SMPS quadfilar transformer construction - Page 1
Forum section: electronics / projects

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 03:11:46 pm
content: Hello all, I am designing an ETD59 running at 25Khz with two secondary's that may be connected in series or parallel. As the primary will fit in a single layer I propose to split the secondary's either side of it. To cover the current density and skin depth the secondary's require two wires each.Now comes the conundrum, I am concerned if the two secondary's are wound as two wires (bifilar) in two layers there may be circulating eddy currents when they are connected in parallel so I am thinking of winding them together in quadfilar (split across the two layers) however that is going to be really hard to do especially on the outer layer wound over a lumpy primary.Am I being to concerned about eddy currents circulating in parallel connected secondary's, has anybody designed a similar transformer and how did they deal with the issue ?

author: sandalcandal
author_rank: Unknown
timestamp: May 17, 2021, 03:39:47 pm
content: To clarify, the wire you are using is single core enamelled copper wire (magnet wire)?I'm not sure what eddy currents in a parallel configuration you are thinking of. Are you concerned current at the termination of the parallel pair will flow back into one of the secondaries? A counter example to your situation would be Litz wire where there are sometimes essentially hundreds of "secondaries" connected in parallel. Also I'm not sure spelling out the maths/physics would make it particularly clearer but I believe a physics analysis should show matched windings are simply parallel sources as one would expect and any mismatch between windings will result in "shorting" extra turns to the lowest effective turns in the paralleled windings.

author: fcb
author_rank: Super Contributor
timestamp: May 17, 2021, 03:53:44 pm
content: Never worried about or had issues with eddy currents or unexpected losses with parallel secondaries, even past 100KHz.ETD59 is quite big, what sort of diameter is your secondary - are you expecting uneven windings?

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 03:58:32 pm
content: Hi SandicalYup, magnet wire, so two secondary's of two parallel wires each, each secondary needs a whole layer.So build-up 1 : Layer 1 secondary 1, Layer 2 Primary, Layer 3 Secondary 2. This is the one where I worry about connecting the two secondary's in parallel.Build-up 2 : Layer 1 1/2 of secondary 1 & 2 in quadfilar, Layer 2 Primary, Layer 3 1/2 of secondary 1 & 2 in quadfilar.I am sorry I did not do a very good job of explaining it in the original post. Build-up 2 is exactly akin to using Litz and I have no worries about it electrically but physically I think it will be a pig to (hand) wind.However I am concerned build-up 1 will have unequalness in some way and result in nasty circulating currents when the two secondary's are connected in parallel.Or am I just overthinking this ?

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 04:03:06 pm
content: Quote from: fcb on May 17, 2021, 03:53:44 pmHi Fcb, Layer 3 will be about 29mm diameter and the problem I foresee is a bit lumpy due to the primary underneath whereas layer 1 (1st secondary) will be on the smooth former). Thank you for your comment, how were your sections/layers arranged ?

author: sandalcandal
author_rank: Unknown
timestamp: May 17, 2021, 04:05:03 pm
content: Quote from: fourtytwo42 on May 17, 2021, 03:58:32 pmFrom a theoretical perspective I can't think of any way that should cause problems. In theory, the outer winding will have greater diameter thus longer wire length thus higher ESR and will take a slightly lower share of the current under load but unless the primary is particularly thick I think it will be negligible. The underlying EMF/open circuit voltage should stay the same however and as long as (effective) number of turns is the same there will be no "leakage"/mismatch current. (penfold below makes a point to be careful of accidental half-turns)That being said I haven't actually built and/or fully analysed a transformer of your description so I can't say with100%certainty. There might be something I've missed that may cause issues. Hopefully some more experienced people will chime in.

author: penfold
author_rank: Frequent Contributor
timestamp: May 17, 2021, 04:06:34 pm
content: In terms of any significant errors that could cause problems, i.e. whole numbers of turns missing, I'd be concerned about the production process if this happened with 5 turns, and at 500 turns the voltage/turn to total resistance of the winding would minimise the problem itself. So, wouldn't usually worry in that regard.However, it is worth being a little careful about which pins you route the windings out to avoid half-turns worth of errors, can add a little extra leakage inductance you'd be better without but probably wouldn't notice, but would ever so slightly imabalnce the current sharing

