noise coming into the source question

[john23]

Hello, in the lecture below and photo they say that noise comes from the circuit into the source.
I dont see how?
Its a buck converter when the inductor is charge the mosfet is closed and the magnetic energy disipates on the load and diode.
How exactly noise coming form the circuit into the source?
Thanks.
15:04

[MariuszD]

To understand advanced courses, you should first take the basic course.

The transistor draws current from the source intermittently. Despite the use of a capacitor at the input that smooths the pulses, the AC component still exists. The AC component is noise.

[john23]

so you say that ripple passes threw gate source and gate drain of the mosfet?
Thanks.

[MariuszD]

so you say that ripple passes threw gate source and gate drain of the mosfet?
Thanks.

No.
Rapid changes in current cause EMI/RFI. It doesn't matter whether the switch is a MOSFET, BJT, or even an electromechanical vibrator (ancient technology from vacuum tube era).  Maybe you didn't notice that he is talking about the power source and not the MOSFET source.