9.2.2 PWM

Pulse width modulation means a device is either fully on (100%) or completely off (0%). The period of PWM is the length of a ``cycle'', and this is fixed at some relatively small amount of time. For DC motor control, a period of 5ms is suitable for smaller motors, while bigger ones can have periods up to 20ms.

Within a cycle, a controller controls the proportion of time in which the device is on. In other words, for a motor not to do anything, the duty cycle is 0%. To utilize all the torque available, the duty cycle should be 100%. A controller can also adjust the duty cycle to any point between 0% and 100% to change the average voltage applied to the motor.

How is this better? Let's try to produce an average of 6V across a motor, just like in the previous subsection.

The available voltage is 12V, this means we want to have a 50% duty cycle. In other words, half of the time, the transistor is fully on. When a transistor, be it N-MOSFET or NPN BJT, is fully on, the power dissipation on the device is at a minimum. A modest N-MOSFET has a resistance of about $0.1 \Omega$. This means the power dissipated at the transistor is $P=I^2R=16\times 0.1=1.6\mathrm{W}$ when the transistor is turned on at 100% duty cycle. At 50% duty cycle, the power dissipation is exactly one half at 0.8W. 0.8W can be dissipated by a TO-220 package without external heatsink at room temperature. We just reduced our power dissipation from 12W to 0.8W!