lecture_05_thyristors.pdf - Power Electronics Lecture-5 Thyristors Dr Imtiaz Hussain Assistant Professor email [email protected]

lecture_05_thyristors.pdf - Power Electronics Lecture-5...

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Power Electronics Dr. Imtiaz Hussain Assistant Professor email: [email protected] URL : Lecture-5 Thyristors 1
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One of the most important type of power semiconductor device. Compared to transistors, thyristors have lower on- state conduction losses and higher power handling capability. However, they have worse switching performances than transistors. Name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR . Introduction 2
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Thyristors are four-layer pnpn power semiconductor devices. These devices switch between conducting and nonconducting states in response to a control signal. Thyristors are used in timing circuits, AC motor speed control and switching circuits. Introduction 3
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Thyristors Bell Laboratories were the first to fabricate a silicon- based thyristor. Its first prototype was introduced by GE (USA) in 1957. Later on many other devices having characteristics similar to of a thyristor were developed. These semiconductor devices are SCR, SCS, Triac, Diac, PUT, GTO, e.t.c. This whole family of semiconductor devices is given the name thyristors. 4
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Thyristor/ SCR SCR is a three terminal, four layers solid state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N, It can handle high currents and high voltages, with better switching speed and improved breakdown voltage . A K G 5
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Thyristor/ SCR Thyristor can handle high currents and high voltages. Typical rating are 1.5kA & 10kV which responds to 15MW power handling capacity. This power can be controlled by a gate current of about 1A only. Thyristor acts as a bistable switch. It conducts when gate receives a current pulse, and continue to conduct as long as forward biased (till device voltage is not reversed). They stay ON once they are triggered, and will go OFF only if current is too low or when triggered off. 6
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Thyristor/ SCR Operation When the anode voltage is made positive with respect to the cathode, junctions J 1 and J 3 are forward biased and junction J 2 is reverse biased. The thyristor is said to be in the forward blocking or off-state condition . A small leakage current flows from anode to cathode and is called the off- state current . 7
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Thyristor/ SCR Operation If the anode voltage V AK is increased to a sufficiently large value, the reverse biased junction J 2 would breakdown. This is known as avalanche breakdown and the corresponding voltage is called the forward breakdown voltage V BO . Since the other two junctions J 1 and J 3 are already forward biased, there will be free movement of carriers across all three junctions.
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