Calculation of power loss in Flyback transformer windings in vicinity of gap fringing flux fields?

Hi,
We wish to do a 240VAC 31W Offline PFC’d flyback LED driver with a Ferroxcube PQ2625 Ferrite core.

The winding cross section is as attached. NP=26, NS = 14. (The “N4” winding was never used)

Secondary is “sandwiched” between two primary halves.
The PQ2625 core is centre leg gapped with gap of 0.42mm. We make no attempt to situate turns on the former so that they are away from the centre leg gap.

The primary current (always DCM) is as attached. It has a peak of 2.9A and Bpk is 0.28T. The switching frequency is approximately 66kHz across the 10ms mains half sine. (its COT with Off-time of 13.4us)

Are there any simulation tools we can use to calculate the power dissipation in the windings due to the fringing flux field in the vicinity of the core gap?

TEX-E Triple insulated wire:
https://www.furukawa.co.jp/tex-e/en/product/texe_series.html

PQ2625 Ferroxcube datasheet:

The height of the fringing magnetic force lines are roughly equal to the gap distance, or 0.42mm. The thickness of the bobbin wall is 0.95mm, which is > twice the gap distance.

As such, the fringing field lines won't reach the winding.

As for the windings themselves, you really only need triple insulated wire on either the primary, or the secondary. Not both. As it is more expensive that regular magnet wire, I would only use it on your N2 winding. (And N4 if it's also a secondary, if you were using N4)

The height of the fringing magnetic force lines are roughly equal to the gap distance, or 0.42mm. The thickness of the bobbin wall is 0.95mm, which is > twice the gap distance.
As such, the fringing field lines won't reach the winding.

Click to expand...

Thanks, now that is interesting...i wonder up to what gap size that holds for?....eg, if it was a 1.3mm gap, does that "rule" still hold?