Methodology to Improve Switching Speed of SiC MOSFETs in Hard Switching Applications

To meet the higher efficiency and power density requirement for power converters, the switching speed of power devices is preferred to increase. Thanks to silicon carbide (SiC) power MOSFETs, their intrinsic superior switching characteristics compared with silicon IGBTs makes it possible to run converters at faster switching speed in hard switching applications. Nevertheless, the switching speed is not only dependent on the device’s characteristics, but also strongly related to the circuit like gate drive and parasitics. To fully utilize the potential of SiC MOSFETs, the impact factors limiting the switching speed are required to be understood. Specific solutions and methods need to be developed to mitigate the influence from these impact factors.The characterization of the switching speed for SiC MOSFETs with different current ratings is conducted with double pulse test (DPT) first. Based on the result, the impact factors of switching speed are evaluated in detail.According to the evaluation, the switching speed of SiC discrete devices with low current rating is mainly limited by the gate drive capability. A current source gate drive as well as a charge pump gate drive are proposed, which can provide higher current during the switching transient regardless of the low transconductance and large internal gate resistance of SiC discrete devices.For SiC power modules with high current rating, the switching speed is mainly determined by the device drain-source overvoltage resulting from circuit parasitics. An analytical model for the multiple switching loops related overvoltage in 3L-ANPC converters is established. A simple modulation is developed to mitigate the effect of the non-linear device output capacitance, which helps reduce the overvoltage and enables higher switching speed operation of SiC power modules.Furthermore, the layout design methodology for three-level converters concerning the multiple commutation loops is introduced. The development of a laminated busbar for a 500 kVA 3L-ANPC converter with SiC power modules is presented in detail.Finally, a SiC based 1 MW inverter is built and tested to operate at cryogenic temperature. The proposed control and busbar above are utilized to increase the switching speed of the SiC power module

Composite power semiconductor switches for high-power applications

It is predicted that 80 % of the world’s electricity will flow through power electronic based converters by 2030, with a growing demand for renewable technolo gies and the highest levels of efficiency at every stage from generation to load. At the heart of a power electronic converter is the power semiconductor switch which is responsible for controlling and modulating the flow of power from the input to the output. The requirements for these power semiconductor switches are vast, and include: having an extremely low level of conduction and switching losses; being a low source of electromagnetic noise, and not being susceptible to external Electromagnetic Interference (EMI); and having a good level of ruggedness and reliability. These high-performance switches must also be economically viable and not have an unnecessarily large manufacturing related carbon footprint. This thesis investigates the switching performance of the two main semiconductor switches used in high-power applications — the well-established Silicon (Si)-Insulated-Gate Bipolar Transistor (IGBT) and the state-of-the-art Wide-Bandgap (WBG) Silicon-Carbide (SiC)-Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET). The SiC-MOSFET is ostensibly a better device than the Si-IGBT due to the lower level of losses, however the cost of the device is far greater and there are characteristics which can be troublesome, such as the high levels of oscillatory behaviour at the switching edges which can cause serious Electromagnetic Compatibility (EMC) issues. The operating mechanism of these devices, the materials which are used to make them, and their auxiliary components are critically analysed and discussed. This includes a head-to-head comparison of the two high-capacity devices in terms of their losses and switching characteristics. The design of a high-power Double-Pulse Test Rig (DPTR) and the associated high-bandwidth measurement platform is presented. This test rig is then extensively used throughout this thesis to experimentally characterise the switching performance of the aforementioned high-capacity power semiconductor devices. A hybrid switch concept — termed “The Diverter” — is investigated, with the motivation of achieving improved switching performance without the high-cost of a full SiC solution. This comprises a fully rated Si-IGBT as the main conduction device and a part-rated SiC-MOSFET which is used at the turn-off. The coordinated switching scheme for the Si/SiC-Diverter is experimentally examined to determine the required timings which yield the lowest turn-off loss and the lowest level of oscillatory behaviour and other EMI precursors. The thermal stress imposed on the part-rated SiC-MOSFET is considered in a junction temperature simulation and determined to be negligible. This concept is then analysed in a grid-tied converter simulation and compared to a fully rated SiC-MOSFET and Si-IGBT. A conduction assistance operating mode, which solely uses the part-rated SiC-MOSFET when within its rating, is also investigated. Results show that the Diverter achieves a significantly lower level of losses compared to a Si-IGBT and only marginally higher than a full SiC solution. This is achieved at a much lower cost than a full SiC solution and may also provide a better method of achieving high-current SiC switche

