The evolution of traction inverter technology is poised to revolutionise the next generation of passenger cars.
Each new generation of electric vehicles faces greater expectations from its prospective customers. They must match or exceed the performance of their internal combustion-engine counterparts, travel further on a single charge, and become more affordable to encourage the transition to e-mobility. This presents challenges for vehicle designers; while battery technology is constantly evolving, which helps to improve performance and driving range. Attention to other design elements is also required. The electric drivetrain must become lighter and more compact; electrical and mechanical efficiencies need to increase; and costs have to be constrained – all without compromising safety.
With this in mind, BorgWarner has updated its traction inverter technology in readiness for the next generation of passenger cars and medium-power commercial vehicles. The power module, cooling module and electrical control system have been redesigned, and production methods rationalised, leading to higher efficiency, enhanced reliability, and easier integration with multiple vehicle architectures.
This next generation traction inverter operates at 800VDC with RMS phase currents of 650A peak/400A continuous, offering scalable power ratings of up to 400kVA. Scalability of the power output can be achieved by modifying component number or size; for example, a lower silicon carbide (SiC) die count or reduced capacitor footprint. Inverter performance and efficiency are stable between -40 and +85°C and up to 5,000m altitude. Environmental protection against dust and water ingress is provided by the IP67-rated enclosure.
The upgrade
To achieve greater compactness and reduce weight, improved component integration, coupled with a revised assembly process, results in an inverter package of around 7kg dry weight with a sub-four-litre active component volume. This is compatible with the US Department of Energy Vehicle Technologies Office 2025 power density target of 100kW per litre.
Assembly of the core elements of the new inverter has been simplified. The largest component is the bulk capacitor, its physical size dictated by system parameters such as operating voltage, peak phase current, voltage ripple and switching frequencies. Employing next generation capacitor technology enables those dimensions to be minimised while retaining the performance characteristics and self-healing properties of a conventional capacitor. Segmenting the bulk capacitor into smaller blocks further improves scalability, increases power density, and improves thermal performance.
The power module
BorgWarner’s SiC based Viper power module is used in a single switch configuration with dual-sided cooling, to deliver higher power density and efficiency for extended range and better performance. The same footprint is retained regardless of power class, allowing the number and type of SiC dies and the cooling module design to be configured to suit the customer’s application.
A die-attach process is used to assemble the high-temperature SiC devices, screw mounting or laser welding for the power connections, and soldering for auxiliary connections. The finished power module is encapsulated with a new type of epoxy overmould compound and can operate with junction temperatures at or beyond the typical 175°C to reduce system losses and improve current handling.
Dual-sided cooling for the Viper power module employs top and bottom heat sinks with a novel fin design that optimises thermal performance and reduces system pressure drop within the cooling channels. Simple stamping and brazing processes are employed in manufacturing the scalable cooling module, which allows efficient handling at high temperatures while keeping costs reasonable. Aluminum is used as standard, but copper may be substituted if necessary to meet thermal performance demands.
High voltages and currents, combined with the potential for rapid accelerative and decelerative forces mean that priority must be given to the electrical and dynamic safety of the electric vehicle drivetrain. With this in mind, BorgWarner set out to achieve Automotive Safety Integrity Level (ASIL) D – the top safety tier defined in the ISO 26262 standard – for its next-generation traction inverter. The modifications needed to render the inverter ASIL D compliant would normally mean increased component count with their associated extra cost, leading to a larger inverter package.
A safe solution
The Inverter Safety System ASIC (INSSA) provides a reliable feedback loop for sensing, control, and actuation functions, while allowing controller and gate driver circuitry to share a single reduced-footprint mainboard. Each INSSA enables an independent power supply for the gate drive circuitry on each side of the high voltage supply, so a single point electrical fault in the inverter causes the system to react independently of microprocessor intervention. This is a major advantage of the latest BorgWarner technology; using voltage and motor speed measurements plus the fault signature ensures that the high voltage bus is always maintained within a safe range.
The innovative design techniques implemented in BorgWarner’s next-generation traction inverter have created a cost-balanced package with industry-leading fault-tolerant control architecture that complies with ISO 26262 ASIL D and satisfies the VTO 2025 goal of 100kW/l power density.
