I did a full bridge phase shift ZVS converter many years ago. I remember u sing some fairly significant RC snubbers on the 2 output rectifiers.
I can't seem to recall the exact explanation for why they are needed. I'm guessing the leakage reactance in the transformer has something to do with it. I draw the converter with a pair of inductors on the transformer outpu t to represent the leakage and I still can't get my head around it.
regards, Bob N9NEO