Q.What is the duty ratio of a chopper?

A DC chopper converts a fixed DC voltage into a variable DC output by switching the supply on and off rapidly. It is a DC-to-DC converter used in variable-voltage DC supplies, battery chargers, switched-mode power supplies and DC motor-speed control.

A power switch — commonly a MOSFET — is placed between the DC source and the load. A gate pulse switches it ON, connecting the supply to the load; removing the gate signal switches it OFF, disconnecting the load. For a resistive load, the output voltage and current follow the shape of the gate pulses, appearing as a chopped DC waveform.

The output is controlled through the duty ratio — the ratio of the ON-time to the total switching period. The average output voltage is

$V_{av} = \frac{T_{ON}}{T}\,V_S,\qquad \text{Duty ratio} = \frac{T_{ON}}{T}$ …