- 1 What is SCR & characteristics of scr?
- 2 Why We Called It 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. 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) mode, forward conduction mode (on state) and Reverse Blocking Mode.
Characteristics Of SCR graph
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.