How to design in SiC MOSFETs to improve EV traction inverter efficiency
power metal oxide semiconductor field-effect transistors (MOSFETs) can yield a more efficient EV drive train than one using insulated-gate bipolar transistors (IGBTs). The article concludes with an example of a SiC MOSFET-based traction inverter, and design tips on how to maximize the unit’s efficiency. What is a traction inverter? An EV’s traction inverter converts the DC-current provided by the vehicle’s high-voltage (HV) batteries into the AC-current required by the electric motor to produce the torque required to move the vehicle. The electrical performance of the traction inverter has a significant impact on the vehicle acceleration and driving range. Contemporary traction inverters are driven by HV battery systems of 400-volts, or more recently, 800-volt designs. With traction inverter currents of 300 amperes (A) or greater, a device powered by an 800-volt battery system is capable of delivering over 200 kilowatts (kW) of power. As the power has climbed, the size of the inverters has shrunk, significantly increasing the power density. EV’s with 400-volt battery systems require traction inverters with power semiconductor devices rated in the 600 to 750-volt range, while 800-
Table 1: Typical 2021 traction inverter requirements; energy density shows a 250% increase, compared to 2009. (Image source: Steven Keeping)
volt vehicles require semiconductor devices rated in the range of 900 to 1200 volts. The power components used in the traction inverters must also be able to handle peak AC currents of over 500 A for 30 seconds (s) and a maximum AC current of 1600 A for 1 millisecond (ms). In addition, the switching transistors and gate drivers used for the device must be capable of handling these large loads while maintaining high traction inverter efficiency (Table 1).
A traction inverter typically comprises three half-bridge elements (high-side plus low- side switches), one for each motor phase, with gate drivers controlling the low-side switching of each transistor. The entire assembly must be galvanically isolated from the low-voltage (LV) circuits powering the rest of the vehicle’s systems (Figure 1).
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