How do you know if Litz wire is OK for offline SMPS transformers? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 14, 2022, 09:18:49 am
content: Hi,I am a hobbyist. I am now making a 4kW SMPS. 100-265VAC input.When I want to specify a power switching FET, I open its datasheet, and check its voltage rating is ok etc. But when I need Litz wire, (which is expensive) there is no datasheet ever available with relevant details.So how can I know that eg “Rupalit classic” Litz wire…https://www.packlitzwire.com/products/litz-wires/rupalit-classic-plus/…is OK for use in an LLC converter with Vin = 390Vdc and Vout = +/-180Vdc?(Being a hobbyist, I just want to know that it won’t suffer insulation breakdown etc, and that it will work, I am not concerned about standards requirements. As a hobbyist, I am finding these companies don’t respond to my questions about these things)There have been previous threads discussing this, but no resolution has ever been found…..https://www.edaboard.com/threads/litz-wire-for-use-in-boost-pfc-inductor-winding-informative-datasheets-available.396262/I happen to know that the following type of enamelled copper wire is ok for use in offline SMPS, but that’s because I have actually used it and it worked…https://www.farnell.com/datasheets/3170015.pdfI admit that i can wind a transformer, and then give it a hipot test (but i dont have a hipot tester, and theyre expensive).....also, maybe the insulation is just managing for the short duration of the insulation test......as you know, insulation testers should not be used at length on samples, as the insulation is actually slightly damaged during an insulation test.Also, eg Do you know which if these Litz wires will be OK for use in UK Mains SMPS transformers?https://www.wires.co.uk/acatalog/litzwire.html

author: Gyro
author_rank: Super Contributor
timestamp: April 14, 2022, 05:38:46 pm
content: What's with the new "I am a hobbyist" / "Being a Hobbyist" narrative treez?QuoteAh, I get it, it's as a ploy to avoid talking about meeting regulatory standards! Ho hum.QuoteAs for using enamelled copper wire coating as primary - secondary insulation.Forget it!- That's the sort of criminal practice that you only find in the crappiest and most murderous Chinese SMPSs!

author: Faringdon
author_rank: Super Contributor
timestamp: April 14, 2022, 09:12:56 pm
content: QuoteThanks, i know what you mean, but this isnt for shipping to the public.In all the test jigs ive done ive always used Pro Power ECW for offline SMPS and no extra tubing or anything.....never had a problem...yet....i do worry about getting a nick in the enamel, but then again, when its only in a test jig, its just going to blow the fuse and send me back to do another one, and hope the enamel doesnt get nicked /blemished again.The thing is, can the Litz as shown above handle mains in the same way?.........................Do you know which if these Litz wires will be OK for use in UK Mains SMPS transformers?https://www.wires.co.uk/acatalog/litzwire.htmlGeneral internet research says that enamel on enamelled wire has an electrical strength of 170V-200V/um......presumbaly there are no varieties of enamel which are weaker than that?Apparently, the coating thickness on enamelled wire is between 3um and 15um.........so that means it should be fine with most mains transformers...having said that, i cant believe that 0.2mm ECW has the same coating thickness as say 1mm ECW.The enamelled wire in Litz is typically 0.04mm to 0.1mm in diameter...i simply cannot believe that this can garantee having an enamel coating thickness that is always >3um?

author: Gyro
author_rank: Super Contributor
timestamp: April 14, 2022, 09:41:14 pm
content: You can make all the "isn't for the public" claims you want but it is NOT OK, ever! Read the regulations.With your background and reputation, I don't trust a word that you say. I don't believe that you aren't trying to sneak something into production.SMPS [EDIT: All] transformers need to be constructed properly, with proper inter-winding insulation, edge margins etc. I pity the unsuspecting users of your test jigs!Of course Silk covered litz wire isn't ok for insulation purposes! It is wrapped in Silk strands just for mechanical bundling purposes. It therefore has air gaps between every strand. It has no insulation advantage over the enamelled wire underneath.You scare me!

