EEL 2003 Electrical / Electronics Department SO1/2 5/15/2016 1 Nasser Thyristor In many ways the Silicon Controlled Rectifier, or the Thyristor as it is more commonly known, is similar to the transistor. It is a multi-layer semiconductor device, hence the “silicon” part of its name. It requires a gate signal to turn it “ON”, the “controlled” part of the name and once “ON” it behaves like a rectifying diode, the “rectifier” part of the name Figure 1. In fact the circuit symbol for the thyristorsuggests that this device acts like a controlled rectifying diode. A thyristoris a solid-state semiconductor device with four layers of alternating N and P-type material. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct while the voltage across the device is not reversed (forward-biased).Figure 1: Thyristor Symbol 1-Thyristor representation However, unlike the diode which is a two layer ( P-N ) semiconductor device, or the transistor which is a three layer ( P-N-P, or N-P-N ) device, the Thyristoris a four layer ( P-N-P-N ) semiconductor device that contains three PN junctions in series, and is represented by the symbol as shown in figure 2. Figure 2: Thyristor representation
EEL 2003 Electrical / Electronics Department SO1/2 5/15/2016 2 Nasser Like the diode, the Thyristor is a unidirectional device, that is it will only conduct current in one direction only, but unlike a diode, the thyristor can be made to operate as either an open-circuit switch or as a rectifying diode depending upon how the thyristors gate is triggered. In other words, thyristors can operate only in the switching mode and cannot be used for amplification. The silicon controlled rectifier SCR, is one of several power semiconductor devices along with Triacs (Triode AC’s), Diacs (Diode AC’s) and UJT’s (Unijunction Transistor) that are all capable of acting like very fast solid state AC switches for controlling large AC voltages and currents. So for the Electronics student this makes these very handy solid state devices for controlling AC motors, lamps and for phase control. The thyristor is a three-terminal device labelled: “Anode”, “Cathode” and “Gate” and consisting of three PN junctions which can be switched “ON” and “OFF” at an extremely fast rate, or it can be switched “ON” for variable lengths of time during half cycles to deliver a selected amount of power to a load. The operation of the thyristor can be best explained by assuming it to be made up of two transistors connected back-to-back as a pair of complementary regenerative switches as shown in figure 3. 2-Thyristor I-V Characteristics Curves Figure Thyristor Characteristics Figure 3: Thyristor Characteristics
EEL 2003 Electrical / Electronics Department SO1/2 5/15/2016 3 Nasser The important points on this characteristic are: Latching Current IL This is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate signal has been removed.