June/July 2021

16 PCIM EUROPE 2021 Issue 3 2021 Power Electronics Europe www.power-mag.com impedance mismatches in case of long motor cables, as well as common- mode ground currents that would reduce the bearings’ lifetime. Furthermore, there are no conducted or radiated high-frequency electromagnetic emissions, i.e., it is not necessary to employ shielded motor cables. Compared to direct connection of inverter and motor, i.e., without an output filter, lower requirements with respect to motor winding insulation and high-frequency losses facilitate a notable cost reduction. In addition, the audible noise typical for IGBT PWM inverters operating with relatively low switching frequencies can be avoided, and an improvement of the part-load efficiency of the overall system by several percentage points can be achieved. Buck-boost DC/AC converters In case of battery or fuel cell supply of a VSD, the DC input voltage widely fluctuates, depending on the load state and the battery’s state of charge. In the simplest case, a DC/DC boost converter stage placed at the drive system’s input can compensate these input voltage variations. However, all bridge-legs then operate with high DC voltage (defined by the maximum input voltage), and the topology requires a total of four inductive components. Furthermore, the continuous operation of the DC/DC stage, i.e., the two-stage energy conversion, degrades the converter’s efficiency. If, in contrast, a boost-type bridge-leg is inserted between the output filter inductor and the filter capacitor of each phase, the switching operation can be limited to either this bridge-leg or the corresponding buck-type bridge-leg of the main inverter stage at any given time. Like any conventional inverter, in the simplest case the system then generates output phase voltages (with respect to the negative DC rail) that consist of a sinusoidal component and a DC offset. In order to realize the low conduction losses of a single transistor, a total of four individual switches must be employed (see Figure 1a). Novel monolithic AC power semiconductor switches (monolithic bidirectional switches, M-BDSs, (see Figure 1b) with bipolar voltage blocking capability and bidirectional current controllability require only a slightly larger chip area compared to a single (unidirectional) switch; consider, e.g., 600 V GaN drain-drain M-BDSs (see Figure 1c) or SiC M-BDSs for higher blocking voltages that are currently under development. The DC/AC stage can then again be realized with only six switching devices and the overhead remains limited to a doubling of the number of gate drives and the implementation of a four-step commutation scheme. Monolithic bidirectional converters GaN M-BDSs, which in addition to a normally-off variant also exist in a normally-on variant that is advantageous regarding the realization of protection concepts, form the general basis for the future use of three-phase DC/AC or AC/AC current DC-link converters. As shown in Figure 2b, an AC/AC converter then requires only twelve M- BDS elements and a single magnetic component, whereas three-phase AC/AC voltage DC-link converters employ the same number of switches, but require a total of six magnetic components in case a PFC rectifier front-end is employed. It is important to highlight that the AC/AC converter topology also is of clear advantage compared to direct or indirect AC/AC matrix converters, because the latter are inherently limited to buck operation and require three filter inductors to form a continuous output voltage. Literature Next-Generation SiC/GaN Three-Phase Variable-Speed Drive Inverter Concepts, Proceedings PCIM Europe digital days 2021, pages 1-5 300 kW Isolated DC/DC SiC Converter With solid state transformers efficiency is an essential criterion underlines the Best Paper.. A sufficiently high switching frequency in order to reduce filter elements, noise pollution, volume and mass of the transformer is useful. Therefore, the concept of soft switching is indispensable for reasonably meet these constraints. Additionally, it is not possible to extrapolate the switching energy curves for zero current switching (ZCS) or zero voltage switching (ZVS) using the device’s datasheet. Therefore, an experimental test bench is fundamental to achieve results far beyond those that theoretical calculations and simulations could provide. Finally, aiming to obtain further efficiency improvement, this awarded paper presents an experimental comparison regarding two different current ratings of 3.3 kV SiC-MOSFETs focusing on the influence caused by the output capacitance of the devices. Gustavo Fortes, Laplace; Université de Toulouse (gustavo.fortes@laplace.univ-tlse.fr ) The manufacturing processes for SiC devices are complex, but considerable advances were made in the last years. New packages for low and high voltages modules (LVM and HVM) have been proposed by several manufacturers. Nevertheless, the low availability and the high cost of these new power modules means that the number of publications is quite small concerning their use in power converters. Moreover, in the datasheets no information concerning the energy losses in soft-switching is available which does not facilitate the converter design. R-SAB prototype The first 3.3 kV SiC-MOSFET power modules samples became available in 2019. This made it possible to evaluate two current ratings (375 A and 750 A), focusing on the influence of the output capacitance and on-state resistance, eg. Mitsubishi’s HBM (H-Bridge Module) which are considered. The same SiC-MOSFET power modules have been used both on the inverter and the rectifier. Accordingly, the encapsulated Schottky SiC-diodes are used as rectifier-bridge, meanwhile the transistors are kept in the off state. A water cooled 300 kW prototype rated to 1.8 kV and 170 A has been developed (Figure 1) based on ABB ‘s water cooled medium frequency oil immersed transformer; Mersen’s water cooled heatsinks; HC5 series resonant capacitors from Illinois; LH3 series DC link capacitors from Electronic Concept; LEM ‘s series LV and DV sensors; Imperix’s BoomBox control with optical interfaces; customized gate-drivers; and Mitsubishi HBM 3.3 kV SiC-MOSFETs. These semiconductor devices are housed in a low inductance package with insulated base plate. Accordingly, there is no need to install a water deionization system that would comply with voltage insulation issues between the different parts of the converter. In order to characterize the R-SAB converter, an opposition method has been used as shown in Figure 2. The voltage source (V DC ) imposes the voltage Figure 1: Resonant Single Active Bridge test bench