Thyristor based switches triggered in impact-ionization wave mode

Commercial thyristors of tablet design with diameters of silicon wafers of 40 to 56 mm and an operating voltage of 2 to 2.4 kV DC were triggered by an external overvoltage pulse applied across the thyristor main electrodes. In experiments a voltage rise rate across the thyristor was changed from 0.5 to 6 kV/ns. 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 to 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, 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 1.1-mF capacitor, which was charged to a voltage of 5 kV, was switched to a resistive load of 0.018 Ω. The following discharge parameters were obtained: discharge current amplitude of 200 kA, maximum current-rise rate of 58 kA/μs, current rise time (0.1-0.9 level) of 5.5 μs, pulse duration (FWHM) of ~25 μs, and switching efficiency of 0.97. It was shown that the voltage rise rate at the triggering stage was the main factor affected on the thyristors switching characteristics.