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In many pulsed-power applications, there is a trend to modulators based on semiconductor technology. For these modulators, high-voltage and high-current semiconductor switches are required in order to achieve a high pulsed power. Therefore, often, high-power IGBT modules or IGCT devices are used. Since these devices are based on bipolar technology, the switching speed is limited, and the switching losses are higher. In contrast to bipolar devices, unipolar ones (e.g., SiC JFETs) basically offer a better switching performance. Moreover, these devices enable high blocking voltages in the case where wide-band-gap materials, for example, SiC, are used. At the moment, SiC JFET devices with a blocking voltage of 1.2 kV per JFET are available. Alternatively, the operating voltage could be increased by connecting N JFETs and a low-voltage MOSFET in series, resulting in a super cascode switch with a blocking voltage N times higher than the blocking voltage of a single JFET. For the super cascode, auxiliary elements are required for achieving a statically and dynamically balanced voltage distribution in the cascode. In this paper, a new balancing circuit, which results in faster switching transients and higher possible operating pulse currents, is presented and validated by measurement results.
Date of Publication: Oct. 2012