2.THYROISTER SCRnew.pptx - Thyristor Devices Silicon...

This preview shows page 1 - 11 out of 49 pages.

Thyristor Devices Silicon Controlled Rectifiers (SCR)
Image of page 1
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.
Image of page 2
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)
Image of page 3
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
Image of page 4
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.
Image of page 5
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.
Image of page 6
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
Image of page 7
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..
Image of page 8
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.
Image of page 9
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.
Image of page 10
Image of page 11

You've reached the end of your free preview.

Want to read all 49 pages?

  • Fall '19
  • SCR

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture