matter on buck chopper
The problem is when the switch is supposed to be in the off-state. The emitter is supposed to go negative, but the base can only go from 0V to Vin, so you won't be able to turn the BJT off. It would make more sense to use a PMOS or pnp as the power transistor. Alternatively, the pwm control can be referenced from the common connection between the switch, diode, and inductor. Your pwm control circuit would need to be able to withstand high voltages, though.