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 04:18:40 pm
content: Thank you all for your very helpful replies, the primary is about 2.5mm thick (0.8mm*7), the secondary's are 29 turns each.I like the comment about bobbin pins, easily overlooked, thank you. I feel considerably relievedI should mention I have never built a ferrite transformer with parallel secondary's before

author: fcb
author_rank: Super Contributor
timestamp: May 17, 2021, 04:20:18 pm
content: Quote from: fourtytwo42 on May 17, 2021, 04:03:06 pmWe tried winding both secondaries at the same time - worked but was awkward, the we tried winding the second ontop of the first - also worked. There was a small but measurable difference in secondary ESR, but not enough to effect the overall design, but we weren't really stuffing the bobbin. The biggest risk - as Penfold says - is unequal secondary winding numbers.25KHz is fairly low - assuming you have some sort of performance/efficiency target to hit? Do you have the budget for real Litz?

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 04:33:04 pm
content: Quote from: fcb on May 17, 2021, 04:20:18 pmFairly relaxed 0.6 fill factor on this oneAhh REAL LITZSomething I find myself drooling over in catalogues but sadly not available in prototype quantities otherwise I would instantly increase frequency, so I just spend my humble life designing without it

author: fcb
author_rank: Super Contributor
timestamp: May 17, 2021, 05:32:26 pm
content: http://www.osco.uk.com/products/cable-litz-wire/litz-wire-winding-wirehttps://www.hi-wire.co.uk/acatalog/litz_wire.htmlOr if you just need to prove it and don't mind the hifi prices:https://www.wires.co.uk/acatalog/litzwire.html

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 05:41:03 pm
content: Quote from: fcb on May 17, 2021, 05:32:26 pmThanks for the links, I have looked before a few years ago but perhaps they are more flexible (pun) now! Any thoughts on a Litz equivalent to TIW again in small volumes (I am trying to work out if the outer servings do the job) ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2021, 06:10:09 pm
content: I wouldn't worry about it. The mismatch between two wires over that many turns will be small. I would be more worried for a shorter winding -- which would also then have problems with leakage so I'd be looking at a foil construction instead.If you have the winding area, you can always use twisted pairs (or quads or whatever). (That is to say, the fill factor will be lower.) Downside, you have to ohm them out at the end(s). Which eh, it's fine for one-offs?Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 17, 2021, 06:45:47 pm
content: Quote from: T3sl4co1l on May 17, 2021, 06:10:09 pmThanks Tim, I am by nature a worrierbut I also have the determination to get off the potI thought of the twisted solution but the window utilisation was getting poor and I was not to keen on an even number (4) in the twist so I am very happy you guy's all helped me understand what I was worrying aboutYes I also figured the buzz it out bit that I will now not have to do!I have been musing about foil too as a solution to the frequency problem and lack of Litz but typically my voltages are to high and consequentially need quite a few turns, the thoughts of long lengths of copper foil and mylar insulation running amok during the winding process, picking up bits of sawdust from the floor and bench put me off

author: fcb
author_rank: Super Contributor
timestamp: May 18, 2021, 09:21:03 am
content: Get to it fourtytwo42! Stop procrastinating and wind yourself a bloody prototype. Your next posting should be a picture of the transformer.

author: fourtytwo42
author_rank: Super Contributor
timestamp: May 18, 2021, 02:59:22 pm
content: Ok the bobbin should arrive this week but I am clean out of 0.8mm wire so that's a few weeks and I am waiting for TIW quotes, loif in the countryside, nothing happens quick around here but to be sure I will keep you posted, now back to sleep while I wait

