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Thanks, I will look closer into "Booster then LLC" option, but for now I already have existing design based on UCC28951 which I really need to try and get working, even if over limited voltage range or reduced current output.  The design is based of a TI webench design (see attached), I have a prototype built and it is working so far at reduced current output (the synchronous fets not firing yet as less than 30% load DCM threshold) my schematic is attached.  Main thing that is concerning me at the moment is ringing on the synchronous fets (even though they not firing yet so the rectification is just through the body diodes).  I have tamed that to a certain extent with a RC snubber across them (4 X 1nf + 33R 5W) but there is a lot of power dissipated by snubber resistors.  So my next step was going to be to look for fets with lower Coss.  Do you guys think that is the correct approach to take?