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transformer secondary winding mode

Posted by: ni.amir on Thu, 05/06/2021


I have a question about secondary winding of flyback transformer. I have to use 4 strands of 0.35mm wire for secondary, but I can't wind 4 strands of wire in the same time (because of instrumental limitation). For this reason we wind two winding separatly that every one is bifilar (two strands) and then we parallel the two windings in the PCB. This winding is the main output of flyback SMPS.

we use TNY280 in this SMPS.

please help me that , is this job correct? (paralleling of two winding for main output)

Thank you

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Submitted by PI-Yoda on Fri, 05/07/2021

Hi  ni.amir,

The answer depends of the number of layers of your secondary winding.

  • Case 1 - single layer secondary - 4 filars (strands) are on the same (one) layer in 2+2 configuration

The actual performance difference between the proposed and build transformer will be tiny. It may show some variations in the EMI performance depending on the actual implementations.

  • Case 2 - your transformer has 1+ layers secondary and you are paralleling 2 separate layers together

That is generally not a good idea. The reason is that the coupling with the primary of each of the layers is different, resulting in internal currents between the two windings. It is still workable solution if some mechanical restrictions limit the 'all strands wound together' technique  but it should not be preferable solution.


One thing to pay attention to when you connect windings 'on the pcb' and flying leads are used.

The flying leads are introducing leakage inductance and you must try to minimize it. It may look tiny, but its value transfers to primary multiplied by the number of turns squared. Minimizing measure in this situation is - twisting both leads from the same winding together. 

The above note is especially critical when those two windings are electrically in parallel.


Best Regards,