How The AC stage of an Adaptor works

[Tarot Superstars]

Hi. I have been watching videos on how a power adaptor works.
There is explanation of the rectifier bridge to the pulse width modulator, the mosfet for switching, and opto-couplers, but there is little to nothing about the AC stage of the transformer the powers the primary coil.

So, I see that the PWM needs DC to work, but I don't see anything about how the AC gets from the mains supply to the transformer stage.
A transformer only works on AC so where is that part of the circuit.
I only see Capacitors on the output. Should I assume that the secondary coil and capacitors is the only thing that changes AC to DC?
I don't see anywhere explained about the mosfet switching polarity between the drain and the source either.

I have watched this video:
https://www.youtube.com/watch?v=IoSaaK71tGY
and I have read basic schematics but they all seem to miss out the AC to primary circuit.

I have attached an example. I am working on learning practical from this adaptor.
I have a schematic from the IC data sheet but it doesn't explain the AC to primary coil. It doesn't explain the AC to DC from the secondary coil.

It also doesn't explain how DC gets from the secondary to the output.

Could anyone explain the mains to AC primary stage for me please?
Could some one explain the AC secondary coil to the DC output stage please?
Could someone explain if and also how the mosfet switches polarity between Source and drain.
Could anyone point me to resources on those stages?

[eccerr0r]

Re: How The AC stage of an Adaptor works

On the primary side, dc is converted to "ac" (note the "quotes") by the transistor - which does this by turning the dc on and off rapidly. Technically this is not "ac" but rather switched dc. The key concept is that transformers can only pass energy to the secondary if and only if the magnetic field is changing. AC clearly is constantly changing and thus the field is changing, but this works fine with switched dc as its field is likewise constantly changing.
Of course the magnetic field is a square wave so the output of the transformer is also likewise a square wave. A diode is still needed as we want to make sure power flows towards the output even when the transformer goes back to 0 volts when the primary switch transistor turns off.

[stj]

Re: How The AC stage of an Adaptor works

watch some psu teardowns from this guy - he's a good teacher
https://www.youtube.com/channel/UCQa...B_Kd54g/videos