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