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 27, 2021, 12:29:24 pm
content: Two and a half months later we finally have a transformerThe major holdup was the TIW, one lot arrived and seemed fine until actually in the wind I discovered an unexpectedly small number of turns per layer and on close inspection discovered the insulation was 0.125mm thick instead of 0.1mmThe cores were a good laugh too, I had to wait for three shipments altogether with much supplier discussion as they kept arriving smashedSo another zzzzzzzzzz time later with the correct TIW winding began again, a layer of tape over the 1st primary layer helped to protect it from the heavy secondary wind. 2 layers of tape were required over this to prevent the next secondary layer dropping between the primary turns.The build-up is a little weird as although the primary is a single layer both secondaries are a layer and a half each (sods law in action)So layer 1 is S1, layer 2 is primary, 3 is S2 and 4 is the remaining halves of both S1 & S2. Resulting leakage 450nH.This is an ETD59, gone up a core size as it was to tight on an ETD54. The two secondaries work very happily in parallel that if anybody can remember was the original questionI also chose to use flying lead terminations as this is a chassis mount transformer rather then pcb and the primary is to big for the little bobbin pins anyway.Ohh I should mention I sweated the slot (pin) numbers carefully to ensure the two secondaries anded up with exactly the same number of fractional turns.Lot's of pictures ...........

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 27, 2021, 12:35:16 pm
content: The rest

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2021, 01:54:09 pm
content: Lisztomania wire?

author: fcb
author_rank: Super Contributor
timestamp: July 27, 2021, 02:06:15 pm
content: Nice job.Have you calculated/measured the difference in resistance between the two secondaries. Presumably you've got plenty of margin to cope with more current contribution from the inner secondary (they are in parallel if I remember?).Although if the inner secondary gets warmer it's resistance will go up, might balance out nicely.

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 27, 2021, 03:15:12 pm
content: Quote from: Terry Bites on July 27, 2021, 01:54:09 pmHand-madePrimary is 7 strands of 0.8mm ecw using electric drill & bench vice.

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 27, 2021, 03:19:56 pm
content: Quote from: fcb on July 27, 2021, 02:06:15 pmThank youI must confess I have not attempted to measure them, I could probably calculate it from the length differance BUT I prefer your last point, the advantage of copper having a positive temperature co-efficient vs silicon, no thermal runaway here.Only running <5A/mm2 so plenty of margin indeed

author: sandalcandal
author_rank: Unknown
timestamp: July 27, 2021, 05:23:41 pm
content: Beautifully done!Would be interested in seeing full performance numbers but it looks quite good already.

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 27, 2021, 05:57:02 pm
content: Quote from: sandalcandal on July 27, 2021, 05:23:41 pmThank you kind Sirbut I dont think as neat as some of yours!It's OK on the bench but thats only capable of ~200W so have to wait till its outside again for a full power test (it's a freaky kind of solar converter), but it's not just that transformer that needed upgrading so I have some more work to do on cooling (uprating again) and a new APU to power the uprated cooling hohoho don't we like lots of work to doPicture is primary ends at ~100W unfortunatly using ubiqutios long ground lead probes! I decide to put up with a bit of ringing to reduce snubber power, the EMC filters stop it getting outside the box anyway.

author: sandalcandal
author_rank: Unknown
timestamp: July 28, 2021, 12:18:50 am
content: Quote from: fourtytwo42 on July 27, 2021, 05:57:02 pmCareful now. Ringing can result in more losses the semiconductor switches and other parts of the circuit with greater total loss. Even though snubbers "lose" energy there can be overall efficiency gains by reducing ringing.

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 28, 2021, 02:01:43 am
content: Quote from: sandalcandal on July 28, 2021, 12:18:50 amYour quite right but heat in the snubber is a real nuisance being on the pcb near capacitors etc and also contributes to poor efficiency at low power, however I will bear it in mind for another lookOf course the real level of ringing can only be estimated with such poor probing with little chance of improvement at such voltages. The secondary is even worse, any attempt to measure that and the laptop mouse freezesmaking it impossible to control the scope (Owon VDS1022I) Ohh for the days of my trusty TEK465 and scope cameraI end up estimating the secondary using LTspice then waving a shorted scope probe near it to verify

