Principle and Working of SCR (silicon controlled rectifier)

This article mentions the detailed working of SCR(silicon controlled rectifier ) with application, characteristics and parameters affecting it.

SCR, It is actually an thyristor which is used to control the high voltage supply as compared to the conventional transistors. It is an four layered P-N-P-N semiconductor three terminal device, such that all the four segment arranged in a single array. it has an anode, cathode and an gate terminal. The upper P-type semiconductor is termed as anode ,usually given an positive voltage, the lower N-type semiconductor segment is termed as cathode and usually given an negative voltage while p-type semiconductor near cathod is gate which supplied with an positive pulse to trigger the device.

Modes of operation of an SCR

There are three modes of opearation for an SCR depending upon the biasing given to it:

a) Forward blocking mode(off state)
b) Forward conduction mode(on state)
c) Reverse blocking mode(off state)

a) Forward blocking mode: In this mode of operation anode is given an positive potential while cathode is given negative voltage keeping gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from anode to cathode till applied voltage reaches it breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it stars conducting but below breakover voltage it offers very high resistance to the flow of current through to it and said to be in off state .
b) Forward conduction mode: SCR can be brought from blocking mode to conduction mode in two way either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it stars conducting no more gate voltage is required to maintain it in on state. Now there are two way to turn it off i.e.
* Reduce the current flowing through it below a minimum value called holding current .
*Apply an negative pulse at gate which will bring it in off state instantaneously.
c)Reverse blocking mode: In this mode anode it supplied with negative and cahtode with positive voltage which reverse bias J1 and J3 and no current passes through it.

Application of SCR

It can be used in many ways dependinding upon its operation in the circuit.
1) As the name suggests it can be used as an rectifier which offers low resistance in direction while high resistance in opposite direction just like an ordinary diode but difference is that for the conduction in one direction it also requires an positive gate pulse. But while using it as an rectifier does'n prove to very useful because the output wave form obtained is distorted.

Working as an rectifier

As per shown in figure:1 the input ac power obtained from the transformer in fed between anode and cathode while gate is supplied with an positve voltage by the application of an diode upon it though the resistor, both togather givev only a small positive voltage to the gate. Now, when positive half half cycle appeares across the transformer it produces positive potential to anode and negative potential at cathode and at the same time small voltage also appear across gate which triggers the SCR and current conducts from anode to cathode while when negative half cycle appears across transformer it make anode negative, cathode positive and gate is given negative pulse which completely revers bias the SCR and no current flows though it. Thus, it produces half wave rectified output. it cannot be used for rectification of samall voltages(2-6V) because in conduction state an voltage appears across it which is around 1-2V so a major drop in output waveform will be seen if used for small voltages.

 2) The second major application is as an switching device, in which when an positive pulse is given at gate the SCR stars to conduct current from anode to cathode. But here it can be used only for DC power source.

V-I characteristics of SCR

It has very different characteristics in forward and verese biases as is seen in figure:2 . in forward bias it has

•   Forward blocking region: In which SCR offers very high resistance.
•   breakover voltage: At this voltage it changes its state form off state to on state.
•   Holding current: This is the minimum current required to maintain it in on state.
•   Latching region: In this region it remains in conduction mode and a voltage of 1-2V appears across it.

In reverse biasing its characteristics are are just like an diode in reverse biased state.

Note One very important parameter affecting working of an SCR is its dV/dt value that rate of chance of anode to cathode voltage with respect to time. Very high value of dV/dt causes the SCR trigerred before its breakover voltage and can result in undesired triggering of the device there is always a maximum possible value of dV/dt for avoiding any undesired triggering of SCR and it can be different for defferent SCRs.