Current Distribution Optimization of Thyristor Over-Voltage Protectors, by Quasi-3D Simulation

Transient over-voltage protectors (TOVP´s) are commonly used in telephone lines to ward off electric transients induced by lightning. The TOVP is basically a four-layer device, similar to a thyristor, but without gate, and with a number of shorting dots between the cathode and the nearest base. The shorting dots, among other things, control the current distribution during fast turn on transients: the more uniform the current, the higher the surge capability of the device. The position of the shorting dots has been optimized in the past by failure analysis methods, and combinations of 2D simulations and analytical methods. 2D simulations, however, give an incomplete picture of turn-on processes, because of their three-dimensional (3D) nature. On the other hand, full 3D simulations are impractical because they would require long computation times in high-end workstations and even in supercomputers. With our quasi-3D Spice-based simulator, transient current density distributions of four layer devices with shorting dots can be quickly simulated. This method consists of dividing the device into four-layered square prisms, and a 1D PNP-NPN transistor pair model associated to each of them. The resulting equivalent circuit is simulated with Spice in a personal computer. In the present paper, a quasi-3D simulation-based optimization method for the shorting-dot positions of a bipolar TOVP, consisting of two four-layer devices connected in anti-parallel, is presented for the first time. It was found that the boundary between the two sections of the device disturbs the transient current density distribution when 10 mus/300 mus current spikes are applied to the device, and that the distribution could be improved by shifting 50 mum the position of the shorting dots. It was also estimated, by quasi-3D simulation, and confirmed with 2D Atlas electro-thermal simulations, that at anode currents of 100 A/cm², temperature rise is less than 5 K