The AC motor is the main load of the high-voltage battery in a Battery Electric Vehicles (BEVs). The motor relies on the traction inverter to convert DC battery power into AC power, making the traction inverter the heart of the BEV, providing the torque and acceleration needed to drive the vehicle forward. Today, many BEVs and Hybrid Electric Vehicles (HEVs) are built on IGBT technology, but with the introduction of Silicon Carbide (SiC) technology, new levels of efficiency and performance are achievable. This article explores the application of traction inverters and the solutions offered by onsemi.
Key design considerations for traction inverters
The two primary design considerations for traction inverters are conversion efficiency and peak power. The higher the conversion efficiency from DC to AC, the longer the range for the same battery. Higher efficiency also means the system can deliver more power while managing less heat. The peak power of the traction inverter determines the overall performance of the vehicle, particularly its instantaneous torque and acceleration capabilities. Efficiency (range) and peak power (performance) ultimately define the vehicle's application and use case.
Traction inverters create the largest demand for SiC in all EV applications, and this demand is expected to increase further as applications shift to 800V battery packs. The advantages of SiC technology will become even more significant compared to IGBT-based systems.
In power applications, power density is important, but reliability is equally critical, and in some applications, it may even be more crucial. While engineers can often address performance limitations, they are less likely to accept actual failures that could lead to unplanned downtime or repairs, as these can have significant unexpected impacts on the business.
Related technologies and design architectures for traction inverters
BEVs rely entirely on the energy stored in high-voltage battery packs and require the most efficient traction inverters and motor engineering. There are hardly any identical architectures in the market, leading to different traction inverter requirements. The main difference lies in how the motor is ties to the wheels - either directly or through a differential.
Direct drive can achieve higher efficiency, less maintenance, and simpler implementation, but it usually requires a larger volume due to low-speed requirements. Differential drive implementation increases power density due to higher RPM operation and the fixed gear ratio of the differential, but mechanical gears require maintenance and suffer from transmission losses.
By improving the efficiency and robustness of the traction inverter, the overall efficiency of the vehicle can be enhanced. However, the inverter is not just a 6-pack of MOSFETs; it also includes protection and monitoring auxiliary circuits to prevent system-level failures. Fast and reliable switching within the inverter, along with monitoring signals to reduce the likelihood of failure, has additional implications for the choice of gate drivers. Moreover, BEVs and HEVs consist of various adjacent power electronic systems responsible for regulating power, battery management, and moving the vehicle forward.
The most critical module of the traction inverter is its power stage, which consists of high-power switches such as IGBT-based and SiC MOSFET-based integrated power modules. The power stage is controlled by a power management IC, a microcontroller (MCU), or a combination of both. The switches are monitored and protected by sensing power stage temperature, voltage, and current during operation.
Switch control is achieved through the MCU, which generates the initial PWM signal. The isolated gate driver amplifies the generated PWM signal and provides sufficient charge to turn the high-power switches on and off. The MCU must control and modify the inverter modulation based on received feedback signals such as voltage, current, and motor position.
MOSFETs are the most critical components in the power stage of traction inverters
MOSFETs in the power stage of traction inverters are the most critical components because they control the current flowing to the motor to generate motion. The three legs of the inverter convert the DC battery voltage into three-phase AC voltage and current to drive the motor. The power stage is monitored and protected by sensing the temperature, voltage, and current during operation.
onsemi offers three approaches to build high-performance power stages using EliteSiC™ devices. The first is the most integrated solution using a single 6-pack module (SSDC39) with a pin-fit heatsink. The second uses 3x half-bridge modules (AHPM15) to achieve higher design flexibility while maintaining performance. The third uses 6x M3e MOSFET in a packageless bare die format to create a custom module design.
The 6-pack EliteSiC power module for traction inverters - SSDC39 - offers increased performance, better efficiency, and higher power density in an industry-standard package solution. The NVXR17S90M2SPB module integrates 900V 1.7 mΩ SiC MOSFETs in the 6-pack configuration, SSDC39 package. For ease of assembly and improved reliability, the next-generation press-fit pins are integrated into the signal terminals of the power module. For direct cooling, the gel-filled package integrates an optimized pin-fin heatsink in the baseplate, designed to meet the AQG324 automotive standard.
The half-bridge EliteSiC power module for traction inverters - AHPM15 - features the NVVR26A120M1WSS power module, which integrates 1200V 1.7 mΩ SiC MOSFETs in a half-bridge configuration. This module has low stray inductance (7.1 nH) and RDS(ON), making it ideal for hybrid and battery electric vehicle traction inverter applications. The AHPM15 family of modules come in two power tab variants, including straight or 90° power tabs. To improve reliability and thermal performance, die attachment uses sintering technology, and the module is designed to meet the AQG324 standard.
The NCS025M3E120NF06X, based on the new EliteSiC™ 1200V M3e MOSFET technology, is onsemi's high-performance third-generation 1200V SiC MOSFET in a packageless bare die format. The 5x5 mm bare die can be implemented in any custom module design. The M3e product family, based on onsemi's latest generation of SiC MOSFET technology, offers the lowest on-resistance (typical) in its class, with VGS = 18V, ID = 135A, TJ = 25℃ at 11.0 mΩ, making it an ideal choice for automotive traction inverters.
