The architecture of a traction inverter varies with vehicle type. Plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs) have a three-phase voltage source inverter topology, with power levels in the 100- to 500-kW range. The battery pack can either directly connect to the inverter DC input or a DC/DC boost converter can be used to step up the battery voltage and supply the inverter with a controlled DC voltage.

The two-level inverter is the most common power converter used in electrified vehicles and in the industry, with the power range of tens of kilowatts up to hundreds of kilowatts. Usually, the switching frequency is in the range of 5 kHz to 30 kHz, Currently, three-level inverters are becoming more popular because the inverters offer higher power capability (beyond 300 kW), higher efficiency, and lower harmonic distortion and allow the use of a smaller electromagnetic interference (EMI) filter. Among many topologies, neutral point clamped and T-type neutral point clamped (TNPC) are the most competitive designs. #FIG_GCM_5CZ_N5B illustrates an example of a three-level TNPC inverter.

A second trend is a dual-motor architecture. As early as in 2012, Tesla introduced the Model S, a rear-wheel drive, full-size luxury sedan with a range of up to 426 km with the 85-kWh battery pack. In 2014, Tesla announced an all-wheel drive version of the Model S with an electric motor on both the front and rear axles. Since then, dual inverters have been implemented by various OEMs such as the Chevy Volt PHEV, Toyota Prius HEV, and Cadillac CT6 PHEV.

A third trend improving system integration is the implementation of e-axles, which combine the power electronics, electric motor, and transmission in a compact system housing. E-axles improve motor performance because this design can achieve higher torque and top speed, for example 20-k RPM. Better cooling and a coil winding structure improve power density and motor efficiency.

Other trends in traction inverter features include:

• Increasing power levels and Automotive Safety Integrity Levels (ASILs) (100 kW to 500 kW, ASIL C to
  ASIL D)
• Shifting towards 800-V technology with increased switching transient voltages
• Easily adjusting the gate-drive strength to reduce overshoot, optimize efficiency, and reduce EMI
• Employing an inductive position-sensing technology instead of a resolver to reduce costs
• Integrating active discharge into a Gate driver integrated circuit (IC) to reduce costs and save space

A traction inverter requires isolation technology, technology implemented on the low-voltage domain, and technology implemented on the high-voltage domain. TI’s capacitive isolation technology, found in isolated gate drivers, digital isolators, isolated analog-to-digital converters, and solid-state relays, incorporates reinforced signal isolation in a capacitive circuit that uses silicon dioxide for the dielectric. #FIG_GCM_5CZ_N5B shows an example of a traction inverter system. The isolation barrier (red dotted line) separates the low-voltage domain and high-voltage domain.

In the low-voltage domain, a microcontroller (MCU) generates pulse-width modulation (PWM) signals to the power switches. The MCU runs the sensing and speed control in a closed loop, and handles host functions to fulfill mandatory hardware and software security and safe code execution requirements. Additionally, implementing a safe power-tree keeps the MCU and critical power rails from losing power. A power-management integrated circuit (PMIC) or system-basis chip connected to the 12-V car battery powers the MCU. The MCU interfaces with the analog front end of the resolver or a Hall-effect sensor.

Key functions in the high-voltage domain include:

• Power switches – usually silicon carbide (SiC) or insulated gate bipolar transistor (IGBT) based power modules, which are controlled by isolated gate drivers with protection and monitoring capabilities
• Isolated gate drivers – an isolation device allows data and power transfer between high- and low-voltage units, while preventing hazardous DC or uncontrolled transient current flowing from the high-voltage domain
• Bias supplies – a galvanically isolated power supply which takes the input from low-voltage side and generates the gate drive voltage to the power switches
• Isolated voltage and current sensing – to sense the DC link voltage and motor-phase current and makes sure that the correct torque is being applied to the motor
• Active discharge – to discharge the DC bus capacitor voltage to a safe voltage. Active discharge is required for the type of motors that can generate back-electromotive force (EMF). United Nations regulation No. 94 of the Economic Commission for Europe of the United Nations requires that the DC bus capacitor voltage drop to a safe voltage (60 V) in less than 5 s. Additionally, diagnostic circuitry is included to perform self-tests on critical functions to prevent system failure.

The inverter control and safety scheme also varies with vehicle type. For example, a permanent magnet synchronous motor (PMSM) can be leveraged because the PMSM has high efficiency, low torque ripple, and a large speed range. PMSMs often use pace vector PWM control, also known as field-oriented control. Controlling the stator current in the way that creates a stator vector perpendicular to rotor magnetics generates torque. Updating the stator currents keeps the stator flux vector at 90 degrees to the rotor magnets at all times. Other popular motor types in PHEVs and BEVs include induction motors, externally excited synchronous machines, and switched reluctance machines.

To reduce the costly rare earth materials permanent magnet, the externally excited synchronous motors (EESM) is growing as not only a secondary axle but also primary axle movers for the vehicle. The goal in using this motor is to reduce cost - for example, 100-kW peak power requires about 1.5 kg magnets, and to reduce the efforts in manufacturing and maintenance. EESM machine types include conductive EESM and inductive EESM (iEESM). Commercial vehicles using EESM include the Toyota Prius, Chevrolet Bolt EV, Ford Focus Electric, VW e-Golf, BMW iX3, and so forth.

Figure 3-1 Traction Inverter System Block Diagram