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Yes from top scope shot is seen a resonant period of some 200ns after the primary is switched in.

Do you use a series blocking capacitor?...if so, what is resonant period of it with the primary leakage inductance?

Have you produced the "representatve simulation" yet in LTspice?...if not, i will send you a PSFB in LTspice eand you can adapt it to your spec?.....the sim is good at doing simple stuff liek LC resonances.

..in fact, ive attached the whole PSFB folder...there are sims in there of PSFB if you want to dig one out?

But my bet, looking at the smoothly sinusoidal resonance, is that its the series cap and the leakage L ringing with each other.

Absolutely expected...big voltage step input to a high Q series LC resoannt cct...will ring like mad....we cannot stop it...its mother nature.