestamp: April 18, 2019, 07:14:52 am
content: I was looking at an article about a 5kw/10kw resonant converter:https://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/04448897.pdfand they seem to have put a lot of effort into cooling the transkormer:1. 90% efficient gives 700W of losses for Tx2. 95% efficient gives 350W of losses for Tx3. 97% efficient gives 210W of losses for TxIt is going to get hot under load. Maybe you need some heatsinking like the above article.