Channel Resistance (RDS(on))
The resistance you see between drain and source while a MOSFET is fully conducting; it is given in the milliohm range. The power dissipated in it is calculated with P = I² × RDS(on): 15 A of current and 10 mΩ of resistance means about 2.25 W of heat per switch. It rises noticeably with temperature.
RDS(on) is the single most important parameter determining how hot a driver runs. Note that the loss grows with the square of the current: when you double the current the heat quadruples. A switch producing 2.25 W at 15 A produces 9 W at 30 A with the same resistance value.
The second trap is temperature. The RDS(on) value in data sheets is typically given at a junction temperature of 25 °C; when the junction rises to 100–125 °C the resistance generally increases by between 50 and 100 percent. For this reason making the thermal calculation with the room-temperature value is misleading, and the normalised curve in the data sheet should be used.
The field equivalent of this behaviour is the following: as the driver heats up the resistance rises, and as the resistance rises it heats up more. Sufficient cooling and a realistic current margin break this cycle. If your application's continuous current is very close to the driver's nominal rating, moving to a higher model (for example the 30 A PT500 instead of the 15 A KS250) is a thermally safer choice.
The context in which this term is used: DC motor and driver fault diagnosis matrix