Design of Gate Drive Circuit for Thyristor Stack in Electromagnetic Railgun by Load-Line Analysis

Abstract

A gate-drive circuit is presented, which simultaneously triggers high power thyristors for surge current operation in a 400-kJ, 2×3 series-parallel stack, to be used as a controllable switch in a pulsed power supply (PPS) for a railgun. This type of dynamic application requires high gate current with a large rate of rise. To arrive at the appropriate gate drive circuit, a load line analysis has been done. The point of intersection of this load line with the gate-cathode characteristics of the thyristor theoretically predicts the operating point. The gate triggering circuit is designed such that its Thévenin equivalent circuit corresponds to the load line intercepts. To verify the design, the gate-cathode characteristics of the thyristor have been behaviorally modeled as a diode, and SPICE simulations are performed on the designed triggering circuit with this diode as a load. A peak gate current and a gate-to-cathode voltage of 9 A and 6 V, respectively, are achieved, resulting in a design compliant with ABB 5STP26N6500 surge specifications. The peak rate of rise of gate current obtained is 10 A/μs. Beyond the 9-A peak, the current pulse starts decaying and remains nonzero up to 160 μs. Circuit configurations having load-lines ranging from (15 V/ 0.96 Ω) to (45 V/4.13 Ω) are examined, and the loadline with slope (45 V/4.13 Ω) is chosen, because it results in an output current ≤ 10 A (the maximum allowed gate current) for all temperatures in between maximum dynamic high temperature and minimum temperature of -40°C.