What are conduction losses in power semiconductor devices?

Conduction losses occur when the device is fully on and carrying current. For MOSFETs: Pcond = I^2 x Rds(on), where Rds(on) is on-state resistance (increases with temperature). For IGBTs and diodes: Pcond = Vce(sat) x I + r x I^2, where Vce(sat) is saturation voltage and r is slope resistance. Conduction losses are significant at high currents. MOSFETs have lower conduction losses at low voltages; IGBTs at high voltages. Total device losses = switching losses + conduction losses.