RDSon

Rds(on), short for drain–source on-resistance, is the resistance between the drain and source terminals of a MOSFET when the device is fully turned on. It is a key parameter for conduction losses in power electronics and is usually expressed in milliohms. Rds(on) is specified by manufacturers in datasheets under a defined gate-drive voltage (Vgs) and temperature, often at 25°C, and can be listed for other temperatures as well.

Rds(on) is not fixed; it depends on several factors. It is specified at a particular Vgs, and higher Vgs generally lowers the on-resistance up to the device limit. Temperature has a strong effect: as junction temperature rises, Rds(on) typically increases, raising conduction losses. Process variations and device geometry also contribute to tolerances around the rated value. P-channel MOSFETs tend to have higher Rds(on) than comparable N-channel devices for the same process.

In use, the power dissipated due to conduction is P = I^2 × Rds(on), where I is the load current. Therefore, lower Rds(on) reduces heat and improves efficiency for continuously conducting loads. Rds(on) must be considered alongside switching losses, thermal management, and the required gate drive.

Selection considerations include ensuring the Rds(on) rating matches the expected ambient and junction temperatures, choosing a suitable Vgs for the drive circuit, and providing adequate cooling. In high-current designs, multiple devices in parallel or devices with very low Rds(on) values may be used to meet thermal and electrical requirements.