Unveiling the Potential of GaN Semiconductor-Enabled Three-Phase Propulsion Inverters for Enhanced EV Performance

As the world strives to transition towards a more sustainable future, electric mobility has emerged as a key solution to reduce greenhouse gas emissions and combat climate change. Electric vehicles have gained significant attention, providing a cleaner alternative to traditional IC engine vehicles. Within the realm of EV technology, the efficiency of Three Phase Propulsion Inverters plays a pivotal role in maximising performance and optimising energy usage.
Optimising Three Phase Propulsion Inverters contributes to the longevity and performance of the battery pack.

Efficient power conversion reduces heat generation and voltage fluctuations, reducing the strain on the battery. This results in prolonging the battery's lifespan and enables stable performance by reducing wear and tear. Moreover, increasing the efficiency of Three Phase Propulsion Inverters extends the driving range of EVs by maximising power utilisation and minimising energy losses, efficient inverters allow for more effective use of the available battery capacity. On the other hand, electric mobility and efficient Three Phase Propulsion Inverters have significant environmental implications. By increasing power conversion efficiency, less energy is wasted during the process, reducing overall energy consumption.

Ⅰ. Exploring the characteristics of Propulsion Drive Modelling and Understanding Loss Modelling of Power Devices

A typical Electric vehicle uses a three-phase traction drive system which is modelled with PMSM and a three-phase VSI that is depicted in Figure 1. PMSM is widely accepted over induction motors and DC in industrial applications, mainly due to its high power-weight ratio and superior efficiency. On the other hand, VSI has three legs with two switches each, moreover, each switch has one transistor connected with its antiparallel diode.

Figure 1: Schematic Diagram of a Three-phase propulsion drive with PMSM and VSI

The propulsion Drive is connected to a battery with a voltage VB of 384 V and a switching frequency of 20 kHz. On the other hand, Lx, Rx, and ex represent inductance, winding resistance and back emf respectively where x denotes the phase of the system. The peak phase voltage Vp required to provide current Ip to the motor is given by equation 1. Here,  represents the angular speed of the motor in radians and km is the motor constant with Ep as the back EMFs.

Equation 1: Equation to determine the peak phase voltage required to provide current to the motor

Power losses in propulsion systems are an important aspect to understand as it is crucial for optimising efficiency, performance, and range in an electric vehicle. Moreover, It helps in designing effective thermal management, reducing costs, and selecting the appropriate battery capacity. Overall, there are 4 types of losses namely, conduction losses, driving losses, switching losses and inverter losses. Conduction losses are one of the most impacting losses in IGBT, MOSFET and diode that are represented in equations 2,3 and 4 respectively.

Equation 2: Power losses in IGBT due to saturation voltage and channel resistance acting on average current

Here, Rce and Vceo are the channel resistance and saturation voltage while Iavg,I is the average current of the IGBT.

Equation 3: Power losses in MOSFET due to channel resistance in the rms current

Here, Rds(on) is the channel resistance of MOSFET whereas Irms,M is the rms current of MOSFET.

Equation 4: Power losses in Diode due to diode resistance and forward voltage on current

Here, Rd and VF are the diode resistance and diode forward voltage respectively. On the other hand, Io is the addition of average and rms current. Apart from these losses, Inverter loss is one important factor contributing to efficiency. The inverter loss is calculated by inputting the above formulae in a Matlab code to simulate the power losses with respect to speed and torque in different Wideband gap devices. The inputs in Matlab consist of the phase angle between current and voltage, sinusoidal motor phase current and voltage. The simulation in Matlab clearly shows that  Si-IGBTs inverters consume way more power than  GaN MOSFETs inverters at high speed and torque.

Ⅱ. Exploring Design, Control, and Energy Optimization Strategies for Enhanced Performance in Propulsion Drive Systems

Typically, the efficiency of a propulsion drive system running on inverters based on GaNMOSFETs and Si-IGBTs are calculated using a predictive current controller or PI controller respectively. The efficiency curves for Si- IGBT and GaN-MOSFET propulsion drive determined by contour plotting based on NEDC speeds is illustrated in Figure 2. Although the efficiency curve is mapped along a wide range of torque and speed of the motor, it is pretty evident that  Si-IGBTs propulsion drive has an efficiency of 96 percent in the high-speed zone and lowers in the low-speed zone. On the other hand, GaN MOSFET using a predictive current controller has an efficiency of 97 percent in the high-speed zone and degrades slightly in the low-speed zone. It is also important to note that the loss formulae are illustrated in the form of voltage and current whereas the efficiency map and loss matrix is illustrated via torque and speed.

Figure 2: A Comparative Graph Between Efficiencies of GaN MOSFET and Si IGBT Propulsion Drive Using Different Controllers

Ⅲ. Conclusion

Three-phase propulsion drives have immense importance in advancing transportation technology towards greater efficiency and technical innovation. Researching motor design, power electronics, and control systems can optimise performance, resulting in enhanced energy efficiency and superior power output.

Therefore, it is vital to understand the impact of GaN-MOSFETs in three-phase propulsion drives, emphasizing their importance and unique characteristics. The loss model for power devices compares inverters built with GaN-MOSFETs and IGBTs, revealing the former's superior power loss matrix. MATLAB contour plots illustrate the efficiency map for both inverter types across speed and torque ranges. Additionally, a predictive current controller is implemented for the GaN-based propulsion drive, reducing switching frequency by 12% compared to the conventional PI controller at rated torque and speed. This combined with GaN MOSFETs' lower power losses and reduced conduction losses due to lower rms phase current ensures highly efficient operation of the propulsion drive. Overall, GaN-MOSFETs present a promising solution to achieve enhanced efficiency and performance in sustainable propulsion systems.