Diode forward conduction loss

What does this quantity describe?

While a diode conducts in the forward direction, forward voltage VF drops across it as current IF flows through it; the instantaneous power dissipated as heat is Pinst = VF·IF. If the diode conducts only part of a periodic switching cycle — the duty cycle D — then, assuming constant VF and IF during conduction, the loss averaged over the period is Pavg = Pinst·D.

Picture a tap that is only open part of the time: instantaneous flow rate is constant while the tap is open, but the flow rate averaged over a longer time falls in proportion to the fraction of time the tap was actually open.

Formula and variables

Pinst = VF · IF; Pavg = Pinst · D

• Pinst = VF · IF
• Pavg = Pinst · D
• D = Pavg / Pinst

Choose the inputs correctly

Forward voltage VF and forward current IF set the instantaneous loss; the additional duty cycle D between 0 and 1 gives the averaged loss. VF depends on load and temperature and should be read from the forward characteristic in the datasheet at the actual operating point (IF and junction temperature) rather than assumed as a blanket constant across the whole current range.

How to use the calculator

Select the target quantity. For instantaneous loss, enter VF and IF; for average loss, additionally enter duty cycle D. To estimate the total loss of several identical, simultaneously conducting diodes, multiply Pavg by the number of diodes afterward.

Worked example

VF = 0.7 V and IF = 5 A give an instantaneous loss of Pinst = 3.5 W. If the diode conducts only 50% of a switching period — for example as a freewheeling diode in a buck converter at a 50% duty cycle — the average loss is Pavg = 3.5 W · 0.5 = 1.75 W.

Understand the result and units

An average loss of 1.75 W must be conducted away through the diode package and any heatsink to the ambient; the linked junction temperature calculator, combined with the diode's thermal resistance RthJA, can be used to estimate the resulting junction temperature.

Forward voltage is given in V, forward current in A, power loss in W, and duty cycle D dimensionless between 0 and 1 or as a percentage.

Conduction loss versus switching loss

Total diode loss in a switching application consists of conduction loss (this calculator) plus switching loss. Conduction loss occurs while the diode conducts and scales with VF·IF·D. Switching loss occurs during the transition between blocking and conducting states, especially from reverse-recovery charge, which briefly causes a reverse current during fast turn-off. At low switching frequencies, conduction losses usually dominate; at high switching frequencies in the hundreds-of-kHz-to-MHz range, switching losses can dominate instead — which is why Schottky diodes, with negligible reverse-recovery charge, are preferred there.

Typical applications

The relationship is used to estimate power loss for rectifier, freewheeling and protection diodes in power supplies and switching regulators, to pre-select diode package and heatsink, and for a rough efficiency comparison between silicon and Schottky diodes, since Schottky diodes have lower conduction losses due to their lower VF.

Assumptions, limits and common mistakes

The model captures only conduction losses under an assumed constant forward voltage and neglects the real VF(IF,Tj) characteristic, which is nonlinear with both current and temperature. Reverse-recovery loss at turn-off, dynamic switching losses at turn-on and turn-off, and reverse leakage loss are not included and must be additionally considered for fast-switching applications.

Common mistake: A common mistake is considering only conduction losses for fast-switching applications such as high-kHz switch-mode power supplies, even though switching and reverse-recovery losses can there make up a substantial, sometimes dominant, share of total losses. VF is also sometimes used as a fixed value independent of actual operating current, even though VF rises noticeably with increasing IF.