Step-down (buck) converter
$$ D = 1 - \frac{V_{out}}{V_{in}} $$
Where:
D = Duty cycle
V_out = Output voltage
V_in = Input voltage
Ideal average output voltage
$$ V_{out} = V_{in} \cdot (1-D) $$
The voltage increases as the duty cycle (ratio) increases, so if
D ≥ 1, the output voltage will increases infinitely.
Resistance in buck converter