The power class is difficult to achieve with integrated switches, and additionally optimum efficiency for 120W is achieved with a quasi-resonant converter instead of a flyback.
You could take a look at the L6566BH, which is designed for 3-phase applications, but can only handle a maximum of 840V instead of 900V (ALTAIR04-900). You can dimension and simulate the design with the online tool eDesignSuite (select Power Supply Design Tool > AC/DC), but it assumes an ideal transformer and cannot calculate it. However, transformer manufacturers should be able to help you with this.
Good luck!
/Peter
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