Diode Forward Loss⚠ unverified

Understanding the Parameters

• $V_f$ — the on-state voltage of the pn junction (or Schottky barrier). Typical Si rectifiers are ~0.7–1.1 V; Schottkys lower; SiC Schottkys higher than Si Schottky but with better reverse recovery. $V_f$ depends on current and temperature (Si $V_f$ falls with temperature; some Schottkys rise).

• $I_{avg}$ — average current, because for a constant voltage drop $p = V_f\,i(t)$ averages to $V_f\langle i\rangle = V_f I_{avg}$. Using RMS here would be the wrong statistic (RMS is for $I^2R$ heating).

• $P$ — heat generated in the diode junction; drives package and heatsink sizing together with switching/recovery losses.

Derivation (Approaching a Proof)

Model the conducting diode as $v(t) \approx V_f$ (piecewise constant). Instantaneous power is $p(t) = v(t)\,i(t) \approx V_f\,i(t)$ while on and zero while off. Averaging over a period $T$,

$$P = \frac{1}{T}\int_0^{T} V_f\,i(t)\,dt = V_f\left(\frac{1}{T}\int_0^{T} i(t)\,dt\right) = V_f\,I_{avg}.$$

This is Watt's Law with a fixed voltage. A more complete model adds a small dynamic resistance: $P \approx V_f I_{avg} + r_d I_{rms}^2$, blending this calculator with Conduction Loss.

History

Diode conduction loss has limited rectifier and freewheel efficiency since the first power diodes. Synchronous rectification (replacing the diode with a low-$R_{DS(on)}$ MOSFET) exists primarily to replace $V_f I_{avg}$ with a much smaller $I_{rms}^2 R_{DS}$ term at low output voltages.

Related Concepts: Conduction Loss, Synchronous Rectifier Loss, Dead Time Loss, Switching Loss, Watt's Law

Notes: Registry calculator diode-forward-loss (unverified). Constant-$V_f$ model; ignores reverse recovery and capacitive switching loss.