Low-Side Switch
It is the power switch that connects the motor terminal to the chassis (negative) line. Because its source is connected directly to chassis, its gate can simply be driven between 0 and 12 V; it therefore needs no additional supply or bootstrap and is the easiest element of an H-bridge to drive.
The reason the low-side switch is easy to drive is its reference point: the source terminal is at chassis and stays there regardless of the conduction state. So the gate–source voltage can be produced directly from the logic supply; typically 10–12 V is enough, and with logic-level MOSFETs even 5 V can do.
In an H-bridge these switches have a second duty as well. If the two low-side switches are turned on at the same time, both terminals of the motor are connected to chassis; the back EMF the motor generates drives current through the winding and a counter-torque arises. This behaviour is what we call dynamic braking. The same arrangement is also used to provide slow decay during the off phase of the PWM.
In simple unidirectional applications (e.g. on–off and speed control of a fan or pump) a single low-side switch is enough. The moment direction reversal is needed, a full H-bridge is essential; AXI's DC driver modules provide forward–reverse direction control through this topology.
Context where this term is used: DC Motor Driver Selection Guide