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.