Full bridge would be total mistake in this application (90Watts and large output voltage range) I think Flyback would be much better, of course the lowest voltage that can be used wouldn't be lower than 20V (Lower Voltages may be unstable) but i don't think that's a problem.
Sorry for my English I'm from Poland, and I'm only 16 But i've made lots of Power supplies
I think Flyback the wrong way..u only think the desing of the transformer..the leak inductance high,becuase u need many windings..mean u have HV spikes..and
the problem are 0-450V the first..u calulate the Dutycycle first.. u need an PWM 0-98%..
i think the best solution ..u make an PFC ..the Outputspannung are 450V..u can use like FAN48** or some St component..after u make an Floating Ground Step Down Converter..there are easy to make the Current limiting and ec...
i think u understand wrong..first u have an PFC Booster..the Outputvoltage are 450V..u connect an Stepdown to this PFC....the Input voltage of the stepdown are 450V..