Gate Drive Design for Paralleled SiC MOSFETs in High Power Voltage Source Converters

High power voltage source converters (VSC) are vital in applications ranging from industrial motor drives to renewable energy systems and electrified transportation. In order to achieve high power the semiconductor devices used in a VSC need to be paralleled, making the gate drive design complicated. The silicon carbide (SiC) MOSFET brings much benefit over similarly rated silicon (Si) devices but further complicates the gate drive design in a parallel environment due to it’s fast switching capability and limited short-circuit withstand time. A gate driver design with proper accommodation of key issues for paralleled 1.7 kV SiC MOSFETs in high power VSC applications is developed.Three of the main issues are current imbalance, short-circuit protection, and cross-talk. By characterizing devices and supporting circuitry an understanding of constraints and sensitivities with regards to current balance between devices is developed for design optimization. A short-circuit detection scheme with adequate response time is employed and mitigation steps presented for issues arising from paralleling devices including large transient energy and instability. Cdv/dt induced gate voltage—cross-talk—is addressed by adapting a mitigation method to multiple devices. Finally, the gate driver is demonstrated in a full scale half-bridge using four devices per switch

An Optimized Switching Strategy Based on Gate Drivers with Variable Voltage to Improve the Switching Performance of SiC MOSFET Modules

This paper proposes an optimized switching strategy (OSS) based on a silicon carbide (SiC) MOSFET gate driver with variable voltage, which allows simultaneous variations in several different parameters to optimize the switching performance of semiconductor devices. As a relatively new device, the SiC MOSFET shines in the field of high power density and high-frequency switching; it has become a popular solution for electric vehicles and renewable energy conversion systems. However, the increase in voltage and current slope caused by high switching speeds inevitably increases the overshoot and oscillation in a circuit and can even generate additional losses. The principle of this new control strategy is to change the voltage and current in the turn-on and turn-off stages by changing the gate driver’s voltage. That is, we reduced the drive’s voltage after a certain time delay and maintained it for a period of time, thus directly controlling the slopes of di/dt and dv/dt. This study focused on the optimization of the SiC MOSFET by changing the time delay preceding the decrease in the voltage of the gate driver, analyzing and calculating the optimal time delay before the decrease in the voltage of the gate driver, and verifying the findings using LTspice simulation software. The simulated results were compared and analyzed with hard-switching strategies. The results showed that the proposed OSS can improve the switching performance of SiC MOSFETs

Contributions to the design of power modules for electric and hybrid vehicles: trends, design aspects and simulation techniques

