Impact of SiC Power Electronic Devices for Hybrid Electric Vehicles

The superior properties of silicon carbide (SiC) power electronic devices compared with silicon (Si) are expected to have a significant impact on next-generation vehicles, especially hybrid electric vehicles (HEVs). The system-level benefits of using SiC devices in HEVs include a large reduction in the size, weight, and cost of the power conditioning and/or thermal systems. However, the expected performance characteristics of the various semiconductor devices and the impact that these devices could have in applications are not well understood. Simulation tools have been developed and are demonstrated for SiC devices in relevant transportation applications. These tools have been verified by experimental analysis of SiC diodes and MOSFETs and can be used to assess the impact of expected performance gains in SiC devices and determine areas of greatest impact in HEV systems. INTRODUCTION Presently, almost all of the power electronics converter systems in automotive applications use silicon(Si-) based power semiconductor switches. The performance of these systems is approaching the theoretical limits of the Si fundamental material properties. The emergence of silicon carbide(SiC-) based power semiconductor switches likely will result in substantial improvements in the performance of power electronics converter systems in transportation applications. SiC is a wide-bandgap semiconductor, and SiC-based power switches can be used in electric traction drives and other automotive electrical subsystems with many benefits compared with Si-based switches. In this paper, experimental characteristics of Si and SiC are used to develop a simulation model for SiC power electronics devices. The main objective of developing these simulation tools is to show some of the systemlevel benefits of using SiC devices in HEVs such as the large reduction in the size, weight, and cost of the power conditioning and/or thermal management systems. Temperature-dependent circuit models for SiC diodes and MOSFETs have been developed. Power losses and device temperatures have been computed for a traction drive in HEVs. Temperature and efficiency profiles have been created for the devices for powering a vehicle over an urban driving cycle. The system benefits in using SiC devices are highlighted through simulation and experimental results. At Oak Ridge National Laboratory (ORNL), a SiC power MOSFET is presently being designed. This power device will be used in power electronics converter systems for automotive applications to demonstrate the benefits of SiC-based power devices. One of the selected automotive applications for this project is a traction drive. New gate drive layouts, circuit topologies, and filter requirements will also be developed to take advantage of the special properties of SiC devices. ADVANTAGES OF SiC COMPARED WITH Si As mentioned earlier, SiC is a wide-bandgap semiconductor, and this property of SiC is expected to yield greatly superior power electronics devices once processing and fabrication issues with this material are solved. Some of the advantages of SiC compared with Si based power devices are as follows: 1. SiC-based power devices have higher breakdown voltages (5 to 30 times higher than those of Si) because of their higher electric breakdown field. 2. SiC devices are thinner, and they have lower onresistances. The substantially higher breakdownvoltage for SiC allows higher concentrations of doping and consequently a lower series resistance. For lowbreakdown voltage devices (~50V), SiC unipolar device on-resistances are around 100 times less; and at higher breakdown voltages (~5000V), they are up __________________________________________________ *Prepared by the Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, managed by UT-Battelle for the U.S. Department of Energy under contract DE-AC05-00OR22725. The submitted manuscript has been authored by a contractor of the U.S. Government under Contract No. DE-AC0500OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish from the contribution, or allow others to do so, for U.S. Government purposes. to 300 times less [1]. With lower Ron, SiC unipolar power devices have lower conduction losses (Figure 1) and therefore higher overall efficiency. 3. SiC has a higher thermal conductivity and thus a lower junction-to-case thermal resistance, Rth-jc. This means heat is more easily conducted away from the device junction, and thus the device temperature increase is slower. 4. SiC can operate at high temperatures because of its wider bandgap. SiC device operation at up to 600°C is mentioned in the literature [2]. Most Si devices, on the other hand, can operate at a maximum junction temperature of only 150°C. 5. Forward and reverse characteristics of SiC power devices vary only slightly with temperature and time; therefore, SiC devices are more reliable. 6. SiC-based devices have excellent reverse recovery characteristics [3]. With less reverse recovery current, the switching losses and electromagnetic interference (EMI) are reduced and there is less or no need for snubbers. Typical turn-off waveforms of commercial Si and SiC diodes are given in Figure 2. 7. SiC is extremely radiation hard; i.e., radiation does not degrade the electronic properties of SiC. MACHINE AND INVERTER MODELING System level simulation tools have been developed to calculate the conduction and switching losses of the power devices in an inverter used as a motor drive in an HEV. The efficiency of the inverter can then be determined from these losses. The simulation tool is also able to estimate the junction temperature of the power semiconductor devices and recommend an appropriate heatsink size. In this paper, an averaging technique [4] is used to model the power electronics switching losses in an HEV traction drive system. The models are compatible with the Department of Energy’s ADvanced VehIcle SimulatOR (ADVISOR) models. The developed system models use torque and speed values from the ADVISOR simulation to determine the current profile of the system over the Federal Urban Driving Schedule (FUDS). Circuit-level simulation is not practical for this work because the device variables are in microor nanoseconds and the system variables are in 1-second increments. Figure 3 shows the block diagram of the system modeling approach, and Figure 4 shows the three-phase inverter and induction machine for the traction drive system. 300 320 340 360 380 400 420 440 460 480 500 0.01 1