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Input capacitor in buck converter
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Red_Micro:
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:

Input voltage: 120-690Vrms AC (three phase) 50/60Hz
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 or film 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?
graybeard:
My issue with that design is the output is not isolated from the mains.
Red_Micro:

--- Quote from: graybeard on May 04, 2020, 05:03:32 am ---My issue with that design is the output is not isolated from the mains.

--- End quote ---

My application does not require isolation. If I put it, it would be a cost adder. But it will be more safe.
tszaboo:

--- Quote from: Red_Micro on May 04, 2020, 12:51:17 am --- What do you suggest?

--- End quote ---
Buy a 10W 48V isolated DC-DC converter from a reputable brand. Why would you even connect it on 3 phase in the first place. Even if you have a 3 phase connection for other reasons, just leave the other 2 out of the design.
jbb:
There’s nothing wrong with a non-isolated converter, but be careful when testing! Your supply design power is small but there might be a lot of fault energy available in your test setup.

The selection of capacitors needs to hit a few things: low impedance against HF switching, high enough ripple current rating, enough energy to keep voltage somewhat stable over 50 / 60 Hz line cycle, and probably other stuff I’ve forgotten.

3x 10uF electrolytic is probably quite cheap. They are in series, so 3.3uF effective. Now derate by 50% for tolerance and ageing, get around 1.7uF effective.

If energy is the limiting factor, a 2.2uF film capacitor would be equivalent but more expensive.

For safety reasons you probably need some bleed resistors in there in any case.

Given the voltages in play, you should try to find operating voltage specifications for your inductor in addition to the usual inductance and current ratings.
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