314 p.En la última década, la protección del medio ambiente y el uso alternativo de energías renovables están tomando mayor relevancia tanto en el ámbito social y político, como científico. El sector del transporte es uno de los principales causantes de los gases de efecto invernadero y la polución existente, contribuyendo con hasta el 27 % de las emisiones a nivel global. En este contexto desfavorable, la electrificación de los vehículos de carretera se convierte en un factor crucial. Para ello, la transición de la actual flota de vehículos de carretera debe ser progresiva forzando la investigación y desarrollo de nuevos conceptos a la hora de producir vehículos eléctricos (EV) y vehículos eléctricos híbridos (HEV) más eficientes, fiables, seguros y de menor coste. En consecuencia, para el desarrollo y mejora de los convertidores de potencia de los HEV/EV, este trabajo abarca los siguientes aspectos tecnológicos: - Arquitecturas de la etapa de conversión de potencia. Las principales topologías que pueden ser implementadas en el tren de potencia para HEV/EV son descritas y analizadas, teniendo en cuenta las alternativas que mejor se adaptan a los requisitos técnicos que demandan este tipo de aplicaciones. De dicha exposición se identifican los elementos constituyentes fundamentales de los convertidores de potencia que forman parte del tren de tracción para automoción.- Nuevos dispositivos semiconductores de potencia. Los nuevos objetivos y retos tecnológicos solo pueden lograrse mediante el uso de nuevos materiales. Los semiconductores Wide bandgap (WBG), especialmente los dispositivos electrónicos de potencia basados en nitruro de galio (GaN) y carburo de silicio (SiC), son las alternativas más prometedoras al silicio (Si) debido a las mejores prestaciones que poseen dichos materiales, lo que permite mejorar la conductividad térmica, aumentar las frecuencias de conmutación y reducir las pérdidas.- Análisis de técnicas de rutado, conexionado y ensamblado de módulos de potencia. Los módulos de potencia fabricados con dies en lugar de dispositivos discretos son la opción preferida por los fabricantes para lograr las especificaciones indicadas por la industria de la automoción. Teniendo en cuenta los estrictos requisitos de eficiencia, fiabilidad y coste es necesario revisar y plantear nuevos layouts de las etapas de conversión de potencia, así como esquemas y técnicas de paralelización de los circuitos, centrándose en las tecnologías disponibles.Teniendo en cuenta dichos aspectos, la presente investigación evalúa las alternativas de semiconductores de potencia que pueden ser implementadas en aplicaciones HEV/EV, así como su conexionado para la obtención de las densidades de potencia requeridas, centrándose en la técnica de paralelización de semiconductores. Debido a la falta de información tanto científica como comercial e industrial sobre dicha técnica, una de las principales contribuciones del presente trabajo ha sido la propuesta y verificación de una serie de criterios de diseño para el diseño de módulos de potencia. Finalmente, los resultados que se han extraído de los circuitos de potencia propuestos demuestran la utilidad de dichos criterios de diseño, obteniendo circuitos con bajas impedancias parásitas y equilibrados eléctrica y térmicamente. A nivel industrial, el conocimiento expuesto en la presente tesis permite reducir los tiempos de diseño a la hora de obtener prototipos de ciertas garantías, permitiendo comenzar la fase de prototipado habiéndose realizado comprobaciones eléctricas y térmicas

The role of power device technology in the electric vehicle powertrain

In the automotive industry, the design and implementation of power converters and especially inverters, are at a turning point. Silicon (Si) IGBTs are at present the most widely used power semiconductors in most commercial vehicles. However, this trend is beginning to change with the appearance of wide-bandgap (WBG) devices, particularly silicon carbide (SiC) and gallium nitride (GaN). It is therefore advisable to review their main features and advantages, to update the degree of their market penetration, and to identify the most commonly used alternatives in automotive inverters. In this paper, the aim is therefore to summarize the most relevant characteristics of power inverters, reviewing and providing a global overview of the most outstanding aspects (packages, semiconductor internal structure, stack-ups, thermal considerations, etc.) of Si, SiC, and GaN power semiconductor technologies, and the degree of their use in electric vehicle powertrains. In addition, the paper also points out the trends that semiconductor technology and next-generation inverters will be likely to follow, especially when future prospects point to the use of “800 V" battery systems and increased switching frequencies. The internal structure and the characteristics of the power modules are disaggregated, highlighting their thermal and electrical characteristics. In addition, aspects relating to reliability are considered, at both the discrete device and power module level, as well as more general issues that involve the entire propulsion system, such as common-mode voltage.This work has been supported in part by the Government of the Basque Country through the fund for research groups of the Basque University System IT1440-22 and the Ministerio de Ciencia e Innovación of Spain as part of project PID2020-115126RB-I00 and FEDER funds. Finally, the collaboration of Yole Développement (Yole) is appreciated for providing updated data on its resources