Thyristor Devices Silicon Controlled Rectifiers (SCR)
Objectives • Draw and explain the i-v characteristics of a thyristor. • Explain the operating principle of a thyristor in terms of the “two transistor analogy”. • Draw and explain the switching characteristics of a thyristor.
Power Semiconductor Switches Power Diodes Power Transistors Thyristors 2 layer device 3 layer Device 4 layer Device • Thyristor devices can convert and control large amounts of power in AC or DC systems while using very low power for control. • Thyristor family includes 1- Silicon controlled Rectifier (SCR) 2- Gate-turnoff thyristor (GTO) 3- Triac 4- Diac 5- Silicon controlled switch (SCS) 6- Mos-controlled switch (MCT)
Introduction SCR is most popular of thyristor family due to its Fast switching action , small size and high voltage and current ratings. (300V, 25A-----6kV, 4kA) SCR has 3 terminals (gate provides control) SCR is turned on by applying +ve gate signal when anode is +ve with respect to cathode. SCR is turned off by interrupting anode current. PNPN structure Symbol
• N2---Very thin and highly doped • P2---Thicker and less highly doped • N1---Thickest and less doped • P1---Same as P2 Junction J3 has low breakdown voltage in either direction.
Operation of SCR The SCR has two states : a high-current low-impedance ON state and a low-current high-impedance OFF state. The basic transistor action in a four-layer p–n– p–n structure is analysed first with only two terminals, and then the third control input is introduced.
Physical Operation and Characteristics: • The physical operation of the SCR can be explained clearly with reference to the current–voltage characteristics. • Forward blocking Forward Conduction Reverse blocking
SCR in Forward Bias: • There are two different states in which we can examine the SCR in the forward-biased condition: (i) The high- impedance or forward-blocking state (ii) The low-impedance or forward-conducting state At a critical peak forward voltage Vp, the SCR switches from the blocking state to the conducting state, as shown in Fig. 8-2. • A positive voltage places junction j1 and j3 under forward-bias, and the center junction j2 under reverse-bias. • The forward voltage in the blocking state appears across the reverse- biased junction j2 as the applied voltage V is increased. The voltage from the anode A to cathode C, as shown in Fig. 8-1, is very small after switching to the forward-conducting state, and all three junctions are forward-biased. The junction j2 switches from reverse-bias to forward-bias..
I–V Characteristics of the SCR : • Forward-Blocking State: • When the device is biased in the forward-blocking state, as shown in Fig. 8- 4(a), the applied voltage appears primarily across the reverse-biased junction j2. Al though the junctions j1 and j3 are forward-biased, the current is small.
I–V Characteristics of the SCR : • Forward-Conducting State of the SCR : As the value of ( α1 + α2 ) approaches unity through one of the mechanisms ,many holes injected at j1 survive to be swept across j2 into p2.
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- Fall '19