PhD defence by Mengxing Chen
27.10.2020 kl. 13.00 - 16.00
Mengxing Chen, Department of Energy Technology, will defend the thesis "SiC-Based 1.5-kV Photovoltaic Inverter: Switching Behavior, Thermal Modeling, and Reliability Assessment"
SiC-Based 1.5-kV Photovoltaic Inverter: Switching Behavior, Thermal Modeling, and Reliability Assessment
Professor Frede Blaabjerg
Professor Huai Wang, Professor Xiongfei Wang
Associate Professor Yongheng Yang
Associate Professor Szymon Michal Beczkowski, Dept. of Energy Technology, Aalborg University (Chairman)
Professor Hans-Peter Nee, KTH Royal Institute of Technology, Sweden
Professor Maria Ines Valla, National University of La Plata (UNLP), Argentina
Power electronic converter based on silicon carbide (SiC) devices is one of the most promising technologies for a breakthrough in renewable generation systems. Owing to its wide band-gap (WBG) characteristics, the SiC features several superiorities compared to its silicon (Si) counterpart, e.g., the increased electric field, the higher thermal conductivity, and the greater melting point. Among the application areas of SiC devices, the emerging 1.5-kV photovoltaic (PV) inverter is one of the most popular applications over the last five years. However, the broad adoption of SiC devices in emerging 1.5-kV PV inverters is still facing several reliability uncertainties. Although manufacturers have been working promptly to enhance the robustness of their SiC power devices, efforts from the application perspective are still highly demanded in several aspects.
First, owing to the high di/dt and dv/dt during the switching transient, the SiC metal-oxide-semiconductor field-effect-transistor (MOSFET) can suffer unfavorable switching behaviors, e.g., critical switching oscillations and voltage overshoots, which lead to a catastrophic breakdown in the early phase of operation. Moreover, this switching-behavior challenge becomes even more considerable in the emerging 1.5-kV PV inverter application, where three-level topologies are commonly adopted. Hence, it is highly demanded to have a switching-behavior investigation in 1.5-kV SiC-based inverters and identify the potential solution.
Secondly, the state-of-the-art thermal models fail to ensure an acceptable level of accuracy (especially in high operating temperatures), since the temperature effect on thermal performance is omitted. As the SiC device's thermal profile is closely related to several failure mechanisms, falsely predicting the junction temperature can lead to thermal runaway or accelerated degradation in both short-term and long-term operations. Therefore, a modified thermal model and its modeling methodology with improved accuracy will be needed to realize a highly reliable SiC-based PV inverter.
Thirdly, the long-term wear-out reliability of SiC MOSFETs is still threatened by the issue of die-solder fatigue inside the module, as the Young's modulus of SiC material is two times greater than Si. It has been reported that the thermal-mechanical lifetime of SiC device with standard solder is only 1/3 of Si. Nevertheless, this lifespan deduction has not been considered in state-of-art reliability assessments and design-for-reliability cases. In this perspective, a reliability assessment considering this solder fatigue effect for the 3L-ANPC 1.5-kV PV inverter is highly demanded to evaluate how much system-level lifetime deduction can be expected.
To address the reliability issues as mentioned above, this Ph.D. project works on the following three aspects of SiC-based 1.5-kV PV inverters -- switching behaviors, thermal modeling, and wear-out assessment.
Subsequently, the switching behaviors of SiC MOSFETs in a 1.5-kV three-level active-neutral-point-clamped (3L-ANPC) PV inverter are studied in a comprehensive way to identify the unfavorable switching behaviors. The issues of multi-frequency switching oscillation and the induced drain-source voltage overshoot are investigated, and the reliability-critical SiC device and root-cause parasitic component are identified accordingly. Specific design considerations are concluded to aid the design for more reliable SiC-based PV inverters with 1.5-kV dc input.
One critical point regarding an accuracy-level enhanced thermal model lies in taking the temperature effect into account. In this regard, the temperature-dependent thermal properties of several SiC power module package materials are integrated into a finite-element-method (FEM) based thermal simulation. On this basis, a temperature-dependent Cauer-type thermal model is extracted, where some of its thermal resistance and capacitance are characterized as temperature-dependent by linear polynomials. It can be concluded that the junction-heatsink thermal resistance is increased by over 10% under high junction temperatures by taking the temperature effect in to account.
Regarding the wear-out assessment of SiC-based 1.5-kV PV inverter, special efforts are paid on identifying a convincing lifetime model for solder-joints in the SiC power module. Based on available power cycling data and simulation results, a die-solder lifetime model is extrapolated. With the electrical and thermal models already built and validated, a mission-profile-based wear-out assessment procedure is conducted to predict the accumulated failure rate due to the wear-our of SiC modules within a lifespan of thirty years. On this basis, the wear-out reliability performance considering the SiC solder-joint wear-out can be concluded.
The defence will be in english - all are welcome
Department of Energy Technology
Pontoppidanstræde 101, Room 1.001