author: Faringdon
author_rank: Super Contributor
timestamp: April 14, 2022, 09:45:51 pm
content: Thanks, its actually looking like the only way forward is to buy some unserved Litz wire, (ie just enamelled strands) and manually spiral rap it with polyimide tape or PTFE tape..........both these tapes are highly expensive as seen on Farnell.Mind you, the following tablehttps://www.elektrisola.com/en/Products/Enamelled-Wire/Technical-Data...says that 0.2mm Enamelled wire is at least 1800V in electrical strength.........theres "grades" 1 to 3.....and 3 is better than 1.....the following pro power enamelled copper wire says "Gr 2", so i presume thats Grade 2?https://www.farnell.com/datasheets/3170015.pdf..so basically that 1800v is whats keeping my test jigs from blowing up...the worry is that 0.04mm ECW has a strength of just 250V (but thats min)...and thats about the diam of most Litz strands.....the worst thing is that no offtheshelf Litz actually has any datasheet stating what grade of enamelled wire has been used.....?Page 42 of this shows that 0.4mm enamelled copper wire can have a breakdown potential of anywhere between 192V and 3000Vhttps://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_53/Philips_Tech_Review/PTechReview-03-1938-039.pdfImagine being told that you have a 48v rail, and you have to use capacitors on it which may be anywhere between 10V and 100V........thats what you are expected to do with enamelled copper wire and offtheshelf Litz.

author: jonpaul
author_rank: Super Contributor
timestamp: April 15, 2022, 09:41:25 am
content: Dear Farrington, "hobbyists" dont normally build 4 KW SMPS. Please clarify your intentions, one off prototypes or mass production?Litz is expensive and difficult to terminate, depending upon the current, frequency and transformer build, litz may,be a needless expense and headaches.all Litz specs available from wire manufacturer like Rubidue, expect 16..60 weeks delivery.Seldom needed, we used foil or bunched wire rather than litz.Suggest that you take time to learn more about SMPS and transformers design and wire sélection.Jon

author: Faringdon
author_rank: Super Contributor
timestamp: April 15, 2022, 03:32:55 pm
content: QuoteThanks, its just dawned on me how right you are.....just bunch the enamelled copper wire...and wrap it with polyimide tape......I reckon theres no need for 80+ strands of hair-width wire, ....skin effect isnt that bad.....here attached is our primary current and the Fourier of it...it went 20dB peak so i want to calc the percentage at the ~100khz fundamental.So yes , i reckon a few strands of ECW and we'll be ok with skin effect.......must admit, im not looking forward to wrapping with polyimide tape, going to take ages, and cost a fortune.Though i assure this is just hobby play.Had a discussion here about skin effect the other day.......kindly received good formula......r^2/[(r^2 - (r-d)^2)]r is radius, and d is skin depth........it shows that skin effect of the fundamental is pretty low.....but then theres the need to calculate the skin effect of the harmonicsI believe that the Fourier peak of 20dB means that the fundamental is approx 10Arms?Since 20log(10) = 20..now, the total irms = 11A...so that means the percentage of harmonics is very small here, would'st you agree?

author: wraper
author_rank: Unknown
timestamp: April 15, 2022, 03:41:31 pm
content: Quote from: Gyro on April 14, 2022, 05:38:46 pmI don't see him asking about using it as primary-secondary insulation.

author: Gyro
author_rank: Super Contributor
timestamp: April 15, 2022, 04:24:18 pm
content: Quote from: wraper on April 15, 2022, 03:41:31 pmHow else would a Hipot test be relevant (or applicable) to the wire enamel thickness? You can't Hipot test a continuous winding ....Quote from: Faringdon on April 14, 2022, 09:18:49 amIf he meant something else, then he did not provide any correction.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 15, 2022, 04:45:41 pm
content: To the OP,I would recommend watching the YouTube channel DiodesGoneWild. He has performed autopsies on many SMPS transformers showing how to incorporated dangerous and unsafe practices into the design.The highest award you can obtain is 'Super Dodgy!!!'.The channel also highlights other money saving tips, like using normal ceramic capacitors where Y1 and Y2 capacitors should be used. This practice is equally unsafe.I would have thought it would be much easier to wind the primary, cover the primary with at least two layers of transformer tape, sleeve the terminations, rather than trying to spirally tape the Litz wire. But what would I know, I am not a hobbyist.Jay_Diddy_B

