Electrical Question: Explain the V-I Characteristics Of an SCR or Thyristor?

Answer:The Static V-I Characteristics of an SCR is shown in the figure. Thyristor V-I Characteristics are divided into three regions of operation. These Three regions of operations are:

• Reverse Blocking Region
• Forward Blocking Region
• Forward Conduction Region

 Reverse Blocking Region: When the Cathode of the SCR is made positive with respect to the anode, then outer junctions J1 and J3 are reverse biased, whereas junction J2 is forward biased. Therefore a small leakage current flows through the SCR. This can be observed from the figure that, in the reverse blocking region small current magnitude is observed and it almost remain constant with the rise in the voltage.This Scenario is observed until certain voltage which is called avalanche breakdown voltage.At this particular voltage the depletion regions of the junctions J1 and J3 are broken down and the thyristor conducts in the reverse bias direction. This can be observed from the figure at certain voltage Vbr current flowing through the thyristor increases suddenly to high value.

Forward Blocking Region: In this region anode is made positive with respect to the cathode and therefore junction J1 and J3 are forward biased and junction J2 is reversed biased. Hence the thyristor is in forward blocking condition and a small leakage current flows during this condition due to drift of charge carriers. In this condition device does not conduct.

Forward Conduction Region: When the thyristor is in forward blocking condition and anode to cathode voltage is increased further and gate circuit is in open condition, then at particular voltage called as forward break over voltage (Vbo) avalanche breakdown occurs and and thyristor starts conducting in forward mode. This conduction process can be seen in the figure through the voltage starts abruptly falling from some hundreds of volts to 1 to 2 volts and suddenly very large current flowing through the SCR. This process of conducting the thyristor damages the device. So in order to conduct the thyristor safely, gate current Ig is provided which results in the reduction in the forward break over voltage less then (Vbo). With increase in the gate current (Ig) the conduction voltage reduces. This can be observed in the figure.