A necessity for all professional power supplies a circuit breaker. Once the power goes above 100W or you accidentally short the output the voltage at the diode goes up because the mosfet cant drop all the voltage and after reaching a certain threshold it becomes high enough to activate the bjt and thus sinking in the gate voltage of the mosfet and disabling it. By putting higher power mosfets you increase the threshold thus allowing the circuit to work at higher powers. Now to engage the short circuit (the overload) close the second switch and then reopen it. You now see that the circuit has kicked in and no current flows through the load. The circuit will remain in this state until its been reset by the first switch. Now usually there is a little downside for these circuit breakers. Imagine you hook up a circuit with capacitive behaviour. When a capacitor is being charged at the first moment its resistance is almost equal to zero and it draws as much as you can offer. This means that every time you try to charge the capacitor the circuit breaker will kick in and cut off the power. Now to fix this just increase the capacitor at the base of the bjt to 100uF. This will delay the breaker just enough so that it doesnt kick immediately and stop you from charging the given cap. So 100nF fast breaker for normal resistive loads 100uF slow breaker for capacitive loads that have a peak draw in the beggining.