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2022, 08:51:15 pm
content: Quote from: Faringdon on April 14, 2022, 09:18:49 amYeah. Riiiight.Well, I suppose treez was little more than that.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 15, 2022, 09:56:00 pm
content: Should be noted that Diode Gone Wild is evaluating this stuff on more of a "good enough" basis than "meets current regulations".As I understand (from various teardowns of legit phone chargers and Google) any mains powered SMPS transformer you see smaller than a golf ball or so is probably noncompliant if the secondary isn't wound with triple insulated wire. The creepage requirements are too great to practically make it work any other way.

author: Faringdon
author_rank: Super Contributor
timestamp: April 15, 2022, 10:06:33 pm
content: The below power integrations App Note shows that in fact, enamelled copper wire has superb electrical strength.....Its concerning a 100-265VAC 60W flyback SMPSApp note DER535https://www.power.com/sites/default/files/documents/der535.pdfSee page 20 if you wish to... (its also attached).....here you see the switching node wire, which goes up to in excess of 500V, layed right over the wire that connects to primary ground. There is no tubing. Its just bare enamelled copper wire. It shows that enamelled wire has superb strength.If you look at other pictures there...you see other cases of High voltage being layed (touching) over low voltage wires.The switching node wire is 0.32mm diam, and the pin 3 (gnd) wire is 0.2mm diam....and yet their enamelled insulation is good enough to handle the 500V thats between these wires.On the same page, if you look at pins 5 and 6, there you see the mid-primary point wire, lying right over the bias coil voltage wire....again...no problem.I think from this, it shows that if your just doing test jigs or hobby work.....its a waste of time using tubing.....and margin tape would also be a waste of time....I bet if that secondary had been wound with plain Enamelled wire, the transformer would work fine.(would not pass regs, but wouldnt be going in for compliance anyway)

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 15, 2022, 10:23:28 pm
content: Nobody dies if there's a short on the primary side, you'll note that, like I said, the secondary is wound with TIW.

author: Faringdon
author_rank: Super Contributor
timestamp: April 15, 2022, 10:29:31 pm
content: QuoteThanks, youre quite right, and the premise here is that we can just use enamelled copper wires for transformers that arent being shipped to the public.From past experience, ive never had offline flybacks blow up on me when i just used enamelled wire for pri and sec ...and no tubing and no margin tape....(just the inter-layer tape).....(woudlnt be going for conformance, or shipped to public..)

author: TimNJ
author_rank: Super Contributor
timestamp: April 16, 2022, 02:57:26 am
content: I have never seen an SMPS app note design which carries a 3rd party approval. It's a little irresponsible for Power Integrations to publish those photos in my opinion, though itprobablywon't end up in a dangerous situation (in terms of shock hazard).Check IEC62368-1 - Annex G.5 Wound ComponentsQuoteSure, do whatever you want in the engineering lab; it will probably be okay. But it's generally not good practice. Plus, is itreallythat hard to add some Teflon tubing?The other benefit of adding tubing is, if you dunk the finished transformer in a solder pot to solder all the pins, you don't need to worry about solder creeping up the wire and burning off too much insulation. The wire will still be well insulated.

author: wraper
author_rank: Unknown
timestamp: April 16, 2022, 07:59:29 am
content: Quote from: Gyro on April 15, 2022, 04:24:18 pmYou can wind it onto metal foil and test the breakdown voltage. It is relevant because short turns within the winding or between separate windings on the primary side are completely possible.

