Three phase utility meter display 80VAC - 500VAC

I am designing an energy meter for industrial applications. It use an ARM processor and 4.7" backlit LCD and LiPo battery.

We need ~ 10-12 watts minimum at 5V Isolated. There is no guarantee that a neutral wire is available so I wish to put L1, L2, L3 & Neutral through 8 diodes as a full wave rectifier and use this as the primary DC feed. Preferred operating range at full power is from at least 80VAC - 500VAC.

I envisage a cascode FET being utilised as per PI's DER-58 3W utility meter application. Can you supply any advice which may help my design - I am thinking of trying the Topswitch JX in this application - is there a better choice for the main control device ?

I would welcome any suggestions or advice if anyone has been down a similar path, or can steer me to which device and design spreadsheet to start with. Transformer design is not my strong point either. Thanking you in advance for any help.