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2021, 05:33:50 am
content: Note that that power goes somewhere, regardless; the only complete solution is not to excite the reactive elements in the first place (not an option), or have them dump somewhere useful (as in resonant topologies). A snubber is a bit blunt, as it has to take some power from the waveform as well. Heh, I suppose you could make a series resonant trap, but still at expense to waveform cleanliness as it necessarily has a longer and taller step response.Sounds like you have functional EMC problems already, so, better get that fixed first...Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 28, 2021, 10:31:26 am
content: Hi Tim,Thank you for your comments.Well that ringing is with both ends of the primary open apart from the vcc/gnd clamp diodes and snubber so the leakage is being dissipated in the snubber and trafo via circuit capacitance.I don't know of any hard switched converter that doesn't have EMC problems in the raw state, that's why they all have filters (except certain far eastern manufactured examples where there linked out). I wouldn't say I have one to fix, more it's something I always keep a beady eye upon. The sharp turn-on/off is worse from an EMC perspective than the remaining ringing providing it is controlled to a degree, the degree being the tradeoff between snubber power and size of filters (all IMOP of course.Roger

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 10, 2021, 09:00:35 am
content: Status update, runs as cool as a cucumber at 650W that limit being imposed by inadequate cooling of the rectifier. Using STTH512FP (x4) and at those voltages not much choice to reduce VF except STTH810FP so messing around with ducting etc to improve airflow. Looked at Sic but VF worse!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2021, 04:22:42 pm
content: Between the coolness and your avatar, immediately brought to mind:You may yet be impressed by SiC -- eliminating recovery losses counts for a lot in most circuits! You do want to use a somewhat oversized diode because of Vf, yes.Tim

author: sandalcandal
author_rank: Unknown
timestamp: August 10, 2021, 08:36:10 pm
content: Quote from: T3sl4co1l on August 10, 2021, 04:22:42 pmDepending on the voltages here, normal Si Schottky might be better if it can be used due to lowerVf/$Vf*$ (lower Vf for less $ is better) The lower junction capacitance of SiC might not be enough to over come differences in Vf depending on switching frequency [probably not important at 25kHz?].

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 01:09:07 am
content: Yeah, Si goes up to 200V or so -- kinda crazy the lengths they've gone to, with field effect stuff and all that. I think 300V is even out there, but not very available? SiC takes over beyond there.I haven't really figured out the difference, when recovery is worse than capacitance; most converters don't mind the capacitance on the main output, and the diodes are comparable to transistors so it's not grossly worse at least. For sure, recovery sucks in hard switching (CCM). So it's worth a try.Schottky also seem to be poorly suited to snubbers, as they have higher Rs*Cp for the same ratings; and usually lower peak current rating (especially SiC, with quite high Rs and low peak ratings*). The flipside is, one must be careful of forward as well as reverse recovery, due to the pulsed operation. (For which, lossy snubbers usually have soft and slow recovery, so it doesn't matter; but this can make quasi-resonant or lossless snubbers a challenge.)*Some datasheets show the extended range on the V-I graph, where the guard ring diode goes into conduction. This is why the tempco and Vf changes at high currents. SiC P-N drop is around 3V so you see the kink above this level or so. This can carry more current, but it has a smaller area than the schottky, so doesn't increase the surge current rating much; and note that the minority carriers thus generated, will cause significant recovery time. In other words, the schottky is nice and fast and, schottk-ey?, at nominal currents, but exhibits P-N recovery at high currents.Tim

author: sandalcandal
author_rank: Unknown
timestamp: August 11, 2021, 04:06:27 am
content: The current best performance for hard switching rectification if you ignore cost seems to be synchronous rectification using GaN eHEMT (or even better GaN-on-Si eHEMT with integrated driver). That's followed by synchronous rectification using SiC MOSFET with parallel Schottky. Not saying either of those are appropriate choices for the power levels being pushed here.Also mistake in the previous post, the figure-of-merit should be Vf*$ not Vf/$. We obviously want lowest Vf for the lowest cost.

author: trobbins
author_rank: Frequent Contributor
timestamp: August 11, 2021, 05:30:33 am
content: How are you driving the primary? It may be worthwhile doing some comparison tests with different slew rates to see if the combination of switching plus snubber losses changes, and if there are any collateral benefits to higher frequency filtering to meet any specs.

