Conduction Loss⚠ unverified
Electrical / Power · Compute the conduction loss in a switching device
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| I_rms | Irms | A | 1.0 | RMS current through the device |
| R_on | Ron | ohm | 1.0 | On-state resistance of the device |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | Pcond | W | Conduction power loss, in watts (W) |
The science & history
Understanding the Parameters
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$I_{rms}$ — the RMS (not average) current, because heating depends on the mean of the current squared. Using average current would under‑count the loss for pulsed/ripply waveforms.
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$R_{on}$ — the device's on resistance ($R_{DS(on)}$ for a MOSFET); it rises with temperature, so loss and temperature feed back on each other.
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$P_{cond}$ — the heat generated in the conducting device.
Derivation (Approaching a Proof)
While on, the device behaves like a resistor $R_{on}$, so its instantaneous dissipation is $p(t) = i(t)^2 R_{on}$. Averaging over a cycle,
$$P_{cond} = \langle i^2 \rangle R_{on} = I_{rms}^2\,R_{on},$$
since $I_{rms}^2 \equiv \langle i^2\rangle$ by definition of RMS. This is Watt's Law specialised to a resistive drop — the same $I^2R$ that governs any conductor, applied to the switch's on‑resistance. (For diodes/IGBTs with a forward voltage $V_F$, a $V_F\,I_{avg}$ term is added.)
History
Minimising conduction loss has driven power‑semiconductor development for decades — from bipolar devices to low‑$R_{DS(on)}$ silicon MOSFETs and now wide‑bandgap SiC/GaN devices, which cut both conduction and switching losses to enable smaller, more efficient converters.
Related Concepts: Switching Loss, Watt's Law, Buck Converter Duty Cycle
Notes: Registry calculator conduction-loss (unverified). Resistive‑device model; add a $V_F I_{avg}$
term for diode/IGBT forward drops.