Gatetriggering

Gatetriggering, or gate triggering, is the method by which a thyristor, such as an SCR or a triac, is turned on by applying a signal to its gate terminal. A gate current must reach a specified value, known as the gate trigger current I_GT, or gate trigger voltage V_GT, to initiate conduction from anode to cathode (or MT2 to MT1 in a triac). Once triggered, the device latches, maintaining current until the load current falls below its holding current I_H.

The minimum current needed to guarantee turn-on is I_GT; the current required to keep the device on after triggering is the latching current I_L. Gate triggering can be accomplished with pulses of current or voltage, commonly using isolated gate drives for safety. Factors affecting triggering include temperature, device quality, and dv/dt sensitivity. dv/dt triggering occurs when a rapid rate of rise of the applied voltage inadvertently drives the device on even without a gate pulse; accordingly, gate drives and snubber networks are used to prevent false triggering. Many practical designs use optically isolated drivers (optotriacs or optocouplers) to provide isolation between control circuitry and the high-power load.

Triacs are bidirectional thyristors that use gate triggering in both positive and negative halves, whereas SCRs conduct in one direction and require gating to start conduction. Gate triggering is central to AC power control, dimming, motor speed control, rectification, and HVDC converters, where precise control of turn-on is essential.

Limitations of gate triggering include limited dv/dt immunity, sensitivity to temperature, and the need for gate drive circuitry tailored to the device's I_GT and I_L specifications. Proper gate design, snubbers, and isolation are important for reliability and safe operation.