3 phase ac to 5V 50mA power supply

I am working on a 3 phase energy metering project using ADE7752 IC. I am stuck into getting the power supply part in a smaller form factor.

Since energy measurement is being done for all 3 phases, the IC must be able to measure energy even if two phases go down and hence VDD supply must come from all three phases.

After some searching, I came to this schematic:

This involves 3 transformers which will be too bulky in size and hence becoming an issue for me.

Is there some alternative compact circuit which can do the job? I don't need isolated power supply here.

I was thinking about using diodes to rectify all phases and combine them at a capacitor but I think that will end up in smoke if not fire and shock.

You can use a three phase rectifier but the problem is that your DC cap will have to be rated for ~500V. Since your output power is just 250mW this will be overkill.

If isolation is not an issue you could add lots of capacitance in series to drop a large part of the voltage before rectifying it. C1, C2 and C5 are AC caps, values to be decided based on load current, with resistors in parallel to drain residual charge in the caps. C6 is the DC cap and R6 provides the minimum load to ensure that voltage across C6 is always limited. You will also need a fuse in all three phases just in case.

simulate this circuit – Schematic created using CircuitLab

• So this thing works? I think I was correct with rectifier idea, then. I primarily chucked it because I thought after rectification, still there will be a phase difference between all three phases and it will cause a short circuit at the capacitor. – Whiskeyjack Apr 18 '16 at 11:19
• The caps are only to drop some voltage. The diodes with the highest input voltage will conduct like in a normal 3ph rectifier. Simulation should give you an idea of the resistor / cap values. – kabZX Apr 18 '16 at 11:24
• This works for small current supplies (look at led supplies with capacitive voltage dropper) ...just make sure R6 is in circuit when you turn it on for the first time. Otherwise you get the high volts. Can also have a zener to limit the voltage. – Spoon Apr 18 '16 at 11:52
• I am planning to go for second circuit. I have already used it for single phase. I am just wondering what's the use of R1, R2 and R3? – Whiskeyjack Apr 18 '16 at 12:00

Check out ST application note on this very subject: http://www2.st.com/resource/en/application_note/CD00074286.pdf

• Interesting! Although the high voltage DC just after the rectifier is probably what OP wants to avoid. – kabZX Apr 18 '16 at 11:15
• The linear reg and the flyback give some carbon footprint. – Autistic Apr 18 '16 at 12:09
• They have that voltage clamp circuit (mosfet in series) which means you don't have to use 630V capacitors. 450V 2.2u elcap isn't physically that big. Of course if you want to have this rated for three phase CAT IV you need biiiiig creepages and airgaps. – Barleyman Apr 18 '16 at 12:22

You may use a Three-phase Transformer with only one core like this http://www.allaboutcircuits.com/textbook/alternating-current/chpt-10/three-phase-transformer-circuits/ But these transformers are not on stock for so small secondary voltages and power as you need. You may order them build for you in larger quantities.