Is building a DC-DC PSU PWM controller from discrete components a terrible idea?
What is your technical definition of a PWM controller? – Andy aka



  @Andyaka A circuit that changes the duty cycle of a constant frequency
  pulse wave outputs in response to analog inputs


OK, just like an LTC6992 then.


  Does anyone knows if it is a useful practice to build a PWM controller
  from discrete components or not and why?


I'd say it's not great practice because there are such devices available. The LTC6992: -