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Old 23rd September 2012, 02:34 AM   #1
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Default how to wind coupled inductor on toroid

I want to wind a toroid as a coupled inductor (it's for a forward smps). There are 4 windings : +/- 40 and +/- 7

It's no so important that the +/-7 cross-regulate, but definitely want the 40 volt windings tight. At the moment, I am experimenting with windings. One thought is to take 4 strands of #24 wire and twist them up with drill. Then wind the toroid, about 50 wraps. Then take the 4 strands, use 2 for +40 and the other 2 for -40. Seems that will couple pretty tightly. But the toroid, T130, isn't quite big enough to do this in one layer - I wound up with about 40 wraps in a single layer. It's okay to go the extra distance and continue wrapping, I think. but . . .

If I wrap a single strand of #24 wire, it fits all 50 wraps with a lot of room. What if I then lay a second wrap along side the first wrap, then the 3rd along side the 2nd and the 4th along side the 3rd. Assuming this fits, does this seem a reasonable method to ensure a single layer and, more importantly, tightly coupled?

The 7V windings will be about 8 wraps each, and will sit on top of the 40V windings.

Does this seem reasonable or are there better ways?
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Old 23rd September 2012, 09:59 AM   #2
AndrewT is offline AndrewT  Scotland
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I would wind the two 40V as a bi-fillar, not twisted.
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Old 23rd September 2012, 12:05 PM   #3
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You mean the 2 wires in parallel, I think, wound at the same time. Is that really different than twisting the 2 together and winding it at once?
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Old 23rd September 2012, 12:17 PM   #4
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AFAIK, it's always a matter of increased magnetic coupling at the expense of capacitive coupling. If you twist the wires you'll increase the copper losses because it will take more wire to make a turn. You'll also increase the capacitance between the wires, though I don't know if that's actually a problem. Bifilar winding (parallel, side by side) is what's normally done. Multiple layers should be no problem.
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Old 23rd September 2012, 05:14 PM   #5
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thanks for the help - I tried it bifilar, just as suggested -it looks good. For now, the wire guage is probably too small - so eventually will try it with larger wire.

If the amount of required turns is 50, and I can fit 40 on one layer - which is correct, wind the remaining 10 wraps on the second layer or wind 25 on each? Seems to me that it should be 25 each.
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