Diode Power Loss Calculator
Diode Power Loss Calculator
Estimate a rectifier or freewheel diode’s losses — conduction, reverse recovery and leakage — and its junction temperature, using either a single forward voltage or the threshold-plus-resistance model from the datasheet.
Diode losses and junction temperature
Freewheel diode, 3 A average, 4 A RMS, VT0 0.7 V, rd 20 mΩ, Qrr 50 nC at 48 V and 100 kHz, k = ½, 10 µA leakage blocking half the time, θJA 25 °C/W, 50 °C ambient
Diode losses
- VT0, rd
- the straight-line model of the forward characteristic at your temperature (ST AN604)
- Qrr
- reverse recovery charge at your conditions
- k
- share of Qrr·VR that ends up in the diode; ½ is a conservative estimate, AN5028’s triangular model gives at most ⅓
- θJA
- junction-to-ambient thermal resistance
Worked example
Freewheel diode, 3 A average, 4 A RMS, VT0 0.7 V, rd 20 mΩ, Qrr 50 nC at 48 V and 100 kHz, k = ½, 10 µA leakage blocking half the time, θJA 25 °C/W, 50 °C ambient
Conduction = 0.7 × 3 + 0.020 × 4² = 2.42 W (an effective VF of 0.807 V)
Recovery = ½ × 50 nC × 48 V × 100 kHz = 0.12 W
Leakage = 48 × 10 µA × 0.5 = 0.24 mW
Total = 2.54 W; Tj = 50 + 2.540 × 25 = 113.5 °C
Where a diode’s losses come from
Conduction. Most of a power diode’s loss is the forward voltage times the current. A single VF times the average current is fine when the current is nearly steady. When it is pulsed — a rectifier feeding a capacitor, a freewheel diode carrying a ramp — the straight-line model is better: a threshold VT0 plus a resistance rd, giving VT0·IAV + rd·IRMS² (ST AN604). The resistive part grows with the square of the current, so a peaky waveform heats the diode more than its average suggests. Read both parameters at a hot junction: silicon’s forward voltage drops by about 2 mV/°C.
Reverse recovery. A PN diode that has been conducting holds stored charge; when the circuit reverses it, current flows backwards until that charge Qrr is swept out, while the voltage across it rises. The energy lost is some fraction of Qrr·VR each cycle. ST’s AN5028 models the tail as a triangle and gets VR·Qb/3, where Qb is the part after the reverse-current peak; so the page’s default of ½ is a deliberately conservative estimate — set it to what your measurements or the manufacturer’s model support. Recovery also costs the switch that turns on against the diode, often more than the diode itself: the MOSFET loss calculator covers the switch.
Schottky or ultrafast? A Schottky diode has no minority-carrier storage, so no recovery loss and a lower forward drop, but its leakage current is far higher and climbs steeply with temperature; in a hot, high-voltage position that leakage can run away. Silicon Schottkys stop at about 100–200 V; above that, ultrafast (fast-recovery) or SiC Schottky diodes take over, with higher VF and, for ultrafast silicon, some Qrr. That is why this page asks for both.
Temperature. The total times θJA gives the rise above ambient. θJA from a datasheet is measured on a JEDEC test board (JESD51) and your layout may be better or worse; with a heatsink, add the path up in the heatsink thermal resistance calculator. The drawing warns amber within 15% of your maximum junction temperature and red above it.
Frequently asked questions
How do you calculate diode conduction loss?
VT0 × Iavg + rd × Irms², or simply VF × Iavg for a steady current. 0.7 V and 20 mΩ at 3 A average and 4 A RMS give 2.42 W.
How do you calculate diode reverse recovery loss?
About k × Qrr × VR × fsw. k = ½ is a conservative estimate; ST’s triangular model gives VR × Qb / 3, at most a third. 50 nC at 48 V and 100 kHz with k = ½ is 0.12 W.
Does a Schottky diode have reverse recovery loss?
Practically none — it has only junction capacitance to charge — but its leakage current is much higher and rises sharply with temperature, so include leakage loss.
How hot will my diode get?
Tj = Ta + Ptotal × θJA. 2.540 W on 25 °C/W at 50 °C ambient is 113.5 °C.
Related calculators
References
- STMicroelectronics. Calculation of conduction losses in a power rectifier. Application note AN604, Rev 3, August 2011. P = VT0·IF(AV) + rd·IF(RMS)².
- STMicroelectronics. Calculation of turn-off power losses generated by an ultrafast diode. Application note AN5028, Rev 1, October 2017. Diode turn-off loss f·VR·Qb/3.
- Vishay Semiconductors. How to Calculate Power Losses in Gen 5 Diodes. Application note 98280, revised May 2022. Forward (VT0, RD) and reverse-leakage losses.
- JEDEC JESD51-2A. Integrated Circuits Thermal Test Method Environmental Conditions — Natural Convection (Still Air). JEDEC Solid State Technology Association, 2008. The conditions under which a datasheet θJA is measured.