# Offline switcher SMPS design with two phase inputs

I am trying to build a power supply circuit which accepts two phases (220 VAC 50 Hz) and outputs 5V 1A. It must keep working if there is at least one phase present. Both phases can be coming from separate sources so I have taken a worse case phase difference of 180 degrees. Essentially the phase difference can be 0-180 degrees.

The two phases can be any two out of the following:

Phase-1,2,3 are three phases of a 3 phase 4 wire system. Inverter is common household inverter. The household appliances are usually wired like as shown above - Common neutral for all loads. Inverter is usually powered using one of the phases (phase-1,2,3). When that phase goes off, the inverter kicks in. When that phase is present, the power to all loads connected on inverter phase essentially is provided by that phase.

I could think of two ways to tackle this:

1) Use a separate bridge rectifier for both phases as shown below:

However the output voltage on bulk cap varies a lot depending upon whether both phases are present or just a single one is present. Please check the waveforms below:

Fig (above) : Both phases present (180 degrees phase difference, 350 V amplitude)

Fig (above) : Single phase present (180 degrees phase difference, 350 V amplitude)

I see two problems with this solution:

(a) 700 V bulk capacitor voltage will be difficult to handle with regular offline switching ICs.

(b) A voltage difference of 350 V whether both phases are present or not will add further difficulty. Since I'd also want it to work in universal voltage range 90-270 VAC, I guess it will add further complexity to the circuit and part selection. Cost is another concern.

2) Use a half wave rectifier for both phases.

Fig (above) : Half wave rectifier for both phases

(Resistor represents the complete SMPS circuit)

Waveforms are as shown below:

Fig (above) : Both phases present (180 degrees phase difference, 350 V amplitude)

Fig (above) : Single phase present (180 degrees phase difference, 350 V amplitude)

This looks like an easier thing to handle. Voltage levels are same in both cases. The only difference is - one has higher ripple than the other.

As such I am leaning towards going the half wave rectifier way. Am I doing this right? Am I missing something here?

• That ripple is negligible compared to the wide-range input voltage variation. Commented Jan 2, 2018 at 11:18
• Are you sure you can safely connect those two neutrals together in your situation? Commented Jan 2, 2018 at 11:31
• @Finbarr - Yes, for all other appliances, same neutral is used for whatever phase is being used for powering up the device. So, I think I can use the same neutral in this case as well. Commented Jan 2, 2018 at 11:57
• Do these two AC source floating with respect to each other, or do they have some common connection? This is very important in deciding the input architecture. Commented Jan 2, 2018 at 12:05
• @olin - added those details in the question. I hope I am clear. All phases are referenced to a common neutral. There can be some cases where an appliance might need to powered using phase and phase but my power supply won't be doing that. Commented Jan 2, 2018 at 13:18