Advanced Power-electronic Converters Based-on GaN Semiconductors

• ZUZENDARIAK: Alberto Rodríguez Alonso eta Asier García Bediaga
• UNIBERTSITATEA: Universidad de Oviedo

Laburpena:

Research on power electronics has significantly grown, mainly due to worldwide interest in sustainable energy and electrification of traction systems. Moreover, as energy transport and conversion systems evolve and advance, power electronics becomes also more complex. Hence, although power converters based on Silicon (Si) present a good balance between performance and cost, along with high reliability and maturity, the limits in terms of power density, operation temperature, and switching frequency of Si power devices are close to being reached. Then, advanced power converter topologies and new semiconductors, especially based on Wide-bandgap (WBG) materials, tend to develop higher efficiency and higher power density solutions. Regarding WBG devices, Silicon Carbide (SiC) and Gallium Nitride (GaN) are the most promising power switches due to a higher switching frequency capability than Si-based devices, among other benefits.

Advantages of using WBG devices on current applications are mainly focused on the development of high-frequency power converters, reducing the size and improving the efficiency. While SiC devices have already been put into practice for high power applications, the trend of GaN devices has been moved from radio frequency power amplifiers and microwave solutions to power applications with higher voltage requirements (>200 V) in recent years. The interest of GaN-based transistors is mainly associated with the high electron mobility layer, also known as the two-dimensional electron gas (2DEG). This structure provides a low condition losses and high-switching capability.

However, GaN devices present specific challenges related to their implementation on power electronics. Differential characteristics related to conduction, high-switching speed capability, gate requirements, and thermal cooling are some of the main design challenges. Therefore, special attention must be devoted to these design considerations in order to take full advantage of GaN semiconductors. Besides, there is a need to establish the benefits and limitations of these power devices. It is expected that GaN will be primarily a potential alternative for medium power solutions (in the 200-600V range), medium/high frequency and particularly in medium/high-end applications. The main objective of this thesis is to answer questions regarding the use of GaN devices and define optimal design considerations. Hence, the thesis presents a comprehensive analysis of differential characteristics, that define the design considerations and challenges. Minimum dead-time and gate driver requirements are defined. Gate driver is designed to achieve high switching speed but without exceeding gate limitations of GaN devices.

Afterwards, the thermal limits of current GaN devices are evaluated. An analysis based on the use of heat-spreading materials and parallelization of various devices is presented, increasing the thermal cooling capability. In addition, detailed analysis of hard- and soft-switching operation, is included, defining the most suitable operation modes. Besides, the converter performance is evaluated and experimentally validated, analyzing the impact of working with high-switching frequency. This analysis includes single-cell and multi-cell topologies, defining the sustainability of implementing then with GaN devices. The study carried out in this thesis is organized into six different chapters, evaluating from differential characteristics and challenges of GaN devices, until its benefits and limitations on developing GaN-based power converters.