SCR (silicon controlled Rectifiers)

SCR (silicon controlled Rectifiers) 

Introduction: SCR is a transistor family device, like LASCR, SCS, LASCS. SCR is a Unidirectional device. 

Definition: SCR is one of the oldest type of solid state power device. It was invented in 1957 by the General Electric Research Laboratories. SCR have the highest power handling capacity of all the power semiconductor devices. They have a four layers construction with three users accessible terminal namely anod, cathode and Gate.  SCR is a latching types device that can be turned on by the control terminal(gate) but once turned on, the gate loses control on it (i.e. it cannot be turned off by the Gate). 

Important feature of SCR: 

  1. It is latching types device. 
  2. It can handle very large power. 
  3. current controlled device, because the gate current controls SCR. 
  4. The on state voltage drop is very low. 
  5. It can handle thousands of Ampere of current. 

Construction of SCR:

Construction: The basic structure of SCR is as shown in Figure. It is a four layer PNPN device, with three terminals brought out for the user, namely Anode, Cathode and Gate The "gate" terminal is the controlling terminal that can turn on the device whenever required. The symbol for SCR is as shown in Fig. It is the symbol of a rectifying diode with a third additional control terminal, i.e. gate. The direction of forward anode current, voltage across the thyristor and direction of conventional gate current are as shown in Figure. 

Number of junctions :  As seen in the Figure. there are three junctions, J1 ,J2 and J3. In order to turn on SCR, the anode must be at a higher positive potential than cathode. That means SCR should forward biased. 

Current directions : The directions of the anode and gate current in Figure. are conventional current directions. It clearly indicates that SCR is a unidirectional device and that the gate current can be only positive. The gate current can flow only in one direction i.e. into the gate terminal.

Diagram: structural details of SCR

I-V characteristics (statics characteristics) :

The 1-V characteristics of SCR is a graph of anode current i, on y-axis and anode to cathode voltage plotted on the X- axis as shown in Figure.The characteristics in the reverse direction (anode to cathode voltage negative) is similar to a reverse biased diode. The I-V characteristics can be split into two parts namely the forward characteristics and reverse characteristics. For small reverse voltage a small reverse leakage current flows until the avalanche breakdown takes place at reverse breakover voltage VBR. As soon as the reverse breakdown takes place due to avalanche breakdown, a large current flows through SCR whereas the voltage across the device remains constant. It is dangerous to operate SCR in the reverse breakdown state because it may get damaged due to overheating. The region from 0 volts upto VBR volts in which the SCR is reverse biased and non-conducting is called as "reverse blocking state". Reverse blocking means that the SCR is reverse biased and in the non conducting (blocking) state. 

Forward characteristics: 

The anode is positive with respect to cathode. 
Hence SCR is forward biased. The forward characteristics is divided into three regions of operation namely: 

  1. Forward blocking state.
  2. Transition state. 
  3. ON state. 

1. Forward blocking state :

This is the high voltage low current mode of operation in which SCR is in the "OFF" state. The current through it is "Forward Leakage Current". This current flows due to thermally generated minority carriers. 

2. Transition state : 

SCR remains in the blocking state as long as the forward anode to cathode voltage is less than the breakover voltage. As soon as VAK becomes greater than the breakover value, the anode current iA increases sharply to a high value and the voltage across SCR reduces sharply to a low value (on state voltage). The switching of SCR from off state to on state and vice versa takes place in a short time. This change over state is called as transition state. As this is an unstable state, it is shown by dotted line in Figure. 

3.The low voltage high current mode or the ON state : 

The voltage across SCR is low in this state. In the on state the SCR anode currents is large and the on state voltage drop is small.

Satish jagtap

Hello iam Satish jagtap Electrical engineer. From: Aurangabad, Maharashtra. Edu: MIT college of Engineering.

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