Claude and wrenchone, thanks a lot for your explanations.
I suspected the thing about CMC now I'm sure to feel it. Yet UC1846 typical circuit looks incomplete to me because it doesn't directly sense primary current.
And magnetizing current which is part of total primary current is not possible to sense on secondary side, right? and this sensing is more important to my mind.
Seems I've got the right clue about my described experiment, that if half-wave load is applied to center-tap push-pull converter than transformer magnetizing current doesn't change much and still alternates during "idling" half-cycle. Obviuosly high-permeability ferrite cannot accept high magnetizing current without saturation; I think it may be in order of 10% of nominal load current. Now I begin to realize that I merely turned my push-pull converter into classical single-ended forward converter. Everything fits and I feel my head clarified 🙂
