I have seen many 3 phase AC/DC flyback power supply specially in energy metering field.

most/all of them have a series resistor in the range of 50 Ohm between the AC lines and the

rectifier bridge, few of them uses only one resistor after the bridge rectifier.

by the way the flyback uses a cascode MOSFET configuration.

Does anyone knows why these resistor(s) are there?