Pulse width modulation explication.
If the switch changes its state, the Vref changes too. Then the duty cycle D changes.
If Vt(triangular carrier) > Vref(modulating) the comparator sets its state in 0V.
If Vt(triangular carrier) < Vref(modulating) the comparator sets its state in 15V.
Try to switch between the two Vref. You Will look the Vo changes.
The variation of Vref sets the D.
D=25% with Vzener=3.75 V (0.25*15)
D=5% with Vzener=7.5 V (0.5*15)
D=75% with Vzener=11.25 V ~ 11.3 V(0.75*15)
