Development of a new modular switch using a next-generation semiconductor

An ultra-high-speed short pulse switch for high power devices has been developed by using SiC MOSFET which is one of the next generation semiconductors. Semiconductor switches using SiC-MOSFETs are expected to substitute the thyratron, and they are designed by connecting many semiconductor switches in parallel-series for high power operation. In order to suppress the commonmode noise caused by switching, it is common to form a symmetrical circuit. However, as the number of parallel connections in the horizontal direction increases, the length of the parallel circuit becomes longer, and the output waveform is distorted due to time lag between the circuits. Therefore, we propose a radially-symmetrical type module switch which can equalize the circuit length regardless of the number of parallel circuit. The design and preliminary test results of two types of switch circuits, radially-symmetrical type and general line-symmetrical type are presented here. INTRODUCTION Transistors such as Si diodes, MOSFETs (Metal-OxideSemiconductor Field-Effect Transistor) and IGBTs (insulated gate bipolar transistors) are used as power elements [1]. The development of next generation semiconductors aimed at improving the performance of the power semiconductor is thriving. One of them is SiC (silicon carbide). SiC power semiconductors can realize "high withstand voltage", "low on resistance", "small switching loss". However, the current SiC device is in the process of development and has high withstand voltage but does not support high current. For this reason, in order to realize the semiconductor switch for high current power supply etc., it is absolutely necessary to construct a circuit by parallel multiplexing of elements [2-4]. Regarding multiplexing circuits of semiconductor switches, the usefulness of symmetric circuits has been confirmed as a method of reducing a commonmode noise caused by switching [5]. However, when the number of parallel connections is increased, since a large number of elements are arranged side by side, the length of the parallel circuit becomes longer and there is a difference in the distance between the elements. In the case of a switch circuit that turns on all the elements at the same time, a deviation occurs in the current propagation time between the parallel circuit. Since the output waveform from the switch is a superimposed waveform of the output waveform from each circuit, the superimposed waveform will be distorted if there is a deviation in the propagation time. Therefore, we have conceived a radially-symmetrical type switch circuit in which parallel circuits are arranged on a circle so that the distances between elements are equal. With this circuit configuration, waveform distortion due to the propagation distance can be suppressed. We fabricated a modular switch assuming versatility combining SiC power semiconductor and radially-symmetrical circuit structure and evaluated its performance. In the evaluation, two types of circuits, radially-symmetrical type and linesymmetrical type, were fabricated and the waveform distortion of the output voltage was confirmed. CIRCUIT DESIGN Base circuits common to both circuits were designed. By changing the parallel connection structure of the base circuit, radially-symmetrical type and line-symmetrical type are constructed. Only for line-symmetrical type circuits, two kinds of distances between the parallel circuits of 50 mm and 300 mm were prepared so that the dependency of the difference in the distances between the circuits could be confirmed. The number of parallel circuits was set to 8 parallels in all circuits. Furthermore, the output voltage was doubled by stacking the boards in two stages, and the evaluation was carried out in the same way. For SiC semiconductor, SCT 3030 KL made by Rohm was used.