App note from ON Semiconductors on their FSL4110LR power switch for SMPS power supplies. Link here (PDF)

Some industrial equipment that are supplied from a threephase AC power source such as industrial drives and energy meters often need an auxiliary power supply stage that can provide a regulated low-power DC source for analog and digital circuitry.

This power supply stage requests special specifications such as;
– Wide AC input voltage: 45 VAC to 460 VAC
– Robust system against high line surge
– Protection against magnetic contact test
– Large output capacitance to keep long hold-up timeafter power-off