# Which transformer winding method gives the least leakage inductance?

1. ## Which transformer winding method gives the least leakage inductance?

Hello,
Which of the following two methods of offline flyback SMPS transformer winding gives the least leakage inductance?
We are used to winding our flyback SMPS transformers (30W offline flyback SMPS’s) by arranging for the total number of turns of each coil to almost exactly fit across the full bobbin length. (so as to help reduce leakage inductance) . The end of the coil is then immediately taken back across the bobbin for termination to a pin of the former.
Our method is shown in pictures on page 18 and 19 of the following document….
https://ac-dc.power.com/sites/defaul...les/rdr295.pdf

However, we had some flyback transformers designed by a Chinese company and they wind them differently. We have noticed that the Chinese specify a number of turns for each coil that is usually four or five turns more than the width of the bobbin. They then wind right across the bobbin length, and then spiral wind the last four or five turns back across the bobbin and terminate to a former pin.
Which method do you think involves the least leakage inductance?
I must admit we never do it like the Chinese do it, because we find that the spiral wound layer bit is uneven and is difficult to wind over the top of with the next layer…even with tape over it.

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2. ## Re: Which transformer winding method gives the least leakage inductance?

I don't believe that the discussed winding detail does influence leakage inductance. It's probably a method to finish the winding by machine.

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3. ## Re: Which transformer winding method gives the least leakage inductance?

Normally Chinese company specify leakage inductance say 150µH max if we measured, it will be above 200µH( Some companys are exceptional)

Full length bobbin winding and end wire taking separately(Cross wire) is good for better coupling and hence low leakage inductance. Interleaved winding as did by Power Integration also good for better coupling it will reduce the leakage as well as ringing in the switching wave form but it will increase intrawinding capacitance.

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4. ## Re: Which transformer winding method gives the least leakage inductance?

One interesting factor that is rarely if ever mentioned are the effects of gap fringing which can create some very dramatic effects of volts per turn right over the gap.
Every type of symmetrical split core geometry places the gap right in the middle where it can do the most harm.

Try the same coil winding techniques on a U/I core, the results will be very different, particularly with respect to cross regulation where multiple outputs are involved. Much less ringing too.
Try it and see...

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5. ## Re: Which transformer winding method gives the least leakage inductance?

I don't believe that the discussed winding detail does influence leakage inductance. It's probably a method to finish the winding by machine.
Yes i tend to agree, its just that i think it becomes much more difficult to wind the next layer when you are winding it over the uneven surface created by the spiral winding of a few turns right across the bobbin. I wonder how they get round this?

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