The forward gate characteristics of a thyristor are shown in Fig. 4.9 in the form of a graph between gate voltage and gate current. Here positive gate to cathode voltage Vg and positive gate to cathode current Ig represent dc values. As gate-cathode circuit of a thyristor is a p-n junction, gate characteristics of the device are similar to that of a diode. For a particular type of SCRs, Vg-Ig characteristic has a spread between two curves 1 and 2 as shown in Fig. 4.9. This spread, or scatter, of gate characteristics is due to difference in the low doping levels of p and n layers. The gate trigger circuitry must be suitably designed to take care of this unavoidable scatter of characteristics. In Fig. 4.9, curve 1 represents the lowest voltage values that must be applied to turn-on the read more

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Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment. Static characteristics of a thyristor have already been examined. In this part of the section; switching, dynamic or transient, characteristics of thyristors are discussed. During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it. The time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor. Here, first switching characteristics during turn-on are described and then the switching characteristics during turn-off Switching Characteristics during read more

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Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment. Static characteristics of a thyristor have already been examined. In this part of the section; switching, dynamic or transient, characteristics of thyristors are discussed. During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it. The time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor. Here, first switching characteristics during turn-on are described and then the switching characteristics during turn-off. Switching Characteristics during Turn-on A read more

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With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques : (a) Forward voltage triggering          (b) gate triggering (c) dv/dt triggering              (d)Temperature triggering (e)Light triggering. These methods of turning-on a thyristor are now discussed one after the other. (a) Forward Voltage Triggering: When anode to cathode forward voltage is increased with gate circuit open, the reverse biased junction J2 will break. This is known as avalanche breakdown and the voltage at which avalanche occurs is called forward breakover voltage VB0. At this voltage, thyristor changes from off-state (high voltage with low leakage current) to on-state characterised by low voltage across thyristor with large forward read more

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An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. 4.2 (a). The anode and cathode are connected to main source through the load. The gate and cathode are fed from a source Es which provides positive gate current from gate to cathode. Fig. 4.2 (b) shows static V-I characteristics of a thyristor. Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. Typical SCR V-I characteristic shown in Fig. 4.2 (b) reveals that a thyristor has three basic modes of operation ; namely, reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of operation are now discussed below : Reverse Blocking Mode: When cathode is made positive with respect to anode with read more

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Thyristor is a four layer, three-junction, p-n-p-n semiconductor switching device. It has three terminals ; anode, cathode and gate. Fig. 4.1 (a) gives constructional details of a typical thyristor. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions J1, J2 and J3 as shown in Fig. 4.1 (a). The threaded portion is for the purpose of tightening the thyristor to the frame or heat sink with the help of a nut. Gate terminal is usually kept near the cathode terminal Fig. 4.1 (a). Schematic diagram and circuit symbol for a thyristor are shown respectively in Figs. 4.1 (b) and (c). The terminal connected to outer p region is called anode (A), the terminal connected to outer n region is called cathode and that connected to read more

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As stated before, Bell Laboratories were the first to fabricate a silicon-based semiconductor device called thyristor. Its first prototype was introduced by GEC (USA) in 1957. This company did a great deal of pioneering work about the utility of thyristors in industrial applications. Later on, many other devices having characteristics similar to that of a thyristor were developed. These semiconductor devices, with their characteristics identical with that of a thyristor, are triac, diac, silicon-controlled switch, programmable unijunction transistor (PUT), GTO, RCT etc. This whole family of semiconductor devices is given the name thyristor. Thus the term thyristor denotes a family of semiconductor devices used for power control in dc and ac systems. One oldest member of this thyristor read more

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An MCT is a new device in the field of semiconductor-controlled devices. It is basically a thyristor with two MOSFETs built into the gate structure. One MOSFET is used for turning on the MCT and the other for turning off the device. An MCT is a high-frequency, high:power, low-conduction drop switching device. An MCT combines into it the features of both conventional four-layer thyristor having regenerative action and MOS-gate structure. However, in MCT, anode is the reference with  respect to which, all gate signals are applied. In a conventional SCR, cathode is the reference terminal for gate signals. The basic structure of an MCT is shown in Fig. 2.20. A practical MCT consists of thousands of these basic cells connected in parallel, just like a power MOSFET ( 7, 8). This is done in read more

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