30W Flyback: Use Litz or 2 starnds of ECW?

[cupoftea]

Hi,
We have designed a 30W PFC’d Flyback with Np:Ns = 26:14 (270uH: 78.4uH).
At 180VAC, I(pri) is 0.5Arms, I(sec) is 1.57Arms.
Sec is sandwiched by two 13 turn primary coils
(180-265VAC)
LED voltage is ~35V

We have chosen a PQ2620 core set with 3C90. Its in Constant off time mode so the Switching frequency at max load, 180VAC in, is 63khz, at all points on the mains cycle. Its at BCM at mains peak.

For the primary, we chose TEX-ELZ 7/0.12mm

For the secondary , we have two options…

1….14 turns of 7/0.11 TEX-ELZ (0.225R at DC)
2…14 turns of 2 pallel strands of 0.25mm ECW. (0.143R at DC)

Skin depth of copper at 63khz sine is 0.824mm at 25degC...but of course, Flyback Pri waveform is hardly sinusoidal....so the skin depth at 63khz is fairly meaningless for us?

Which of the two secondary windings would suffer the worst conduction loss?

 

[FvM]

Skin depth of copper at 63khz sine is 0.824mm at 25degC

Actually about 0.26 mm.