author: Faringdon
author_rank: Super Contributor
timestamp: April 19, 2022, 09:55:54 am
content: QuoteThanks yes, the termination is where i struggle, as i dont have solder pots.Though as you know, DIY Litz with multiple ECW strands is also hard to terminate.One big advantage for Litz as you know, is when you have a winding near a wide gap on a ferrite core......the Litz really helps reduce heating from it.But whether i use Litz or ECW.......for the offline version i am going to have to manually spiral wrap (at least some of it) it in polyimide tape...which i believe will be difficult.I may just use Litz anyway as at least you know its exact Outer diameter....which helps to measure out the coils across the bobbin.The Litz here looks cheap so i believe ill get a sample in....https://www.wires.co.uk/acatalog/litzwire.html..its a shame because i dont really want the silk coating, but there you go...Also, from looking at it, it looks silver...i hope the individual strands are enamelled copper wire....otherwise i am going to have to do precision manual polyimide tape wrapping....not easy for my club-handedness.

author: TimNJ
author_rank: Super Contributor
timestamp: April 26, 2022, 05:52:22 pm
content: Quote from: Faringdon on April 19, 2022, 09:55:54 amA small solder pot is like $30 on eBay. If you need bigger, maybe $100 - 150.I am not convinced litz is really all that expensive and don't think it's really difficult to terminate in most case...Maybe back in the day there were less suppliers and the price was higher; I don't know for sure. If you are using bobbins, it's as easy as wrapping ~1.5 turns around the bobbin pin, adding a little flux to the pin/litz, and dunking it into the solder pot. Once the solder starts to wick up the litz a little bit, there's a very high likelihood the solder has penetrated and that all the strands are soldered. If you are using flying wires, same story, just do a little experimenting to figure out the timing.Conceivably for huge litz bundles, it get's a little more complicated to verify whether inner strands are indeed soldered. For these type of windings, probably best to fork the massive bundle out to a few smaller bundles for easier soldering.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: April 26, 2022, 06:24:25 pm
content: Quote from: TimNJ on April 26, 2022, 05:52:22 pmI fell in the cheap solder pot trap. It worked for awhile and then stopped. I opened it up and the hot wire had come loose. By the way the ground was not connected. So at minimum if you buy one of these open it up and carefully inspect.We ended up buying a real solder pot.

author: Faringdon
author_rank: Super Contributor
timestamp: May 01, 2022, 11:21:26 pm
content: Another thing that tells us that enamelled copper wire is good in transformers and inductors to 100’s of volts is RDR-737 by power.com.....RDR-737https://www.power.com/sites/default/files/documents/rdr-737_1point44_watt_non-isolated_buck_converter_using_linkswitch-tn2_900v.pdfThis shows a high voltage input Buck converter, stepping down from 620Vdc to 12Vdc.It uses a small RLB9012-152KL inductor....RLB9012-152KL datasheethttps://www.bourns.com/data/global/pdfs/RLB9012.pdfThis is a very small inductor, featuring no tubing, simple enamelled copper wire, and a datasheet which doesn’t actually even mention voltage rating.Would you agree, that this is indicative that if you are doing a low cost prototype, then there is no need for triple insulated wire, margin tape, tubing, or anything like that in offline transformers…..just plain enamelled copper wire will do?---___---___--___---___--___--___--Also, page 4 of the LNK302 datasheet shows a Buck converter which is stepping down from potentially 370VDC to 12VDC.....yet on the same page, it simply says that the inductor for this should be a "standard offtheshelf inductor"......it says absolutley nothing about any voltage rating for this inductor.LNK302 datasheethttps://www.mouser.com/datasheet/2/328/lnk302_304-306-179954.pdfI think we can say that any offline SMPS needs more than just enamelled copper wire to pass regulatory testing....but if just doing for a prototype, then there is no need whatsoever to use TIW, or tubing, margin tape etc etc...simple enamelled copper wire will do.