Thyristor Semiconductor Products Page 24 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1002 Gating, Latching, and Holding of SCRs and Triacs Introduction Gating, latching, and holding currents of Thyristors are some of the most important parameters. These parameters and their interrelationship determine whether the SCRs and Triacs will function properly in various circuit applications. Gating of SCRs and Triacs Triacs (bilateral devices) can be gated on with a gate signal of either polarity with respect to the MT1 terminal; however, different polarities have different requirements of I GT and V GT . Figure AN1002.2 illustrates current flow through the Triac chip in various gating modes. Figure AN1002.1 SCR Current Flow P N P Anode Cathode Gate (+) (-) (+) I T I GT N P Gate(+) MT1(-) I GT N I T MT2(+) QUADRANT I Gate(-) MT1(-) MT2(+) QUADRANT II I GT N P Gate(+) MT1(+) I GT QUADRANT III Gate(-) MT1(+) MT2(-) QUADRANT IV I T I T I GT MT2(-) Figure AN1002.2 Triac Current Flow (Four Operating Modes) This application note describes how the SCR and Triac parameters are related. This knowledge helps users select best operating modes for various circuit applications. Three general methods are available to switch Thyristors to on-state condition: This application note examines only the application of proper gate signal. Gate signal must exceed the I GT and V GT requirements of the Thyristor being used. I GT (gate trigger current) is the minimum gate current required to switch a Thyristor from the off state to the on state. V GT (gate trigger voltage) is the voltage required to produce the gate trigger current. SCRs (unilateral devices) require a positive gate signal with respect to the cathode polarity. Figure AN1002.1 shows the current flow in a cross-sectional view of the SCR chip. In order for the SCR to latch on, the anode-to-cathode current (I T ) must exceed the latching current (I L ) requirement. Once latched on, the SCR remains on until it is turned off when anode-to-cathode current drops below holding current (I H ) requirement.

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