Wolfspeed, A Cree Company, a leading global supplier of silicon carbide (SiC) power products, won a 2016 R&D 100 Award for its high-temperature, wide-bandgap (WBG) underhood inverter for hybrid and electric vehicles.
Wolfspeed’s high-temperature, WBG underhood inverter was developed in response to the need for smaller, lighter, and more efficient systems with higher power density in the electric vehicle market, and in collaboration with the Toyota Research Institute of North America (TRINA); the National Renewable Energy Laboratory (NREL); the University of Arkansas National Center for Reliable Electric Power Transmission; and the Department of Energy Vehicle Technologies Office (VTO).
This wide bandgap inverter product resulted in a smaller, lighter, higher power density and more efficient system in comparison to Toyota’s present Prius inverter design implementing silicon technology. The product operates by taking a two-level high and low input DC voltage (i.e., taken from the vehicle’s battery pack), and through precise switching of the inverter’s power semiconductor devices using space-vector pulse-width modulation, synthesizes a balanced set of three-phase voltages having a specific amplitude and frequency for any given instant of time. This allows the company to change the torque and speed applied to the vehicle’s wheels from its traction motor.
Our SiC inverter is the first traction drive optimized for wide bandgap devices that utilizes a commercially available SiC power module. By increasing power in a smaller footprint, Wolfspeed is enabling hybrid and electric vehicles to become more attractive to end consumers, contributing further to a reduction in the domestic use of fossil fuels and greenhouse gas emissions. Additionally, through our partnerships with other industry leaders, Wolfspeed ensures that our technology is readily adoptable in vehicle applications.—John Palmour, Wolfspeed’s chief technology officer
Underhood inverters convert the DC power stored in hybrid, plug-in hybrid, or all-electric vehicle battery packs to three-phase AC power that can be used to energize one or more electrical loads, and traditionally employ industry standard silicon semiconductors.
Utilizing Wolfspeed’s WBG semiconductor devices and advanced packaging techniques in an underhood inverter allowed engineers to achieve faster switching with reduced system-level losses during high ambient temperature operation (140 °C). This WBG-based system significantly outperforms silicon technology and meaningfully extends the realm of possibility for vehicle inverters, for which it has been earned the achievement of being named one of the top technical breakthrough products released in 2015.
The core of Wolfspeed’s WBG underhood inverter consists of three commercial Wolfspeed CAS325M12HM2 SiC half-bridge power modules, which are rated for 1200V and 325A of continuous RMS current at high temperatures.
The inverter assembly also includes:
a liquid-cooled cold plate, which provides the thermal conduction path for energy losses;
low inductance power bussing, which minimizes parasitic losses and maximizes switching efficiencies;
snubber and filter components, which dampen over-voltage and over-current spiking and dampens resonances;
control and drive circuitry, which performs the dynamic switching and provides users with feedback, control signals, and high current gate drive signals; and
an enclosure, which provides the unit with EMI shielding, electrical cabling, and liquid cooling inlet/outlet connections.
By utilizing WBG materials, Wolfspeed eliminated the need for the secondary radiator and thermal management system in the vehicle. This inverter allows the onboard power electronics to be cooled by the same coolant loop as the primary radiator and combustion engine, which significantly decreases the overall mass and volume of the system.
In addition to reducing the system footprint within the car, Wolfspeed’s WBG technology also increases the peak power delivery of the unit by 2–3x what is currently achievable, while operating at overall higher ambient temperatures.