Catch diodes in switch mode power
The "catch" diode on the primary side of a transformer-coupled SMPS serves the same purpose it does on any other inductive load such as a relay. Its reverse voltage rating needs to be at least as high as the voltage across the primary when the switch is on. Its (peak) forward current rating needs to be at least as high as the peak primary current at the moment the switch turns off. The average forward current and the total power dissipation will depend on the switching frequency and the value of the leakage inductance of the transformer.

Note that this assumes that we're talking about a "forward" converter, in which the transformer itself is not intended to store significant energy. In a "flyback" converter, you do not put a catch diode directly across the primary.