V-I characteristics of SCR
The VI characteristics are also classified into Static and Dynamic characteristics of SCR as follows:
Static V-I Characteristics of SCR
The V-I characteristics of SCR is a graph between the anode current IA and the anode-cathode voltage VA for different values of gate current IG. This characteristics can be drawn by considering the basic operation of the SCR. The below figure shows the V-I characteristics which is also called as static-cathode characteristics. It basically consist of three regions, They are
- Region 1
- Region 2
- Region 3
Static V-I Characteristics of SCR
Region 1: When the positive terminal of the supply is connected to cathode and the negative terminal is connected to anode with gate circuit open then SCR operates in region 1. In this region junction J1 and J3 becomes reverse biased, whereas the junction J2 becomes forward biased. The reverse biased junctions (J1 and J3) acts as open circuit and the forward biased junction(J2) acts as a short circuit, as shown in figure.
From figure it is clear that, the SCR does not conduct any current and it is said to be in reverse blocking state or OFF state. However, a very small amount of leakage current flows through it. Now, if we increase the reverse voltage to the value called as reverse breakdown voltage, then an avalanche will occur and breaks the junction J1 and J3. Due to this a very huge amount of the current flows through the circuit and hence the SCR starts conducting. But this method of conducting SCR or the circuit in which it is working. Hence, we must ensure that the reverse voltage does not exceed the reverse breakdown voltage VAK.
Region 2: When the positive terminal of the supply is connected to anode and the negative terminal is connected to cathode with gate circuit open then SCR operates in region 2. In this region junction J1 and J3 becomes forward biased, whereas the junction J2 gets reverse biased. The forward biased junctions (J1 and J3) acts as short circuit and the reverse biased junction(J2) acts as a open circuit, as shown in figure. Even in this region, the SCR does not conduct any current expect a very small value of the leakage current. This mode of SCR is called as forward blocking mode. Just as the region 1, i. e., reverse blocking mode, the SCR can be made to conduct in the forward blocking mode by increasing the anode-cathode voltage to a value called as forward breakdown voltage(VBO). Even this method is not recommended as it may also damage the SCR. Hence, the SCR does not conduct even in this mode and is treated as open switch.
Region 3: When the positive terminal of the supply is connected to anode and the negative terminal to cathode with gate circuit closed the SCR operates in region 3. In this region, all the three junctions (J1, J2 and J3) act as short circuit shown in figure and hence conducts current. In this region SCR is said to be in a forward conduction mode and hence acts as a closed switch. This method of conducting the SCR is the most efficient, as it requires a voltage which is very much less than VBO. The only extra thing we require is a gate signal for a small period of latching current. Once the anode current attains this value, the gate losses the control and hence can be removed. The removal of the gate signal will not have any effect on the SCR conduction. However, if the anode current decreases to a value called ad holding current, the SCR will once again go back to the forward blocking gate. Hence, care must be taken that, the anode current should not drop below the holding current after the gate signal is removed.
Dynamic or Switching Characteristics of SCR
During turn ON and turn OFF process, thyristor is subjected to different voltages across it and different currents through it. The time variation of the voltage across a thyristor and the current through it during turn ON and turn OFF constitute the switching Characteristics of a thyristor.
Turn ON Switching Characteristics
A forward biased thyristor is turned ON by applying a positive gate voltage between the gate and the cathode, as shown in figure(1).
Circuit Diagram of a Turn ON Switching Characteristics of SCR
Turn ON Switching Characteristics of SCR
Figure(2) , shows the waveforms of the gate current(IG), anode current(IA) and anode to cathode voltage(VAK). The total switching period being much smaller compared to the cycle time, IA and VAK before and after switching will appear flat.
As shown in figure , there is a transition time “Toff” from forward OFF state to forward ON state. This transition time is called the thyristor turn ON time and can be divided into three separate intervals namely, They are
- Delay time (Td)
- Rise time (Tr)
- Spread time ( Tp)
Delay Time (Td)
It is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value. It is the time taken by the anode voltage to fall from VAK to 0.9 VAK
Rise Time (Tr)
For a resistive load, “rise time” is the time taken by the anode current to rise from 10% of its final value to 90% of its final value. At the same time, the voltage VAK falls from 90% of its initial value to 10% of its initial value. However, current rise and voltage fall characteristics are strongly influenced by the type of the load. For inductive load the voltage falls faster than the current. While, for a capacitive load, current rises rapidly.
Spread Time ( Tp)
It is the time taken by the anode current to rise from 90% of its final value to 100%. During this time conduction spreads over the entire cross-section of the cathode of the thyristor. The spreading interval depends on the area of the cathode and on the gate structure of the thyristor.
Silicon Controlled Rectifier
The SCR or thyristor is one type of semiconductor device and using in high-power switching applications is exceptionally planned. The working of this device should be possible in a switching mode only and acts as a switch. When the SCR is triggered by its gate terminal into the transmission, then it will supply the current constantly. While planning a SCR or Thyristor circuit, exceptional focus ought to be expected for enacting the circuit. This article examines various techniques for SCR setting off or SCR turn ON strategies or setting off of Thyristors. There are different setting off strategies are accessible in light of different substances which incorporate temperature, voltage, and so on.
Table of Content
- Silicon Controlled Rectifier
- Key Terminologies
- Construction
- Operation
- Modes of Operation
- Types
- Advantages of SCR