Design Priorities in EV Traction Inverter WithOptimum Performance

Abstract

This technical white paper explores key system trends, architecture, and technology for traction inverters. The devices and technologies used to enable traction inverters, including isolation, high-voltage domain, and low-voltage domain technology, are also covered. Finally, the document focuses on the system engineering concepts and designs to accelerate traction inverter design time.

1 Introduction

The traction inverter is the heart of an electric vehicle (EV) drivetrain system. As such, the inverter plays a vital role in increasing the adoption of EVs worldwide. The traction motor provides excellent torque and acceleration by converting DC power from the batteries or generator to AC power to power traction drive motors such as permanent magnetic machines (PMSM), induction motors (IM), externally excited synchronous motors (EESM), and switched reluctance motors (SRM). A traction inverter also converts recuperation energy from the motor and recharges the battery while the vehicle is coasting or braking.

There are several key design priorities and trade-offs to consider when measuring the performance of the traction inverter:

Functional safety and security – Functional safety design usually follows ISO 26262 or an e-safety vehicle intrusion-protected applications process that includes safety diagnostics; system-level failure mode and effects analysis; failure modes, effects, and diagnostic analysis; and a hardware security module (HSM).
Weight and power density – The wide band-gap switch and powertrain integration are the key technologies enabling high-power density inverter design. The inverter power density target of OEMs continues to, for example, 100 kW/L in the US market by 2025. The use of SiC enables 800-V DC bus voltage, reduce the current rating and wiring harness. An MCU with fast control loop enables the use of high-speed, lighter motor, and powertrain integration such as an inverter integrated with DC-DC converters.
Efficiency – System efficiency includes traction inverter efficiency, motor efficiency, and inverter efficiency in regenerative braking mode.
Performance and reliability – Performance of the inverter system is measured through motor torque control, a current-sensing loop, and the motor torque transient response. Reliability includes power module reliability, motor reliability, and isolation, and so forth.
System cost – Apart from the electric machine and wiring harness, the main components include:
- EMI filter
- DC link capacitor
- Busbar
- MCU and control electronics
- Power modules and drive stage electronics
- Current sensor
- Inverter housing and cooling

2 Architectures and Trends

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.

Figure 2-1 Three-Level T-Type 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