watson wrote on 2023-10-03, 10:46:

The two caps are not in parallel, they are in series, and they are typically rated at 200V. This enables you to get a higher vol […]
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The two caps are not in parallel, they are in series, and they are typically rated at 200V.
This enables you to get a higher voltage out of 120 V line voltage. You rectify the positive half-wave on one cap, and the negative half-wave on the other.
This mode of operation is adjusted by the "115/230" switch. The resulting DC voltage is then ideally 120*sqrt(2)*2.
When operating in 230V mode, the DC voltage is simply 230*sqrt(2) stored across an effective "400V" capacitor.

Active PFC is basically a boost converter, meaning that it can theoretically boost any input voltage to (most often) slightly below 400V.
That's why you have a single 400V or 420V rated cap in PSUs with active PFC.
The only thing that I don't understand is why not all PSUs with active PFC have a full range input (110V-230V). Some are rated only for 230V. Maybe it is to save a few cents on some components?

I live in place where mains is ~230 V and I have not even one old or new PSU with voltage switch (110/230 V), so when i mean two caps in parallel I mean two caps in parallel in therms of higher capacitance, rated at 250+ V. And there is bridge rectifier before the caps, what they will "rectify" in this case?

analog_programmer wrote on 2023-10-03, 11:45:

I live in place where mains is ~230 V and I have not even one old or new PSU with voltage switch (110/230 V), so when i mean two caps in parallel I mean two caps in parallel in therms of higher capacitance, rated at 250+ V. And there is bridge rectifier before the caps, what they will "rectify" in this case?

I am sorry for using the wrong term. Capacitors are obviously not rectifying anything, they are just storing a charge.

You obviously cannot store rectified 230V AC in a 250 V rated capacitor.
Here is a better explanation of what I was trying to say: http://www.righto.com/2021/05/teardown-of-pc- … wer-supply.html

watson wrote on 2023-10-03, 12:04:

I am sorry for using the wrong term. Capacitors are obviously not rectifying anything, they are just storing a charge.

You obviously cannot store rectified 230V AC in a 250 V rated capacitor.
Here is a better explanation of what I was trying to say: http://www.righto.com/2021/05/teardown-of-pc- … wer-supply.html

Aaaaah, I see. A schematic with half bridge rectifier for half the input voltage. I don't have any PSU built upon such scematic, but yes, you're right in this case for the two caps with lower voltage rating in series for half the input voltage. Seems fully reasonable for "universal" mains PSU schematic.

I own PSUs with one or two filtering caps on the input and I've no clue if there's a rule if one cap usually means active PFC scematic. See the Rhombu-sh*t #5 pics - two caps, and the last FSP #9 is with one cap (probably both PSUs are made with active PFC).