UCC28951 Questions

OK thanks, but coupling means exactly that....and if 3 (pri) turns is perfectly coupled with 4 turns (sec), then there is no leakage inductance....if there is L(leak), then you cannot possibly have the full 3turns to 4 turns coupling....we simply cannot have it both ways.
And this is why soffware is needed for planars...because planars always use low turns numbers, and so they need the soffware to do the coupling and get the ns/np proper.

your sentence is non sequitur,

how many planars have you designed and got into production above 100 watts ?

Ive never had access to the special software that they use to design the planar, as it costs a fortune as you know, and they always charge people so that they can use it for us. I went to Himag for an interview and the engineer showed me him using the software. It was sotware that was not available to be bought as i understood it.....unless you pay a fortune.
I also sent specs off to a place in Scotland. That was for 200W two tran fwd. DCDC
I think they charged £6000 up front...cost of their software used on our behalf, and the tooling. And that was just for a few peices...but they then had the tooling or whatever to allow many more to be made for us without that upfront cost for the further batches.
Just give them the Np/Ns spec and the L(pri) and the L(leak) max, and the size etc, and ask what they can come up with is the normal way with planars as i am lead to believe.

This stuff about full no.s of turns came up recently because we had to do flyback on PQ20/16 , but had to turn the bobbin "sideways" otherwise it couldnt fit in due to the "jutty out" termination bits....but then your final turn doesnt hug so closely around the 360 degs of the bobbin...and its a partial turn kind of thing.
This could affect OP as they have also low turns...and a partial turn of a low turn coil makes quite a big difference to the inductance thats coupled in there to the sec.

All you need is KiCad and and excell spreadsheet ( or calculator ) to design a planar - they are no different to any transformer - you need to take care of the edges of the wdgs - and usually think about the thermals as they are normally run hard and rely on heat-sinking thru the core.