proximity effect in flyback

Can anyone explain me what does the proximity effect depends on ? (in flyback transformer) As far as i know it has the influence on Rac/Rdc ratio in the windings. I made 50W flyback SMPS; it works very well, windings temperature is about 57 C. It is acceptable value, although i'd like to calculate copper losses more precisely than using only RMS currents and known windings resistance (skin effect is neglible in my case). I suppose that proximity effect has a great influence on copper losses

The proximity effect causes the current to stay even closer to the surface of the wire than the AC frequency would cause. Another view is that the current in one wire induces eddy currents in adjacent wires. Sum total is that more loss (heat) is generated in the copper wires.

Two solutions sometimes used are stranded wire or using several smaller diameter and insulated wires in parallel for the windings. Experimentation is usually easier than theory to predict the improvement, if any, in using these solutions.

u can read ti seminar and fundamental of power electronics to get detail info. as design ,we can check the donwell curve to split the wire