I know I shouldn't. But I just wanted to ask due to a specific requirement. I also searched the forum but couldn't find any similar question. Please give me a link if there's any.
I'm asked to modify one of our own design commercial three-phase electric meters according to a big customer's requirement: The e-meter should never affected by outer magnetic flux densities up to 300mT, and it should be as cheap as other cheaper brands' e-meters.
I decided to modify and "cheapen" the power supply first. In the original design the power supply consists of a flyback regulator following a three-phase full-wave rectifier (simplified):
VDD is the supply for digital blocks (MCU, ADCs, communication interfaces etc.) and, as can be seen from the simplified schematic above, its ground (shown as SEC-GND) is tied to the NEUTRAL through a ferrite bead to guarantee accurate phase voltage measurements.
So I decided to make a transformerless power supply since it is relatively cheaper and is relatively resistant to magnetic fields due to the lack of magnetic components (i.e. inductor/transformer):
The problem is that I don't know what should I do to guarantee accurate phase voltage measurements. I know I shouldn't connect the rectifier's GND to the NEUTRAL even if the system is ideally balanced (The system wil never be balanced because the phase voltage differs in tougher environments here in Turkey).