Specification of the diode D2 in MP171

AC-DC Power Converters
#1

The diode D2 can experience very high voltage stress when the FET turns on, hence it is recommended that the diode is rated for above 600V. Peak current can reach 600mA, so the diode should handle that and should be rated for at least 800mA.
One of the recommended part for both the diodes D1 and D2 is STTH1R06A.

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#2

Also it will help if the diode D2 is an ultra fast recovery diode with Reverse recovery time below 35 nsecs. Also if care is taken such that the two diodes have equal diode drops at their respective average currents then feedback sampled will equal the output voltage with higher accuracy.