Electronic – Why are the transformer windings in series with line and load in this circuit

common-mode-chokeinductorpower supplytransformer

I came across this switching power supply circuit and I noticed the transformer is connected in a way I had never seen before.
What is the benefit of connecting it like that? And why does the circuit use a capacitor voltage divider?

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Best Answer

In order to understand the circuit, you need to understand what it is trying to accomplish. Primarily, it is a high frequency (low pass) line filter. This is accomplished with a capacitor and an inductor, for each line (4 components).
Some smart person figured out that the windings of a 1:1 transformer could be used as the inductor with the advantage of reducing the component count (3 now) and the additional benefit of cancelling out any common mode "noise" on the lines. It is this last benefit, that requires the use of a "transformer," to keep each "inductor's" characteristics, as equal as possible (same core, materials,number of turns, etc.).
In conclusion, the answer is, that each transformer winding is being used as an inductor, that's why they are connected as shown.