Conduction Loss
The conversion into heat, in the channel resistance, of the current flowing through a switch while it is fully conducting. It is calculated with the relation P = I² × RDS(on) and grows with the square of the current: doubling the current quadruples this loss. Unlike switching loss it is independent of frequency.
Conduction loss makes up most of a driver's heat budget in continuous operation. A switch with 10 mΩ channel resistance produces 2.25 W at 15 A and 9 W at 30 A. Since in an H-bridge the current flows through two switches at every instant, this value has to be multiplied by two when calculating the total loss.
The fact that the loss is quadratic explains why leaving current headroom matters so much. A driver running at 80 percent of the nominal current produces not just 20 percent but about 36 percent less conduction heat than one running at nominal. This difference is reflected directly in the junction temperature that determines the driver's life.
Its field equivalent is a selection decision. If your motor continuously draws 14 A, a module rated at 15 A continuous is sufficient on paper but leaves no thermal headroom; the PT500 with 30 A continuous runs much cooler at the same load. If starting pulses are repeated often this headroom becomes even more critical.
The context in which this term is used: DC Motor Driver Selection Guide