Characteristics of SCR & it’s theory

SCR is an unidirectional device that blocks the current flow from Cathode to Anode. Unlike the diode, a SCR also blocks the current flow from anode to cathode until it is trigger into Conduction by a proper gate signal. Characteristics of SCR has 3 basic modes of operation : forward blocking mode(off state), forward conduction mode (on state) and Reverse Blocking Mode

What is SCR & characteristics of scr?

  • SCR or thyristor is a four layer, 3 junction, pnpn semiconductor power electronics device.
  • It has 3 terminals – anode, cathode and gate.
  • SCR Consists of 4 layer of alternate p-type and n-type Silicon semiconductor forming 3 junctions J1, J2, and J3.
  • The terminal which is connect to outer p region is called Anode (A), the terminal connect to outer n region is called cathode (K) and that connect to inner p region is called the gate (G). 

Now SCRs of voltage rating 10 kV and rms current rating of 3000 A with power handling capacity of 30 MW are available. Such a high power SCR can be switch on by a low Voltage Supply of having current of 1 A and 10 W. For such power handling, we need to mount heat sink on SCR.

Why We Called It SCR ?

An SCR is so called because silicon is used for its construction and its operation as a rectifier can be control.

Characteristics Of SCR :

SCR is an unidirectional device that blocks the current flow from Cathode to Anode. Unlike the diode, a SCR also blocks the current flow from anode to cathode until it is trigger into Conduction by a proper gate signal.

Typical SCR Characteristics has 3 basic modes of operation : forward blocking (off state) modeforward conduction mode (on state) and Reverse Blocking Mode.

Characteristics Of SCR graph
Characteristics of scr
Characteristics of scr
SCR Characteristics Circuit diagram
Forward Blocking Mode of characteristics of scr: 
  • In this mode anode is positive with respect to cathode, with gate circuit open.
  • The junction J1 and J3 are forward bias but junction J2 is reverse bias.
  • In this mode a small current called forward leakage current flows, due to high impedance. Therefore, SCR works as open switch. 
Forward Conduction Mode Characteristics Of Scr :
  •  When anode to cathode forward voltage increase with gate circuit open, reverse junction J2 will have on avalanche break down at a voltage called forward break over voltage VBO.
  • After this breakdown SCR gets turned on with anode current (IA) sharply increase and voltage (VAK) sharply decreased.
  • We can decrease forward break over Voltage by increases gate pulse.
Reverse Blocking Mode Characteristics Of Scr :
  • In this mode cathode is made positive with respect to anode with gate open.
  • Junction J1 and J3 are reverse bias whereas junction J2 is forward bias.
  • The device behaves as if two diodes are in series with reverse biasing.
  • A small leakage current of order of few micro ampere is flows.
  • This mode is called off state.
  • If we increase reverse bias voltage, then at a certain level reverse breakdown voltage occur due to avalanche breakdown at J1 and J3 and current increases rapidly. This may lead to damage of the SCR due to over heating. 

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