There is a potential for high voltage ringing on the secondary rectifiers, caused by the difference in current between the transformer and the shim inductor when the transformer comes out of freewheeling. Diodes DB and DC provide a path for this current and prevent any ringing by clamping the transformer primary to the primary side power rails. Normally these diodes do not dissipate much power but they should be sized to carry the full primary current. The worse case power dissipated in these diodes is:
The diodes should be ultra-fast types and rated for the input voltage of the converter – VIN (410 VDC in this case).
A MURS360 part is suitable at this power level.