In switching mode power supplies like this, the mains AC is rectified and then chopped up into higher frequency pulses and fed to the transformer. The higher fr

In switching mode power supplies like this, the mains AC is rectified and then chopped up into higher frequency pulses and fed to the transformer. The higher frequency allows for smaller transformer. It also allows for voltage regulation because the input pulse width (in this case via the UC3842B controller IC) can be altered based on the sensed output voltage.