The problem
The traction inverter converts battery DC into controlled three-phase AC and is one of the main sources of loss and heat in an EV. Students often treat it as an ideal block. Designing one on paper and in simulation, with real device data, teaches how current rating, switching frequency and DC-link capacitance interact.
Objectives
- Translate drive requirements into inverter voltage and current ratings.
- Select example devices and estimate conduction and switching losses.
- Size the DC-link capacitor for ripple current and voltage.
- Simulate the inverter with a motor-like load and SVPWM.
- Compare simulated losses with analytical estimates.
What you need
Tools and toolboxes
- Simulink — Modulator, load, scripting and logging.
- Simscape Electrical — MOSFET switching models, DC source, DC-link capacitor and three-phase load.
- Simscape — Physical-network solver and thermal elements for a simple heatsink estimate (installed with Simscape Electrical).
Software
- MATLAB R2023b or later
- Simulink
- Simscape Electrical
- Example MOSFET datasheet values
Extra parts and data
None beyond the software above.
Key concepts
- Phase current from power
- For a sinusoidal drive, P = √3·VLL,rms·Iph,rms·cosφ, so phase current follows from power, line voltage and power factor.
- Conduction loss
- For a MOSFET, P = Irms,switch²·Rds(on), with Rds(on) taken at the expected junction temperature, which can be well above the 25 °C datasheet value.
- Switching loss
- Approximately fs·(Eon + Eoff) scaled from datasheet test conditions to the actual voltage and current, averaged over the output cycle.
- DC-link ripple current
- The inverter draws pulsed current from the DC link; the capacitor must carry the ripple RMS current, which depends on modulation index and power factor.
Step-by-step guide
- 1
Set the drive requirements
Choose labelled example values: a 48 V battery (40–54 V), 5 kW peak output, power factor 0.9, maximum fundamental frequency 400 Hz, switching at 16 kHz. With SVPWM the maximum line voltage is about Vdc/√2 RMS. Compute the peak phase current from P = √3·VLL·I·cosφ at minimum battery voltage and add a margin. Note the assumed power factor because it changes the current.
Checkpoint: A requirements script giving phase current RMS and peak at worst-case battery voltage.
- 2
Choose devices
Choose example MOSFETs with a voltage rating with margin above the maximum battery voltage plus switching overshoot, for example a 100 V device. From an example datasheet, record Rds(on) at 25 °C and at a hot junction temperature, switching energies or rise and fall times, and body-diode forward voltage. Decide how many devices to parallel per switch to keep conduction loss manageable.
Checkpoint: A device table with ratings, losses per device and the chosen number in parallel.
- 3
Calculate losses analytically
Using standard sinusoidal-PWM loss expressions, compute switch RMS current for the chosen modulation index and power factor, then conduction loss per switch from Rds(on) at hot temperature. Compute switching loss as fs·(Eon + Eoff) scaled to operating voltage and averaged current. Sum losses for six switches and calculate inverter efficiency at peak and at a typical cruise point. Record which expressions you used and their source.
Checkpoint: A loss table at two operating points with inverter efficiency for each.
- 4
Size the DC-link capacitor
Compute the capacitor ripple current from the standard expression involving modulation index and power factor, and choose capacitance so that DC-link voltage ripple is below a target, for example 2 per cent. Check the ripple-current rating of example capacitors and decide the number in parallel. Include the battery cable inductance as an example value, since it decides how much ripple the battery sees.
Checkpoint: Capacitor choice with ripple current, voltage ripple and number of parts justified.
- 5
Build the switching model
Build a three-phase bridge with Simscape Electrical MOSFET blocks, a DC source with series resistance and inductance, and the DC-link capacitor. Drive it with SVPWM at 16 kHz and a balanced RL plus sinusoidal back-EMF load set to draw the rated current at the chosen power factor. Use a fixed-step local solver with a step of 50–100 ns. Halve the step once to confirm that results are converged.
Checkpoint: Phase currents are sinusoidal at the expected RMS value and the load draws the expected power.
- 6
Measure ripple and stresses
Log DC-link voltage, capacitor current, battery current and switch voltages. Compute capacitor RMS ripple current and DC-link voltage ripple from the logs and compare with step 4. Check peak switch voltage against the device rating. Also compare battery current ripple with and without the cable inductance, since that inductance and the capacitor form a filter that decides how much switching ripple reaches the battery. Note any resonance between them and whether it lies near a harmonic of the switching frequency.
Checkpoint: Simulated capacitor ripple current within about 10 per cent of the analytical value.
- 7
Compare simulated and analytical losses
Estimate conduction loss from logged switch current and Rds(on), and switching loss by counting switching events and applying scaled energies. Compare with the analytical results at the same points. Sweep switching frequency (8, 16, 24 kHz) with sim() and plot loss and current ripple against frequency to show the trade-off. Explain any difference between the two estimates, for example from temperature assumptions or the averaging in the analytical expressions.
Checkpoint: Simulated and analytical losses agree within a stated margin, and the frequency trade-off plot is complete.
- 8
Write the design report
Present requirements, device choice, loss calculations, capacitor sizing, simulation checks and the switching frequency study. Include a short thermal estimate of the heatsink needed to keep junction temperature below a limit. State simplifications such as ideal gate drive, no layout inductance and no dead time. Add a one-page summary of the final design values, such as devices, capacitors and expected losses, so that a reader can pick up the design without reading every section.
Checkpoint: A design report with every number traceable to a script or datasheet value.
Testing & validation
- Phase current and power match the requirement calculation.
- Capacitor ripple current from simulation matches the analytical value.
- Switch voltages stay below the device rating with margin.
- Simulated losses match analytical losses within a stated margin.
- Loss versus switching frequency trade-off plotted.
Expected results
A documented low-voltage traction inverter design with device selection, loss estimates, DC-link capacitor sizing and simulation confirming ripple and losses, ready to feed a thermal or efficiency-map study.
Take it further
- Add thermal networks to estimate junction temperature over a drive cycle.
- Add dead time and measure its effect on output voltage at low current.
- Compare SVPWM with discontinuous PWM for switching loss.
- Scale the design to a 400 V car inverter with IGBTs.
Viva and review questions
Why use Rds(on) at hot temperature?
Rds(on) rises considerably with junction temperature, so using the 25 °C value would underestimate conduction loss at the conditions the inverter actually runs at.
What sets the DC-link capacitor size in a low-voltage inverter?
Usually the ripple current rating rather than the capacitance, because the capacitor must carry the inverter's pulsed current without overheating.
How does switching frequency affect the design?
Higher frequency reduces current ripple and audible noise but increases switching loss in proportion, so it is a trade-off between motor quality and inverter efficiency.
Learn it at DIYguru
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