Flyback SMPS transformer...interleave wound or bifilar wound?

Hello,
We need to wind a flyback transformer on a Ferroxcube PQ3535 former. We need 20 primary turns interleaved between two layers of 10 secondary turns (ie Np = 20, Ns = 20)

Irms(sec) = 0.77Arms
Irms(pri) = 1.4Arms

The PQ3535 bobbin is 20.8mm in winding length.
As such, we would need the Secondary wire diameter to be 2mm in order to arrange its layers to span along the full length of the bobbin (as you know this is necessary to reduce leakage inductance). However, 2mm Enamelled wire is too unwieldy to wind easily. Therefore, we will instead simply scrap the idea of winding right across the bobbin, and simply wind 20 turns of bifilar TEX-ELZ triple insulated wire for primary and secondary. (we will use 7/0.17 TEX-ELZ)
Do you think that this bifilar winding (primary and secondary twisted together then wound together) will give as low leakage inductance as interleave winding of primary and secondary, even if it doesn’t quite span the full length of the bobbin.?

Ferroxcube PQ3535 bobbin
https://www.ferroxcube.com/FerroxcubeCorporateReception/datasheet/pq3535.pdf

TEX-ELZ triple insulated wire
https://www.furukawa.co.jp/tex-e/en/product/texelz.html

I believe that for safety, you will need to have the windings separated. Get a bobbin with two separate sections and wind the primary on the bottom and the secondary on the top. This way, if the primary windings fail, there is little chance of the primary voltage getting on the secondary windings.

Thanks, as you now Triple Insulated wire (TEX-ELZ) gives full mains isolation by dint of its insulative coating alone, not needing any extra separation i thought?

Search " Winding Capacitance and Leakage Inductance". This sight is the only one I found that briefly mentions a comparison of bifilar vs interleaved as far as leakage is concerned. It is on page 7 or 8 I believe. All the information I've found suggest that bifilar greatly reduces leakage. I think the trade off is that capacitive effects are greater. If I remember correctly I believe the insulation thickness increases these capacitive effects. Since your twisting the primary and secondary together you probably don't want to skimp on insulation though.

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The site also says Warning : Do not use bifilar wire. In case there's some confusion they are talking about the two conductor wire that is already attached together. Kind of like a lamp cord.

If I remember correctly I believe the insulation thickness increases these capacitive effects

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Is it really true? I cannot think of any reason for this effect...

Playing devil's advocate here?

1) Why do you need 20 turns?

2) Have you looked at a smaller bobbin?

3) Have you thought of interleaving all of the turns on one layer? Seems like they may fit considering you say 2mm/turn across the bobbin. The primary-secondary capacitance does increase when you wind like this, but may not be the end of the world. Since you are using triple insulated wire your capacitance will be less. With only 20 turns your capacitance will be even more less.

Just throwing it out there. Maybe will spawn an idea for you.

Bob

You are correct that the TEX-E wire gives you all of the insulation you will need unless some esoteric application we do not know of.

Probably not, I only have a vague recollection of it and probably should of left it unsaid. Thanks,

1) Why do you need 20 turns?

2) Have you looked at a smaller bobbin?

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Thanks, 20 turns gives the required 250uH with the PQ3535 core set with AL value = 630.
The primary peak current is 3.66A...this gives a B of 0.278

The primary peak current is 3.66A...this gives a B of 0.278 ...

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I am just curious: what was the final decision? Going with the triple insulated (they are just three passes on the varnish bath; the default is two passes- if my memory is good) bifilar wound OR they are going to different beds?

Sorry we havent done it yet.....but we are going to do interleave winding, as we feel that in production the bifilar ends would get mixed up if we did bifilar winding. We would need different color TIW for pri and sec and i dont think it comes in different colors.

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I know its old hat, and well known, but in UK, there is certainly no rush to design anything to do with electronics….specially the run of the mill stuff. This is because if UK companies do design such things, then the only reason to do it is to make whichever company look like a designer/manufacturer……once the uk company has this status, they can then play “Middle man” and secretly import stuff from China, and then re-sell it in the UK whilst avoiding the Trade Tariff… they avoid the Tariff because they pretend that they designed/manufactured it themselves.
I once worked for a UK company that designed/manufactured completely ludicrous electronics products just so that they could masquerade as a designer/manufacturer.

Bifilar winding gives the greatest coupling and lowest leakage ( twisting not necessary) if you cut the wires to length before hand you can mark the start / finish of one with a marker pen to avoid confusion or simply measure when finished, the Furukawa TEX-E is good stuff we use it on a lot of products, you could in fact go double insulated on each wire (teflon) and still meet standards as you have 4 layers between wires...