In this chapter, a brief overview of three fundamental aspects of power electronics was provided, namely the application of power devices in systems, the fundamentals of power devices performance, and the emergence of SiC as a replacement for the conventional semi- conductor materials. Power devices are seen as the beating heart of the future electrical systems by enabling power electronics circuits to provide an enhanced control over system parameters. The applications where these can be utilized could be very high voltages such as in transmission systems, or medium voltages such as in electric vehicles. For differ- ent utilizations, depending on the rating and switching characteristics required, different power devices are to be used. Hence, the variety of these structures were also explored by studying the fundamentals of their performance. Last but not least, the potential impacts of emerging wide bandgap material as a replacement for the silicon wafers was discussed. In the later chapters, these impacts are discussed in further details.
3
Transistors and Diodes
Different applications of power semiconductor devices require certain characteristics, hence not all device categories are suitable for all areas. An example would be the fact that de- vices such as Thyristors are not fully controllable, meaning that not both the turn-on and turn-off transients can be initiated and controlled on will through the gate driver pulses. This is a major disadvantage for applications in medium voltage converters. Addition- ally, some devices are not voltage driven, and hence require a significant gate current to turn-on, i.e. BJTs are falling in this category. This complexes the gate drive design for high power applications. As a result, until recently the major power transistor of choice for medium voltage applications has been silicon IGBTs and the main power rectifiers has been silicon PiN diodes. As explained in former chapter, with emergence of SiC technology it is now possible for unipolar devices to also withstand higher voltages, while providing a comparable on-resistance as the lack of conductivity modulation has been compensated by narrower drift regions. To this end, to explore these theoretical breakthroughs, these devices must also be characterized and their performance to be evaluated by means of experimental measurements; this includes exploring both the benefits and challenges.
To understand the superiority and drawbacks of the latter devices, they have been evaluated in the context of their performance in a VSC. The VSCs are important since they provide a higher controllability and hence it is necessary to understand how devices can impact their operation. An interesting and rapidly growing area of power converters where VSCs can be useful is the medium voltage applications such as in an electric vehicle (EV) drive-train. Hence, this chapter provides a comparative analysis between the 1.2 kV SiC MOSFET/Schottky diodes and similarly rated silicon IGBT/PiN diode technologies. In this regard, the switching performance of devices have been tested in a wide range of temperatures while the measurements are repeated at different switching rates for each temperature. The temperature impact on the electromagnetic oscillations in SiC technolo- gies and reverse recovery in silicon bipolar technologies is analyzed, showing improvements with increasing temperature in SiC unipolar devices whereas those of the silicon bipolar technologies deteriorate. These measurements are then used in an EV drive-train model where the temperature rise and conversion efficiency is studied. It is seen that at a given switching frequency, the SiC unipolar technologies outperform silicon bipolar technologies in terms of switching losses, operating temperature and conversion efficiency. These can enable lighter cooling and more compact vehicle systems.
3.1
Principles of Performance Evaluation
Power electronics for electric vehicle drive-trains is essential for high efficiency energy conversion [11] and transiting from slow switching silicon bipolar to fast switching SiC unipolar technologies is theoretically expected to improve the performance and efficiency of EVs [12]. The higher thermal conductivity of these devices will make them more temperature rugged and the wider bandgap means less leakage currents and reduced
probability of thermal runaway in high temperature applications. As a result 1.2 kV SiC MOSFETs and Schottky diodes have been manufactured by CREE® and ROHM® (amongst others) while devices with blocking voltages as high as 10 kV [13, 14] and with increasingly higher currents have also been demonstrated [15]. The feasibility of SiC technology for energy conversion in fully rated power converters in applications such as wind turbine energy conversion and fuel-cell systems was also explored in [16] and the implementation of SiC devices in motor control drives has been explored in [17]. The SiC unipolar devices might impact the EVs on the system level performance by enhancing the compactness of the drive trains and the reduction in passive sizes [18]. The use of SiC devices in on-board chargers can reduce the charging time and the thermal stress [19].
In this chapter, impact of SiC unipolar devices in EVs has been explored. To ensure high fidelity of results, the power devices in the converter have been parameterized by results of extensive experimental measurements. The temperature dependency of tran- sients is analyzed and the impact of switching rate on oscillations and reverse recovery is discussed. A 3-phase NPC VSC is used as in [20] together with a PMSM motor model as in [21,22] to emulate the EV drive-train. In this regard, first the experimental test set-ups are provided along with the devices transients, and then the impacts of implementation of these results in the EV drive-train model are presented.