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Some guides suggest using multiple very thin strands in parallel to "reduce the skin depth". Does this make any real difference?

When I apply their formulas I end up with something like 14x31SWG, which is rather a lot of headache to wind. Obviously for industrial design every watt counts, but for hobby is it OK to use single wire of appropriate diameter?

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  • \$\begingroup\$ Does your enter button make a new post instead of a new line? If not, what's going on? \$\endgroup\$ – Harry Svensson May 29 '18 at 1:27
  • \$\begingroup\$ What is going on is that I posted several questions here and was immediately castigated for not following rules. I tried to be a nice guy and broke them in pieces, which seems to have exactly the spam effect I was worried about. Please, accept my profound apologies. \$\endgroup\$ – Maple May 29 '18 at 1:37
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You often use multiple strands of wire to reduce the effect of skin depth. The skin depth itself will stay the same.

The way this is normally done is to use 'Litz' wire, which is made up from multiple strands, sometimes more than 100 individual strands. You can also make your own by twisting together a number of strands, though it probably won't be as tidy as commercial Litz.

The wire used has to have self-fluxing solderable enamel. Although most wire has this these days, you may pick up some very old stock that doesn't solder well. This is essential as it's impractical to terminate all strands manually. With self-fluxing, you heat the end of the wire together with some multicore solder, and the enamel burns back and allows the wire to tin.

Litz wire is only beneficial over a limited range of frequencies. At low frequencies, audio and below, skin depth is rarely a problem for practical wire thickness. At high frequencies, more than a few MHz, the skin depth is so small that Litz can't be made fine enough to benefit, given that its construction includes much insulation and air, and it's best to go back to a single wire. Within that frequency range, check skin depth tables for the wire diameter you want to use, to see whether it's worth using.

All Litz wire does is to reduce the copper loss. This is only one component of the system loss, the other important contributors being ferrite core loss, and semiconductor conduction and switching losses. It will make your transformer run slightly cooler. Whether that's needed, or a 'nice to have', is down to the detail of your system. Unless you are shooting for the ultimate in efficiency, it's usually possible to use single core.

A slightly different application where you sometimes must use Litz is when winding inductors intended for LC filters. The loss impacts the acheivable Q, a much more fundamental spec point than flyback efficiency.

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  • \$\begingroup\$ This actually reminded me of one time years ago when I needed thin and flexible wire and pulled it from headphone cable. I was surprised to find it was just two multi-stranded wires (one red, one green) with no insulation between them. I was even more surprised when I wasn't able to solder it, even after rigorous attempts to clean enamel with fine sandpaper. Anyway, the chip I am planning to use works in 20-380 kHz range. For the efficiency I am likely to achieve in one-off hobby experiment I think I'll go with single core. Thanks for detailed and educational answer. \$\endgroup\$ – Maple May 29 '18 at 6:09

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