Power Electronics

Power electronics are the translators between battery (DC) and motor (AC). They control hundreds of kilowatts with microsecond precision, enabling the smooth torque delivery that makes EVs special.

Three-Phase Inverter

The heart of the EV powertrain is the traction inverter — it converts battery DC to variable-frequency AC for the motor.

Topology

• 3 half-bridges (one per phase)
• 6 power switches (IGBTs or MOSFETs)
• 6 freewheeling diodes
• DC link capacitor

Each leg has two switches:

• High-side: connects phase to DC+
• Low-side: connects phase to DC-

Switching States

Dead Time

When switching states, a brief dead time (0.5-2 μs) is inserted:

• Prevents shoot-through during transition
• Both switches OFF momentarily
• Current flows through freewheeling diodes
• Causes voltage distortion (must compensate)

DC Link Capacitor

• Smooths DC bus voltage ripple
• Sources/sinks high-frequency current
• Provides energy during switching transients

$$C_{dc} = \frac{I_{rms}}{2\pi f_{sw} \cdot \Delta V_{ripple}}$$

• Film capacitors: High reliability, large volume
• Electrolytic: Compact, limited life
• Modern trend: Film capacitors for automotive

Power Flow

Motoring Mode (Battery → Motor)

$$P_{battery} = V_{dc} \times I_{dc}$$

$$P_{motor} = \sqrt{3} \times V_{line} \times I_{line} \times \cos\phi \times \eta_{motor}$$

$$P_{inverter} = P_{battery} \times \eta_{inverter}$$

Regeneration Mode (Motor → Battery)

During braking:

• Motor acts as generator
• Current direction reverses
• Inverter acts as rectifier
• Energy flows back to battery

Efficiency in regen typically 85-92% (motor + inverter).

Inverter Losses

Conduction Losses

When switch is ON:

$$P_{cond} = I_{rms}^2 \times R_{DS(on)}$$ (MOSFET)

$$P_{cond} = V_{CE(sat)} \times I_{avg} + I_{rms}^2 \times R_{CE}$$ (IGBT)

Switching Losses

Energy lost during each transition:

$$P_{sw} = f_{sw} \times (E_{on} + E_{off})$$

• Switching frequency (higher → more losses)
• DC bus voltage (higher → more energy per switch)
• Current magnitude
• Switch technology (SiC << Si)

Loss Breakdown (Typical 150 kW Inverter)

DC-DC Converters

EVs use multiple DC-DC converters:

High-Voltage DC-DC (400V → 12V)

Powers 12V accessories (lights, ECUs, infotainment):

• Input: 250-450V (battery range)
• Output: 12-14V regulated
• Isolation: 2500V minimum

Boost Converter (Battery → Higher DC Bus)

Some EVs boost battery voltage for higher motor performance:

• Battery: 800V
• Boost available from lower voltage

Bidirectional DC-DC (for 48V Mild Hybrid)

Transfers energy between 48V and 12V systems:

• Boost during acceleration assist
• Buck during regeneration

Power Semiconductor Comparison

Silicon (Si) IGBT

Silicon Carbide (SiC) MOSFET

• 50% lower switching losses
• Higher efficiency at part load
• Smaller heat sink
• Higher switching frequency → smaller passives

Gallium Nitride (GaN)

• Very fast switching
• Low gate charge
• No reverse recovery

Comparison Table

Onboard Charger (OBC)

Converts AC mains to DC for battery charging:

Topology

• Boost converter topology
• Shapes input current to sine wave
• Achieves PF > 0.99

• LLC resonant converter
• Provides galvanic isolation
• Regulates output voltage

OBC Specifications

Bidirectional OBC (V2G)

Emerging technology:

• Can export power from EV to grid
• Requires bidirectional inverter stage
• Enables vehicle-to-home (V2H)

Thermal Management

Power electronics generate significant heat:

Cooling Methods

• Coldplate under power modules
• Shared coolant loop with motor/battery
• Most common for automotive

• Coolant contacts both sides of module
• 2× heat extraction
• Emerging technology

Junction Temperature Limits

EMI Considerations

High dV/dt switching creates EMI:

Sources

• Switch turn-on/off (10-50 kV/μs)
• Diode reverse recovery
• Parasitic ringing

Mitigation

• Shielded cables: Keep noise inside
• Common mode chokes: Filter HF noise
• Snubbers: Dampen ringing
• Gate resistors: Slow down switching (trade EMI vs losses)

Standards

• CISPR 25: Automotive EMC limits
• ISO 11452: Immunity testing

Indian Context

Two-Wheeler Inverters

• Power: 3-8 kW
• Voltage: 48-72V
• Device: Si MOSFETs
• Cost target: ₹8,000-15,000

Passenger Car Inverters

• Power: 80-150 kW
• Voltage: 350-450V
• Device: Si IGBT or SiC MOSFET
• Cost: ₹80,000-1,50,000

Suppliers in India

• Sona Comstar: Inverters for Tata
• KPIT: Software for PE
• Mahle: Thermal management
• Continental: Power electronics

Key Takeaways

• Six-switch inverter converts battery DC to motor AC
• SVPWM synthesizes desired voltage using 8 switch states
• Losses = conduction + switching (2-4% of rated power)
• SiC MOSFETs offer 50% lower losses than Si IGBTs
• OBC uses PFC + isolated DC-DC stages
• Thermal management is critical — liquid cooling standard

What's Next

In the next lesson, we'll explore the complete EV Powertrain — how motor, inverter, gearbox, and differential work together to propel the vehicle.