Superfast Thyristor-Based Switches Operating in Impact-Ionization Wave Mode

The process of triggering thyristors by an overvoltage pulse with a short rise time was investigated. Low-frequency commercial thyristors of tablet design with diameters of silicon wafers of 32-56 mm and an operating voltage of 2-2.4 kV dc were used in the experiments. An external overvoltage pulse was applied across the thyristor main electrodes, which ensured a voltage rise rate from 0.5 to 6 kV/ns within a few nanoseconds. Under such conditions, the thyristor closing process occurred due to initiation and propagation of a fast ionization front across the semiconductor structure. The time of switching the thyristor from the blocking state to the conducting state was within 200-400 ps. The thyristor-based switches operated in this mode were tested in two discharge circuits. In the first circuit, an assembly of nine 2-kV and 40-mm thyristors connected in series switched a 2-μF capacitor, which was charged to a voltage of 20 kV and to a resistive load of 0.17 Ω. The following results were obtained: discharge current amplitude of 45 kA, maximum current-rise rate of 134 kA/μs, current rise time (0.1-0.9 level) of ~0.4 μs, pulse duration (FWHM) of ~1 μs, and switching efficiency of 0.85. In the second circuit, the switch contained two 2.4-kV and 56-mm thyristors connected in series. A 400-μF capacitor, which was charged to a voltage of 5 kV, was switched to a resistive load of 0.026 Ω. The following discharge parameters were obtained: discharge current amplitude of 114 kA, maximum current-rise rate of 32 kA/μs, current rise time (0.1-0.9 level) of ~5 μs, pulse duration (FWHM) of ~20 μs, and switching efficiency of 0.96.