Input capacitor in buck converter

Question

I've chosen this Reference Design as starting point. Here the schematic. I'm using full wave rectifier instead.

I'm designing a buck converter with these requirements:

1. Input voltage: 120-690Vrms AC (three phase) 50/60Hz
2. Output: 50V/200mA (max)

To meet the high voltage requirements, the reference design uses a balancing network of three 400V capacitors, which may be expensive.

For my application following the controller datasheet, I calculated a minimum capacitance of 10 uF. However, I'm considering if I really need it. For example, this another Reference Design based on flyback topology doesn't use bulk input capacitors, just ceramic ones. But I think that applies if I always have three phase, right? If it happens that only two phases are available, then may I run into issues? What do you suggest?

Answer 1

If you have three phase power, you always at have at least 50% your peak voltage available. Single phase power you go down to 0 volts at every zero cross so you need some amount of bulk capacitance.

1 Phase:

3 Phase:

Images from https://www.elprocus.com/difference-between-single-phase-and-three-phase-ac-power-supply/