INFLUENCE OF p-n CAPACITY BARRIER ON OPTIMAL RC PROTECTION OF THYRISTORS

Branislav Kuzmanović - Zoran Baus - Petar Biljanović

   Safe operation of the thyristor depends strongly on the critical rate of rise of the off-state voltage, the rate of on-state current, as well as on the recovery overvoltage. The snubber or commutation circuits ensure the required protections of the thyristor. Nevertheless, the design of the snubber circuits is not simple, the protective circuit designed to satisfy the requirements of one quantity will not necessarily perform optimally for another quantity, and a compromise solutions have to be sought. The paper deals with the optimal thyristor snubber design assuming the real thyristor parameters characteristic of the turn-on and turn-off switching intervals of the thyristor. For both modes of operation, the thyristor was replaced by a parallel RC equivalent model, unlike in previous studies, which treated the thyristor as an ideal switch. The analysis study was performed in normalize form to provide a generalized approach. The analysis results are presented by diagrams, for simplification of the selection of required RC elements for particular thyristor protection. The objective of the paper was to evaluate to what extent the choice of protective elements is affected by the RC model parameters of the thyristor and to provide optimal snubber protection for the thyristor. It was demonstrated that the thyristor equivalent capacitance together with other parasitic capacitances, significantly influence the choice of protective components.

Keywords: thyristor, reverse overvoltage, reverse mod, protective circuit, p-n junction, conducting losses, numerical procedures

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