Design of SiC MOSFET transistors to improve the efficiency of traction converters in electric vehicles.

An efficient traction converter is key to compromising a car's performance and range, and one of the key ways to improve efficiency is through the use of silicon carbide semiconductor devices with wide bandwidth (WBG).

13.10.2021
 

Engineers have to compromise the performance and range of modern electric vehicles. Faster acceleration and higher cruising speeds require more frequent and time-consuming recharging stops. A greater range, in turn, is possible at the expense of less dynamic driving. To increase range and offer drivers greater performance, engineers must design drivelines that ensure that as much energy as possible is transferred from the batteries to the driven wheels. Equally important is the need to keep the drivetrain small enough to fit inside the vehicle. These dual needs require components with both high efficiency and high energy density.

A key component in an electric vehicle's powertrain system is a three-phase voltage source converter (or "traction converter"), which converts the DC voltage from the batteries into the AC voltage required by the vehicle's engine or electric motors. Building an efficient traction converter is key to finding a trade-off between performance and range, and one of the key avenues to improve efficiency is the proper use of silicon carbide (SiC) wide band gap semiconductor (WBG) devices.

This article describes the role of a traction converter in an electric vehicle. It then explains how the use of SiC metal semiconductor field effect transistors (MOSFETs) can provide a more efficient electric vehicle drive system than insulated gate bipolar transistors (IGBTs). The article ends with an example of a traction converter based on SiC MOSFET transistors and design guidelines to maximize the efficiency of the device.

What is a traction converter?

An electric vehicle's traction converter converts the direct current supplied by the high voltage batteries into alternating current that is required by the electric motor to generate the torque necessary to move the vehicle. The electrical parameters of the traction converter have a significant influence on the acceleration of the vehicle and its range.

Modern traction converters are powered by 400V high voltage battery systems, and recently also 800V. With a traction converter current of 300A or more, the device powered by the 800V battery system is able to deliver over 200kW of power. As the power increases, the drive size is reduced, which significantly increases the power density.

 

Electric vehicles with 400V batteries require traction converters with power semiconductors with a rated voltage ranging from 600 to 750V, while vehicles with a voltage of 800V require semiconductors with a rated voltage ranging from 900 to 1200V. The power components used in traction converters must also be able to withstand peak AC currents of more than 500A for 30s and a maximum AC current of 1600A for 1ms. In addition, the switching transistors and gate drivers used in the device must be able to handle such heavy loads while maintaining high efficiency of the traction converter (Table 1).



Table 1: Typical requirements for traction converters in 2021; energy density shows a 250% increase compared to 2009. (Image credit: Steven Keeping)

The traction drive typically consists of three half-bridge elements (high and low side switches), one for each motor phase, with gate drivers controlling the switching of each transistor on the low side. The entire assembly must be galvanically isolated from the low voltage (LV) circuits that power the other vehicle systems (figure 1).


Figure 1: The electric vehicle requires a three-phase voltage source inverter (traction converter) to convert the DC current from the high voltage battery to the AC current required by the vehicle's motor or electric motors. The high voltage (HV) system, including the traction converter, is isolated from the vehicle's conventional 12V wiring. (Image credit: ON Semiconductor)

The switches in the example shown in Figure 1 are IGBTs. They have been a popular choice for traction converters because they are capable of handling high voltages, switch quickly, offer good performance, and are relatively inexpensive. However, as prices fell and the commercial availability of SiC MOSFETs increased, engineers began to turn to these components due to their significant advantages over IGBTs.

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