MOSFET Conduction Loss

Formula

Description

When a MOSFET is fully turned on, it behaves like a small resistor (Rds_on) between drain and source. The power dissipated during conduction follows the I-squared-R relationship. Modern power MOSFETs have Rds_on values ranging from milliohms to hundreds of milliohms depending on voltage rating and die size. Lower Rds_on means less conduction loss but typically comes with higher gate charge and switching losses. Conduction losses dominate at low switching frequencies, while switching losses dominate at high frequencies.

Variables

• P — Conduction power loss (W)
• I — Drain current (A)
• Rds(on) — On-state drain-source resistance (Ω)

Practical Notes

Rds_on increases with temperature, typically doubling between 25°C and 125°C for silicon MOSFETs. Always use the Rds_on value at your expected junction temperature, not the room temperature value from the datasheet summary. For paralleled MOSFETs, Rds_on divides by the number of devices, naturally sharing current due to positive temperature coefficient.