I

I copied the buck converter in the picture and turned the simulation around.

But the resulting waveform was not what I wanted.

Shouldn't a dc waveform of 7.5V come out? What's the problem

What role do transformers, diodes, resistors, capacitors, etc. play on the MOSFET gate input side, and is there a separate name for that circuit?

 

What is the duty cycle of the 20khz waveform? Maybe 50%? Try 33%.