What is the purpose of this AC trace in this SMPS (schematic)

[Beamin]

I watched the video but he doesn't explain two things the purpose of the cap I circled in red or why they have AC going through the switched part of the circuit (red line also goes trough mystery cap). Does this go through that first coil on the bottom right and provide power to the chip that is the oscillator seen on left side?

How does the 60 Hz AC stay in step with the FETs feeding those other coils? I would think you would want that line isolated and its strange tat it terminates at the neg of the box shaped rectifier not the other AC line is tis because of that cap?

Full video:
fhttps://www.youtube.com/watch?v=eq5YpKHXJDM

[Dukov Ahzrukhal]

The red trace is not actually AC, notice its connected to the big capacitors. The reason it has the AC in connected to it is because that rectifier can operate as a voltage doubler depending on the voltage selection switch. You should look online for voltage doubler circuits if you are not already familiar with them. The capacitor you circled appears to be a DC blocking capacitor but to be honest im not really sure of what its doing there. I will attach the schematics of an AT computer power supply, its somewhat similar to what you have there so you may find it helpful to look at it.

[free_electron]

look up 'resonant converter'.

this has all the smell of that type of converter

[T3sl4co1l]

Nah, that's bog standard half bridge, capacitor coupled, forward converter.

Capacitor coupling is used to prevent "flux walking", a silly term when the culprit is DC bias in the core.  Cap blocks DC, all is well.

Tim