Offline flyback transformer with nylon "wire" to pack out bobbin? (gap issue) - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: May 03, 2021, 02:14:50 pm
content: Hi,The attached is a former cross section of a 140W Flyback transformer.Vin = 228VDCVout = 24VIout = 5AFsw = 43kHz(BCM)Core = Ferroxcube E42/21/15 3C90 gapped to 0.85mm (centre leg) giving AL = 315.It is a single primary coil, which is sandwiched inside two secondary coils, SEC1 and SEC2.SEC1 and SEC2 are in parallel with each other.NP/NS = 24/10PRI = 24 turns of 7/0.24mm TEX-ELZSEC1 = 10 turns of 2 strands of 7/0.3mm TEX-ELZSEC2 = 10 turns of 2 strands of 7/0.3mm TEX-ELZLP = 181uH, LS = 31.5uHI(pri) peak = 6A.I(sec) peak = 14.4AThe core is packed out with 0.6mm diameter fishing line to a depth of 1.2mm (to keep turns away from centre leg gap)I am wondering how awkward practically its going to be to wind and terminate the thick secondaries? As you know, TEX-ELX isnt purchaseable in small quantities so we cant even "have a go". Usually send off to winder to ask their advice but this proj doesnt have go -ahead yet.Do you know?TEX-ELZTriple Insulated Wire - Litz Type TEX-ELZ｜Insulated Winding Wires｜Furukawa Electric Co., Ltd.E42/21/15http://ferroxcube.home.pl/prod/assets/e422115.pdf

author: fcb
author_rank: Super Contributor
timestamp: May 03, 2021, 02:44:29 pm
content: Quote from: treez on May 03, 2021, 02:14:50 pmYes it is.https://powermagneticsstore.co.uk/shop/furukawa-triple-insulated-wire.html

author: ocset
author_rank: Super Contributor
timestamp: May 03, 2021, 07:19:23 pm
content: Thankyou very much.Also, any thoughts on the use of fishing line to pack put the bobbin, as in top post? (reduce eddy currents in the wire due to fringing field from the core gap)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2021, 09:05:05 pm
content: Traditional is just using margin tape. It doesn't melt in operation or when curing varnish, and is porous to soak up varnish.I would expect fishing line per se has a lot of shrinkage when heated, or maybe melts easily or something.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: May 04, 2021, 12:17:56 am
content: Try 3M 44T-A if you want thick margin tape. Highly unlikely that the fishing line is allowable under your magnetics' vendors UL insulation system. Check with them for what insulation systems they have access to, and then you'll know what you can and can't use.Also, unless I'm missing something from your layer stackup, you don't have to use TIW for both primary and secondary. Often times, use normal enamel wire on the primary and use TIW on the secondary. For a step-down converter, you'll need more turns on the primary, so the extra TIW thickness adds up more. You can definitely do it the other way around too.You, of course, can use TIW for everything, but your bobbin copper utilization will be much worse, and the cost will be higher.

author: fcb
author_rank: Super Contributor
timestamp: May 04, 2021, 08:31:47 am
content: Quote from: treez on May 03, 2021, 07:19:23 pmI didn't think you were serious about the fishing line. Don't use it for all the reasons others have pointed out.

author: jonpaul
author_rank: Super Contributor
timestamp: May 11, 2021, 01:48:01 pm
content: Use only UL/CE ETS rated insulating materials for safety and compliance.In 50 years PSU design we never had any issues with gap flux requiring such a spacerj

author: ocset
author_rank: Super Contributor
timestamp: May 15, 2021, 09:04:15 pm
content: QuoteThanks, yes, but was that because you always used say 38 strand of #38 AWG Litz?....in that case, as you know, no need for the spacer.As you know, The probelm is that that kind of Litz is difficult to source, especially with offline insulation ratings.