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 11, 2021, 08:38:54 am
content: Thank you all for your interesting commentsThe voltages here are either 300 or 600 depending on the transformer secondary configuration (switched by relay). Unfortunately to high for vanilla Schottky hence fast recovery instead, switching frequency is a mere 25Khz due to lack of Litz supplies (TIW Litz is even worse). Primary is driven by a full bridge, slew rates moderate IMOP IRFB4227's with 10R on the gates.I have never used Sic, I can see it's advantages at much higher frequencies, but insofar as static losses are concerned they seem slightly worse than good fast recovery types, in that respect I should have used overrated diodes originally to bring down Vf but at the time I was not expecting thermal problems.Here is a picture of the fan/heatsink arrangementThe most severe problem is the connecting leads for the transformer & inductor run across between the fan and heatsink! The exhaust from the box is at the top right & left, the lower heatsink is for the primary switches with a 120mm/100cfm fan blowing air from outside over it. The upper fan is an 80mm/30cfm unit.I am presently considering moving the connecting leads out of the way (over the top instead) and changing the 80mm fan to another 120mm, then try and add some baffling to constrain the airflow a bit more over the heatsinks.Please don't laugh to hard about my construction you might get hiccupsAhh the picture has loaded sideways, left side is actually top in reality, ie airflow is bottom to top.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 03:41:12 pm
content: Yeah, ducting will help a lot there. Middle of the fan is a stall, and the airflow around it is swirling. Nothing really inclined to stick down between the fins. Easily constructed from cardboard, plastic, fiberglass, etc.(Cardboard obviously not great for flammability, though in a wood case it seems that may not be much of a priority..There is at least "fishpaper", a hard-bonded paper with a self-extinguishing rating. I have a few sheets, it's pretty handy stuff.)Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 12, 2021, 06:48:34 pm
content: Thanks all.So right, I wish I had made room for plenum space, however instead I have made baffles (cardboard atm but easily re-cut in fibreglass later), re-organised some of the major components to reduce airflow blockage and extended the litz leadouts of the magnetics so they are not blocking the heatsinks & changed the 80mm fan to 120mm. All this seems to have improved matters quite a bit but it has been very dull today in our part of the UK so waiting better weather for a meaningful test.1st Picture is end on view down the exhaust end of the troublesome heatsink, 2nd is overview of new installation.Roger

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 31, 2021, 04:15:29 pm
content: Transformer ran beautifully at 1Kw, no serious heating at all, but fairly short lived till a couple of secondary circuit mosfets let go (d-s s/c & low resistance to gate) these were venerable FCPF400N80Z's that have survived many faults but were probably just worn out, fortunately I have already trialed some IXGP20N120B3 IGBT's in the same location and these will be fitted asap. Meanwhile I was impressed by the TLP350 integrated opto-gate drivers that I am using for the first time, they survived this particular fault without failure. Somewhat off the original subject but part of the story of the transformer trials.

author: fourtytwo42
author_rank: Super Contributor
timestamp: September 10, 2021, 08:30:56 am
content: IGBT's work a treat, just a bit more voltage headroom for all those nasty spikes!Thermal equilibrium reached at Tamb+11C on the heatsink at 1Kw with 100cfm 120mm fans.Transformer rise insignificant (few degC) as so much more copper in it than old one and series/parallel winding switching allows much closer matching to load impedance.So I think what started off as a transformer question is done & dusted, thanks for all the help everybody

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2021, 01:20:18 pm
content: And that's with the kinda thrown together setup, nothing special about the fans, heatsinks, ducting, etc.? Pretty generous flow rate in any case (even if just nearby, not pinpointed); if I had to guess, the temp rise isn't nearly so low that you could get away without the fan -- but it's more than good enough with. Nice!Tim