October / November 2021

18 EPE ECCE 2021 REVIEW www.epe2021.com Issue 4 2021 Power Electronics Europe www.power-mag.com Inverter design with different WBG devices In the paper “Comparative Analysis of High Speed Drive Inverter Designs using different Wide-Band- Gap Power Devices” by Technische Universität Braunschweig (www.tu-braunschweig.de/ imab) , a 15 kW drive inverter design with increased switching frequency demand for the high-speed drive of an automotive electrical turbo- compressor unit is described. After simulation- based evaluations eight inverter prototypes using different WBG power semiconductor devices are built and operated. The selection of the most promising inverter designs and power semiconductor devices was performed using a detailed evaluation matrix with weighted criteria like estimated efficiency, estimated volume or power density, system complexity, qualification state of the devices and assembly aspects. A 3-level T-type inverter using Silicon IGBTs and diodes in a power module package is used as a reference design. Due to the prospective increase of power density, reduction of system complexity and the good availability of AEC-Q101 qualified devices the most selected 2- level WBG inverter designs are using discrete devices, preferably in SMD packages for easy assembly. Using SiC MOSFET power modules can result in inverter designs with good efficiency and high power density. The total space occupied by the SiC modules is about 1/3 to 2/3 of the area of the T- type modules used as reference. Using power modules also allows an easy and robust thermal design with a ceramic DCB as electrical insulation. Unfortunately, there is still no suitable module available with AEC-Q grade. The widest range of available devices can be found for SiC MOSFETs in TO-247 packages. It is recognized that there are still significant differences in terms of RDSon and switching losses for different suppliers. Using two devices in parallel, most of the MOSFETs will outperform the efficiency of the reference solution at 60 kHz. At 120 kHz the switching losses of the devices in the large leaded standard package turn out to be a problem. SiC MOSFETs in a standard SMD package like the D2PAK-7L seem to be a very convenient solution to the presented application. With two devices in parallel the losses are low and the achievable power density is very high. There are qualified devices available from different suppliers and the assembly process is simple. This also makes the solution very interesting in terms of cost. Compared to SiC, the available GaN devices have higher R DSon but lower switching losses. Therefore paralleling of devices is inevitable to handle the peak current. In return especially the solutions with SMD devices result in the highest efficiency at 120 kHz. There is still a lack of suppliers for devices in compatible packages, but the technology will definitely be an option for high- speed drives with special requirements in terms of switching frequency. Five different PCB layouts for different device technologies were designed, optimized, built and set into operation for further evaluation. Taking into account all defined evaluation criteria, the most promising solution is the one based on SiC power MOSFETs in a standard SMD package. Summary of semiconductor losses for selected solutions Series connection of 10 kV SiC MOSFETs The paper “Analysis of Quasi-Two-Level Modulation for Neutral-Point-Clamped Three-level Converter with 10 kV SiC MOSFETs” presented by the Center for Power Electronics Systems (CPES) at Virginia Polytechnic Institute and State University (www.cpes.vt.edu.cn) proposed a solution for two series-connected SiC MOSFETs - a quasi-two- level modulation based on the neutral-point- clamped (NPC) three-level (3L) converter. The proposed solution is very attractive for the recent 10 kV SiC MOSFETs due to several reasons: 1) the blocking voltage could be increased to 20 kV, suitable for most medium voltage applications; 2) with NPC 3L structure, the voltage balancing of series-connected devices is avoided; 3) with Q2L modulation, the total loss on clamping diodes is significantly reduced so the clamping diodes don’t significantly increase converter volume. With Q2L modulation, series-connected 3.3 kV SiC diodes with much smaller volume are chosen as an example as the clamping diodes to showcase the benefit of Q2L modulation. In recent years, the SiC MOSFETs gained increasing popularity in medium voltage (> 1 kV) powerconversion applications due to the potential to improve efficiency and power density by adopting the simpler topology and fewer conversion stages. Despite the recent progress in 10 kV or 15 kV SiC MOSFET, three-level (3L) converters or two- Different approaches to increase the blocking voltage of converter: (a) direct series connection of devices; (b) 3L topology with clamping diodes; (c) 3L topology with flying capacitor