I am using flyback for my designs but with 650V input its not possible to implement the supply with an integrated flyback IC like tny 268 or so.
I suggest an auxilliary winding on your main transformer to make a 15 to 30v dc and then regulate with a linear regulator like 7812.(and of course a 3W start up resistor)
personally I used two switch fly back for my 3 phase 380v input 10kw converter with ucc3895 but for weight and volume sensitive design its not recommended.
by the way the power integration website has an app. note for using their ICs on higher input voltage that you can find it easily.
