MOSFET Conduction Loss
Calculate the conduction loss in a MOSFET, P = I² × R_ds(on), from the current through it I and the on-state channel resistance R_ds(on). The result, in watts, is the power dissipated as heat while the transistor is on, due to the channel resistance. It is one of the two main losses in power switches (the other being switching loss). MOSFETs with lower R_ds(on) reduce these losses, important at high currents and low frequencies. The conduction losses, plus the switching losses, set the heating and the heatsink requirement. Enter the current and the on-state resistance.
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MOSFET conduction losses
A power MOSFET, when it is on (conducting), is not a perfect short circuit: it has a small channel resistance called R_ds(on) (drain-source on-resistance). The current flowing through that resistance dissipates power as heat — the conduction losses: P = I² × R_ds(on), where I is the current and R_ds(on) the on-resistance. Note the squared current: conduction losses grow with the square of the current, which makes them dominant in high-current applications. Together with the switching losses (which occur during the on-off transitions and grow with frequency), they form the two main heat sources in a power switch, and their balance sets the optimum operating frequency: at low frequency conduction dominates, at high frequency switching takes over. The war among MOSFET makers is precisely over ever lower R_ds(on) (from ohms down to milliohms and even microhms), since every reduction cuts conduction losses directly, allowing higher currents with less heat — and wide-bandgap semiconductor technologies (SiC, silicon carbide, and GaN, gallium nitride) push this further, with very low resistances and operation at high voltages and frequencies. Computing the conduction losses (added to the switching losses) is the first step in thermal sizing: the total dissipated power, together with the thermal resistance, sets the junction temperature and tells whether a heat sink is needed. Careful: R_ds(on) rises with temperature (a positive coefficient, which helps when paralleling MOSFETs but requires using the hot value in the calculations). Enter the current and the on-resistance.
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