The waveform required by the transformer is a square wave, so the voltage waveform after passing through the transformer is a square wave, not a sine wave
PWM control currents are often integrated circuits, usually isolated from the primary side by a small transformer, and sometimes isolated from the primary side by a coupling chip (a small IC chip with LEDs and phototransistors)
The PWM control circuit controls the closing of the switch tube of the power supply according to the output load condition of the power supply. If the output voltage is too high or too low, the PWM control circuit will change the voltage waveform to adapt to the switching tube, so as to achieve the purpose of correcting the output voltage