I've seen a design sheet of an isolated off-line 5V/2A flyback converter.

In the "Calculating Wire Sizes" step the designer uses 4A/mm2 for primary and 10A/mm2 for secondary.

This seems quite interesting to me. Because I generally use 4.2A/mm2 (or 500 circular mils per Ampere) for both primary and secondary(ies) on my low power (<50W) designs.

Is this normal, or at least applicable? What can be the motivation of using different "J"s for different windings on the same transformer?

PS: IIRC f=70kHz and the number of turns for secondary was 5 or 7. Maybe using a few number of turns allows the designer to select higher J. I don't know.

  • \$\begingroup\$ We haven't seen that design sheet. Link to it. \$\endgroup\$
    – Neil_UK
    Commented Feb 27, 2019 at 6:32
  • \$\begingroup\$ @Neil_UK it was on paper and now I can't find it. If I could, I would put it in my question. And that's why I said "IIRC ...". Thanks anyway. \$\endgroup\$ Commented Feb 27, 2019 at 6:55
  • \$\begingroup\$ It could be all sorts of reasons - design borrowed from a multiple secondary design, skin depth reduces thick wire effectiveness more than thin, secondary is on outside so gets better cooling, striving to get complete layers 'chooses' an unusual wire size, and I'm not even going to attempt to think what effect the proximity effect might have. \$\endgroup\$
    – Neil_UK
    Commented Feb 27, 2019 at 7:57

1 Answer 1


You want the winding temperatures to remain below a certain limit, and therefore, when cooling conditions (heat conduction path, heat transfer coefficients) are different for primary and secondary coils, you can go for different current densities. I have seen this several times in transformer designs.


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