Thyristor Semiconductor Products Page 18 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1001 Fundamental Characteristics of Thyristors Introduction The Thyristor family of semiconductors consists of several very useful devices. The most widely used of this family are silicon controlled rectifiers (SCRs), Triacs, SIDACs, and DIACs. In many applications these devices perform key functions and are real assets in meeting environmental, speed, and reliability specifications which their electro- mechanical counterparts cannot fulfill. This application note presents the basic fundamentals of SCR, Triac, SIDAC, and DIAC Thyristors so the user understands how they differ in characteristics and parameters from their electro-mechanical counterparts. Also, Thyristor terminology is defined. Figure AN1001.3 shows cross-sectional views of an SCR chip and illustrations of current flow and junction biasing in both the blocking and triggering modes. SCR Basic Operation Figure AN1001.1 shows the simple block construction of an SCR. The connections between the two transistors trigger the occurrence of regenerative action when a proper gate signal is applied to the base of the NPN transistor. Normal leakage current is so low that the combined h FE of the specially coupled two-transistor feedback amplifier is less than unity, thus keeping the circuit in an off-state condition. A momentary positive pulse applied to the gate biases the NPN transistor into conduction which, in turn, biases the PNP transistor into conduction. The effective h FE momentarily becomes greater than unity so that the specially coupled transistors saturate. Once saturated, current through the transistors is enough to keep the combined h FE current (I T ) so that the combined h FE is less than unity and regeneration ceases. This threshold anode current is the holding current of the SCR. Gate J1 J2 J3 P N P N Schematic Symbol Block Construction Cathode Anode Cathode Anode The operation of a PNPN device can best be visualized as a specially coupled pair of transistors as shown in Figure AN1001.2. Figure AN1001.1 SCR Block Construction N P N P N P Gate Cathode J1 J2 J3 Anode N Cathode Gate Anode Load P Two-transistor Schematic Two-transistor Block Construction Equivalent Figure AN1001.2 Coupled Pair of Transistors as a SCR Gate Cathode (-) (+) I GT P N P (+) Anode I T Forward Bias and Current Flow Gate Cathode P N P (-) Anode Reverse Bias Reverse Biased Junction (-) Anode Equivalent Diode Relationship Forward Blocking Junction Cathode (-) (+) Anode Equivalent Diode Relationship Cathode (+) Reverse Biased Gate Junction Geometric Construction Figure AN1001.3 Cross-sectional View of SCR Chip

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