presented by:
GAMI DHAVAL

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Abstract—
Thyristor is common name given to the family of devices. It is very important member of power ele. Devices. It has large application now a day. This report gives introduction and explains characteristics of thyristor devices. It also contain basic introduction of power electronics. It contains basic information about power electronics devices like IGBT and Power MOSFET. At last it includes future applications of thyristor devices in different fields.
I. INTRODUCTION TO POWER ELE.
Power electronics is the study of electronic circuits for the control and conversion of electrical energy. The technology is a critical part of our energy infrastructure, and is a key driver for a wide range of uses of electricity. It is becoming increasingly important as an essential tool for efficient, convenient energy conversion, and management. For power electronics design,
` We consider only those circuits and devices that, in principle, introduce no loss and achieve near-perfect reliability. The two key characteristics of high efficiency and high reliability are implemented with switching circuits, supplemented with energy storage. .
This is driving tremendous expansion of their application. Personal computers, for example, would be unwieldy and inefficient without power electronic dc supplies. Portable communication devices and laptop computers would be impractical. High-performance lighting systems, motor controls, and a wide range of industrial controls depend on power electronics. Strong growth is occurring in automotive applications, in dc power supplies for communication systems, in portable devices, and in high-end converters for advanced microprocessors. In the near future, power electronics will be the enabler for alternative and renewable energy resources. During the next generation, we will reach a time when almost all electrical energy is processed through power electronics somewhere in the path from generation to end use.
II. HISTORY OF POWER ELE.
III. INTRODUCTION OF THYRISTOR
Thyristors are usually three-terminal devices that have four layers of alternating p-type and n-type material (i.e. three p–n junctions) comprising its main power handling section. In contrast to the linear relation which exists between load and control currents in a transistor, the thyristor is bistable. The control terminal of the thyristor, called the gate (G) electrode, may be connected to an integrated and complex structure as a part of the device. The other two terminals, called the anode (A) and cathode (K), handle the large applied potentials (often of both polarities) and conduct the major current through the thyristor.
The anode and cathode terminals are connected in series with the load to which power is to be controlled. Thyristors are used to approximate ideal closed (no voltage drop between anode and cathode) or open (no anode current flow) switches for control of power flow in a circuit. This differs from low-level digital switching circuits that are designed to deliver two distinct small voltage levels while conducting small currents (ideally zero). Thyristor circuits must have the capability of delivering large currents and be able to withstand large externally applied voltages. All thyristor types are controllable in switching from a forward-blocking state (positive potential applied to the anode with respect to the cathode, with correspondingly little anode current flow into a forward-conduction state (large forward anode current flowing, with a small anode–cathode potential drop). Most thyristors have the characteristic that after switching from a forward-blocking state into the forward-conduction state, the gate signal can be removed and the thyristor will remain in its forward-conduction mode.
Almost all power semiconductor devices are made from silicon (Si). Research and development continues in developing other types of devices in silicon carbide (SiC), gallium nitride (GaN), and related material systems. However, the physical description and general behavior of thyristors is unimportant to the semiconductor material system used, though the discussion and any numbers cited in the chapter will be associated with Si devices.
I. BASIC STRUCTURE & OPERATION
A high-resistivity region, n-base, is present in all thyristors. It is this region, the n-base and associated junction, J2 of Fig. 1, which must support the large applied forward voltages that occur when the switch is in its off- or forward-blocking state (non-conducting). The n-base is typically doped with impurity phosphorous atoms at a concentration of 1013 to 1014 cm
Operation of thyristors is as follows. When a positive voltage is applied to the anode (with respect to cathode), the thyristor is in its forward-blocking state. The center junction, J2 is reverse biased. In this operating mode the gate current is held to zero (open circuit). In practice, the gate electrode is biased to a small negative voltage (with respect to the cathode) to reverse bias the GK-junction J3 and prevent charge-carriers from being injected into the p-base. In this condition only thermally generated leakage current flows through the device and can often be approximated as zero in value (the actual value of the leakage current is typically many orders of magnitude lower than the conducted current in the on-state). As long as the forward applied voltage does not exceed the value necessary to cause excessive carrier multiplication in the depletion region around J2 (avalanche breakdown), the thyristor remains in an off-state (forward-blocking). If the applied voltage exceeds the maximum forward-blocking voltage of the thyristor, it will switch to its on-state. However, this mode of turn-on causes non-uniformity in the current flow, is generally destructive, and should be avoided.
When a positive gate current is injected into the device, J3 becomes forward biased and electrons are injected from the n-emitter into the p-base. Some of these electrons diffuse across the p-base and get collected in the n-base. This collected charge causes a change in the bias condition of J1. The change in bias of J1 causes holes to be injected from the p-emitter into the n-base. These holes diffuse across the n-base and are collected in the p-base. The addition of these collected holes in the p-base acts the same as gate current. The entire process is regenerative and will cause the increase in charge carriers until J2 also becomes forward biased and the thyristor is latched in its on-state (forward-conduction).