High-voltage circuit do not know how to choose the topology?

If the output range is narrow, let's LLC.

It is recommended to use LLC with secondary rectification and then series connection. For example, there are two windings on the secondary side. One winding generates 350V and the other winding generates 350V. There is no center tap, and each uses bridge rectification. This will require 8 fast rectifier diodes. As shown:

Yes, series connection after secondary rectification is a good choice. If the secondary does not have a center tap, the secondary transformer can be directly connected in series and then rectified, which can reduce rectifiers,