Thyristors and Triacs Control Gate Trigger Circuits

The thyristor is a power semiconductor device with the control terminal called grid or gate (G). The device name comes from the similarity of operation with the gas triode: thiratron transistor. Because the thyristors are mainly used in the controlled rectifier structures, the technical literature calls them as SCR (Semiconductor Controlled Rectifiers) devices. After the series manufacture start in 1957 until the ‘80s of the twentieth century the thyristor was the leader of power semiconductor devices with control electrode. Despite the fact that the thyristor is a half controlled device (through the gate terminal can be only turned on) this switch remains favourite in a wide range of applications because it is robust, can be easily controlled and can operates in high power structures at high voltages and currents with low power conduction losses. To operate, every thyristor require a drive (trigger, control) circuit that provides gate current pulses to induce the on-state. These pulses must have a certain amplitude and duration. The device remains latched in on-state until it is reverse biased. Thus, in forward biased conditions, after the turn-on process, the gate current can be interrupted. Thyristors can be found in various applications such as phase-controlled rectifiers, AC voltage variators, starters, static breakers, cycloconverters, etc. Depending on the requirements, are available on the market several types of thyristors: common thyristors (used in phase-controlled converters at low switching frequency, high voltages and currents), fast thyristors (with a small blocking time), optical thyristors (light-activated through optical fibers), etc. A triac can be equivalent with two antiparallel connected thyristors. Thus, the control principles and the control circuits of the triacs are the same as those of the thyristors taking into consideration that the triacs are bidirectional devices (the current can flow in both directions through them).