Re: Core effective permiability and gapping
Some time ago I built a reasonably high powered flyback mode converter that had two interleaved 1:1 ratio copper foil windings.
This was a quick one off project for myself, and I wanted to use ferrite pieces that I already had on hand, and the choice was between a large UI combination, and an EE core pair.
I started out with the EE pair which required a 3 mm gap. This obviously produces a lot of fringing, and much ringing on the waveforms which seemed pretty normal, and generally a very poor result.
The peak flyback voltages on each of these two windings were supposed to generate equal +ve and -ve supply rails. But I could only regulate one side. The very poor cross regulation caused by this ringing was its most objectional feature.
I then placed the exact same windings onto the U core, and this had the effect of moving the air gap well away from the windings. The change was dramatic.
I could pull 2Kw from one supply rail (either one) and zero from the other, and the voltage cross regulation was an astonishing 0.5% difference. The voltage waveforms were text book clean too.
Ever since that I have treated placing large air gaps between identical core halves with great doubt and suspicion. Its a really bad concept, although everyone seems to do it.