Thanks for these replies.
Yes, the LTC3723-1 controller was used, and in the back end of its datasheet, there are examples of 100W+ pushpulls. Why they would tout such a lousy converter is beyond me.
I believe the pushpull should be condemned. Any datasheet touting it should stamp the page with "warning", like they do with "not for new designs".
Planar:
The top post shows the planar transfomer spec as written by the Planar Designer company themselves....so they knew it was for push-pull useage....and yet still served up a Planar transformer with 1.2uH of leakage between pri and sec.....(there own spec shows it).....they are one of the well known Planar designers....so how they didnt realise this is beyond me......maybe they just thought the myriad of mitigation circuits needed for this, would be used here.
Mitigation:
In my view the correct mitigation of the hard-switched push-pull, is "choose another topology instead". If you hear the word "push-pull"....Run!
Push-pull term:
Bizarrely, almost all Full Bridge, and Half Bridge drive controllers are labelled "push-pull controllers"...you literally have to use "push pull" in your search term, even though you wouldnt want to touch the pushpull with a barge pole.
Inter-primary coupling:
The pushpull , as kindly suggested elsewhere, needs really supertight coupling between each primary half, so that the magnetising current can be kept flowing after a primary fet turns off......the magnetising current then flows through the diode of the opposite primary fet....and great coupling (between primary halves) is needed so that the reset of the magnetising current can proceed unhindered in this way.
LTC3723-1