Additionally, onsemi has introduced the EliteSiC B2S and B6S power modules for EV traction, based on the new 1200V SiC M3e technology. The B2S module is a sinterable half-bridge, and the B6S is a larger 6-pack module with integrated heatsink. The B2S is pushing the boundaries of high performance and traction inverter efficiency, offering design scalability from 160 to 400 kW. The unit cell pitch of the M3e MOSFET is reduced by more than 60% compared to the M1 family. SiC planar MOSFETs have accumulated trillions of hours of field experience with low failure rates, and 100% defect screen, accelerated electrical tests, and gate oxide stress verification address SiC weaknesses. SiC die and heatsink are attached via sintering technology.
IGBT technology remains the cornerstone of EV technology
Insulated Gate Bipolar Transistors (IGBTs) remain the cornerstone of EV technology, driving improvements in efficiency, reliability, and sustainability. As the backbone of EV power management systems, IGBTs can handle large electrical loads with remarkable stability, ensuring long-term, consistent performance. This reliability is crucial for EVs, as power demands fluctuate significantly from rapid acceleration to regenerative braking.
IGBTs remain a cost-effective option and have been the go-to choice for EV powertrains for many years. However, SiC MOSFETs are gaining traction for their advantages in efficiency and thermal performance, making them an increasingly attractive option for the next-generation of EVs.
onsemi continues to improve and expand its IGBT product portfolio, introducing new IGBT technologies such as the Narrow Mesa Field Stop (FS4 and FS7), which shows lower power losses during lighter loads and overall improved system efficiency for automotive applications.
onsemi's IGBT 6-pack power module = NVH660S75L4SPFB - integrates six FS4 750V Narrow Mesa IGBTs in a six-pack configuration. This module excels in providing high current density while offering robust short-circuit protection and higher blocking voltage. It uses the low stray inductance SSDC33 package with direct cooling and a flat base heatsink.
Another IGBT half-bridge power module - NVG600A75L4DSC2 - integrates two FS4 750V IGBTs in a half-bridge configuration. The module integrates chip-level temperature and current sensors, and the dual-sided cooling package AHPM15 improves thermal performance.
Additionally, onsemi offers two evaluation hardware kits (reference designs) for EV/HEV traction inverter applications (up to 150 kW) based on the VE-Trac IGBT power module family. These evaluation kits (EVKs) allow customers to evaluate the performance of the VE-Trac Direct power module in the early stages of inverter development. There are two EVK variants based on 6-pack and half-bridge power modules. The kits can be used as a double pulse tester to measure key switching parameters or as a three-phase inverter for motor control.
Fully specified EliteSiC and IGBT power modules and evaluation boards
The switching characteristics of IGBT and SiC modules are influenced by many external parameters, such as voltage, current, temperature, gate configuration, and stray elements. DC-link loop inductance and gate loop inductance affect the switching characteristics of IGBT and SiC power modules. A double pulse test setup is used to measure the switching characteristics of two modules, including the 900V EliteSiC 1.7 mΩ class NVXR17S90M2SPB and the 750V VE−Trac IGBT NVH950S75L4SPB, both in the ultra-low stray inductance (8 nH) SSDC33 package. onsemi's discrete solution for traction inverters is the EliteSiC™ MOSFET, supporting 650V~1200V, continuously pushing the boundaries of RDS(ON), QG, RSP, etc.
onsemi also provides a discrete double pulse tester (DPT) evaluation board - EVBUM2897 - designed for comparative measurements of SiC, Si MOSFETs, and IGBTs in discrete packages. The main usage of the tester is to testing of switching performance and comparison of different device types or packages.
onsemi also offers a hotplate and double pulse generator extension board - EVBUM2901 - which provides HOT temperature testing conditions and variable 10-pulse PWM generation for the discrete DPT board. The EVBUM2901, together with the DPT board, supports the use of daughter cards for HOT temperature testing of all onsemi discrete packages (SiC, Si) within a 1200V breakdown voltage.
onsemi's NFVA25012NP2T is a 1200V, 50A intelligent power module (IPM) for high-voltage auxiliary motors, providing a fully-featured, high-performance inverter output stage for hybrid and electric vehicles. The module integrates a 1200V, 50A three-phase IGBT inverter with optimized gate drive, control, and protection features. It is recommended for auxiliary applications such as HVAC electric compressors, high-voltage oil and water pumps, high-voltage superchargers, and variety of fans.
onsemi's high-current single-channel gate drivers, NCV57001 and NCV57100, are designed for high system efficiency and reliability in high-power applications such as traction inverters. On the Miller plateau, NCV57100 can deliver up to ±7A, while NCV57001 can deliver +4/-6A. These drivers feature internal galvanic isolation and include complementary inputs, open-drain fault and ready outputs, active Miller clamp, accurate UVLOs, DESAT protection, and DESAT soft turn-off.
Additionally, there is an evaluation board equipped with the NCV51752 (NCV51152) driver, which features internally integrated negative bias control, eliminating the need for the system to provide an external negative bias rail to the driver. The evaluation board has multiple PCB layout variants for testing all onsemi-supported discrete SiC MOSFET packages.
Conclusion
The traction inverter is the core component of the powertrain in Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs), responsible for converting high-voltage DC power into AC power to drive the motor. Its performance directly affects the vehicle's efficiency, power output, and range. Looking to the future, automakers and supply chain manufacturers must continuously optimize the design of traction inverters to ensure higher power density, lower losses, and superior thermal management performance, while also considering lightweight and reliability to meet the needs of next-generation EVs. onsemi offers fully specified traction inverter solutions that will further enhance the performance of EVs, promote intelligent transportation and sustainable energy development, and help the global automotive industry move towards a zero-carbon future.