TIGHTpak toroid winding technology

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Most Class D filters contain few turns

I don't really see a need to pack the toroid with extra wire most of the time. A 20 uHy coil has 13-14 turns. The rather large toroid that the wiring will go on will usually easily accomodate all the required turns in a single layer, which is as it should be. The wire size should be increased until the number of turns fills the single layer on the chosen toroid form. You want to lower the DC resistance of the output toroid(s) so that the resistive losses are as small as possible.
 
T106-2 cores need 31 turns

I don't really see a need to pack the toroid with extra wire most of the time. A 20 uHy coil has 13-14 turns. The rather large toroid that the wiring will go on will usually easily accomodate all the required turns in a single layer, which is as it should be. The wire size should be increased until the number of turns fills the single layer on the chosen toroid form. You want to lower the DC resistance of the output toroid(s) so that the resistive losses are as small as possible.
You must be using some really heavy cores. The Micrometals T106-2 cores that are the most common Class D toroid cores require 31 turns to make it to 13uH. 20ga. wire barely fits.
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T106-2
 
Ferroxcube

Using a Ferroxcube TN26/11-3C20-A113 gapped toroid cores.
OD is 26 mm, ID in 13.5 mm. Height is 11.6 mm. At least 20 turns, single layer, of #16 wire will fit, I believe.
See their app. note on these cores at their website. Ferroxcube - ferrite cores, bobbins & accessories

What size is the Micrometals core?? Specifically the ID, which controls how many turns are possible, single layer.
The link to the T106 cores is right there in my last post. It barely holds 13uH of undersized wire. Nobody has the A113 ferroxcube cores so I ordered some TN23-4C65 to try in the mean time.
 
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