DER484:...Heating of PFC inductor coil near to the core gap?

PI recommends using Litz wire for gapped ferrite inductors - this is standard practice on our PFS-2 evaluation boards, which all use gapped ferrite inductors for higher efficiency. The Litz wire reduces the effect of heating due to the fringing flux next to the gap.

It is possible to put a tape barrier on the bobbin next to the gap when using magnet wire to keep the wire away from the gap, but this might involve too much manual labor, and may be more expensive in practice than just using Litz wire. It may also be possible to tool a bobbin with an exclusion zone, but I haven't seen that done in practice, either. The exclusion zone technique may also not be popular because it limits the amount of copper you can load into a given core size, important where small size is paramount.

Thanks, would you say Furukawa TEX-ELZ Litz wire has enough strands? (to avoid significant heating due to fringing fields). It has seven strands
https://www.furukawa.co.jp/tex-e/en/product/texelz.html

I would not go as fancy as that. We generally use something like 40/38 served Litz - no need for triple insulated, esp if you varnish the inductor afterwards.

Thanks, i see you use 38 strand Litz, so is the 7 strands of Furukawa TEX-ELZ not enough strands (i appreciate it doesnt need to be triple insulated..however, TEX-ELZ is widely obtainable and always in stock somewhere)

Probably not enough strands - if the wire is too coarse, the fringing flux will heat it up. I found out about this the hard way in my first attempts at an LLC converter...

MWS and New England Wire are good sources for Litz wire. You can also get triple insulated Litz with a proper number of strands from NEW, but the thick coating would make it bulky. Litz with nylon serving (solderable) is the best all-around solution. The serving helps maintain insulation between turns.

Thanks, how would you specify the nylon serving's insulation rating?
eg 500VAC for 1 minute and 1mA?
I contacted mwswire.com to ask what Litz part number got used in DER484 and they responded as follows...
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"Thank you for your inquiry! Sorry if I am misunderstanding, but it looks like you need to check if Power Integration’s document DER484 covers the 60/40 SPNSN that they order from MWS. This need to be directly inquired for with them. "
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Please may i ask for a part number or a spec for this Litz wire.?

Insulated Litz wire, for use with eg PFC inductors, definetely appears to be a nightmare for finding a source who can sell it.
The following shows the great difficulty surrounding insulated Litz wire....
https://www.eevblog.com/forum/renewable-energy/triple-insulated-litz/msg2749224/#msg2749224

You would simply specify nylon serving. The serving is mainly used to help keep the Litz strands bundled, and to protect against mechanical abrasion., not as an insulator -no voltage standoff or hipot rating.
Nylon is probably the default option these days for served Litz, as it goes away when the Litz wire is solder tinned. At one time cotton serving was also used, but that would need to be peeled away before tinning, a fiddly extra step that costs money.
If you want to add moisture resistance and a little extra insulation capability, varnish impregnation is an option, It also binds the winding turns and the core halves together, reducing any tendency for audible noise.

Thanks, though if the Nylon is not being used for insulation, then surely the individual strands of the Litz would need to be enamelled? As you know, or a PFC inductor, the voltage across the inductor is some 385V when near the mains zero crossing and the diode is conducting, so surely some specification of insulation rating is necessary.

For example, i just used PIXls for a 300W PFC design, and it has specified 31 turns of Litz made up of 43 strands of AWG38 wire, with a Nylon serving. Are you saying that all we need to do is email New England wire and ask for literally "Litz wire made up of 43 strands of AWG38 with a nylon serving"?

Litz strands are always enameled, usually single coated, as it reduces the amount of insulation that needs to be expelled from the interior of the Litz bundle when the wire is solder tinned. What you don't want is the solder penetrating only partially into the Litz bundle, leaving a portion of the strands unconnected.

Yes, all you need to do is contact the wire vendor and ask for XX strands of XX AWG, nylon served.