High-Side Switch
The power switch that connects the motor terminal to the positive supply line. Because its source is not at a fixed reference and rises to the supply voltage while conducting, it is harder to drive than a low-side switch; the gate voltage typically has to be held 10-12 V above the supply.
Each leg of an H-bridge contains one high-side and one low-side switch. When the high-side switch turns on it connects the motor terminal directly to the positive supply line; working together with the low-side switch of the opposite leg it applies the full voltage to the motor and sets the direction of rotation.
The difficulty is on the drive side. When an N-channel MOSFET is used, the gate must be positive with respect to the source terminal; and the source terminal rises to supply level while conducting. That is why in a 24 V system a voltage on the order of 34-36 V is needed at the gate. The solution is either a bootstrap circuit, an isolated high-side driver, or the use of a P-channel MOSFET. A P-channel is simpler to drive but typically requires a higher RDS(on) and a larger package at the same current.
On the user side the visible consequence is this: when the high-side switch fails, the motor either does not turn at all in one direction or stays permanently in that direction. On a driver that does not work in one direction, first verify the command input and the cable; if both are sound, the fault is most likely in that leg of the power stage.
The context in which this term is used: DC Motor Driver Selection Guide