BLDC Inverter Loss Calculator
BLDC Inverter Loss Calculator
Losses in a three-phase, six-MOSFET motor inverter with sinusoidal phase current: conduction, switching and dead-time diode losses per device and in total, and the inverter’s efficiency at your output power.
Three-phase inverter loss
48 V bus, 20 A RMS phase current, 20 kHz, 3 mΩ × 1.5, 60 ns + 40 ns transitions, 0.8 V body diode, 500 ns dead time, 800 W out
Per-device losses, sinusoidal current
- Î
- peak phase current = √2 × RMS
- Î/π
- the phase current’s average over a full cycle of the half-wave one device hard-switches (Infineon’s equivalent DC current)
- td
- dead time; the body diode conducts in each of the two per PWM period
Worked example
48 V bus, 20 A RMS phase current, 20 kHz, 3 mΩ × 1.5, 60 ns + 40 ns transitions, 0.8 V body diode, 500 ns dead time, 800 W out
Î = √2 × 20 = 28.28 A
Conduction per MOSFET = 0.0045 × 800 ÷ 4 = 0.90 W
Switching per MOSFET = ½ × 48 × (28.28/π) × 100 ns × 20 kHz = 0.432 W
Dead-time diode per MOSFET = 0.8 × (28.28/π) × 1 µs × 20 kHz = 0.144 W
Total = 6 × 1.476 = 8.857 W; efficiency 800 ÷ 808.9 = 98.90%
Losses in a three-phase motor inverter
A brushless motor drive’s inverter has three half-bridge legs, six MOSFETs in all. This page models sinusoidal phase current — field-oriented control, sine PWM or space-vector PWM — because that has closed-form averages. Trapezoidal (six-step, block-commutated) drive has rectangular phase currents, with only two phases conducting at a time and losses that depend on which switch does the PWM chopping; these formulas do not describe it.
Conduction. With complementary switching, one MOSFET in each leg is always on and its channel carries the phase current in either direction. Over a full electrical cycle each leg dissipates R·Irms², and by symmetry the two devices share it equally: R·Î²/4 each, independent of the modulation depth and the power factor. It is the same result as Infineon’s IGBT and diode terms with equal resistances, and a PWM-period-by-period simulation over random modulation depths and phase angles agrees.
Switching. In each PWM period the leg makes one hard turn-on and one hard turn-off, by the device the current flows forward through; the other device switches softly, into its own diode. Averaged over the cycle, each device switches an equivalent DC current of Î/π (Graovac & Pürschel, Infineon, 2009). The ½·V·I·t overlap model assumes linear edges, so use transition times measured with your gate drive, not the datasheet’s test-circuit figures.
Dead time. Both dead times in each PWM period push the current through a body diode at VF. In the example they take 2.00% of each period and cost 0.864 W across the inverter; a Schottky, a GaN device or a shorter dead time reduces this. Body-diode reverse recovery at each hard turn-on, Coss and gate-drive losses are left out; the MOSFET loss calculator estimates the last two for one device.
Heat. The per-device total is what each MOSFET’s thermal path has to carry. Enter it, with your maximum junction temperature, in the heatsink thermal resistance calculator — remember that all six usually share one heatsink, so it sees the inverter total. The chart shows why current matters most: conduction grows with the square of the current, switching and dead-time loss only in proportion.
Frequently asked questions
How do you calculate conduction loss in a three-phase inverter?
For MOSFETs with sinusoidal current and synchronous conduction, RDS(on) × Î² ÷ 4 per device, Î being the peak phase current. 20 A RMS through 4.5 mΩ gives 0.90 W per MOSFET.
How do you calculate switching loss in a motor inverter?
½ × Vbus × (Î/π) × (turn-on + turn-off time) × PWM frequency per device. For 48 V, 20 A RMS, 100 ns and 20 kHz that is 0.432 W.
How much loss does dead time cause?
About Vf × (Î/π) × 2 × dead time × PWM frequency per device: 0.144 W each, 0.864 W in total, for 500 ns at 20 kHz in the example.
Does this work for trapezoidal (six-step) BLDC drive?
No. It assumes sinusoidal phase current. Six-step drive has rectangular currents and losses that depend on the chopping scheme.
Related calculators
References
- Graovac D, Pürschel M. IGBT Power Losses Calculation Using the Data-Sheet Parameters. Infineon application note V1.1, January 2009. Three-phase inverter losses with a sinusoidal output: conduction terms in Io²/8 and the equivalent switched current Io/π.
- Balogh L. Fundamentals of MOSFET and IGBT Gate Driver Circuits. Texas Instruments application report SLUA618A, revised October 2018. Switching loss V·I·(t2 + t3) ÷ 2T per transition; gate-drive power QG·VDRV·fDRV, dissipated in the gate-drive path, not in the MOSFET.
- Erickson RW, Maksimović D. Fundamentals of Power Electronics, 3rd ed. Springer, 2020. Ch. 2 (inductor volt-second and capacitor charge balance, ripple), Ch. 5 (the CCM–DCM boundary), Ch. 8, Table 8.2 (right-half-plane zero: D′²R/L for the boost, D′²R/(DL) for the buck-boost).