V-I characteristics of thyristor
The V-I characteristics of thyristor 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 thyristor. 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
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 thyristor 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.
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 thyristor 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 thyristor does not conduct any current expect a very small value of the leakage current. This mode of thyristor is called as forward blocking mode. Just as the region 1, i. e., reverse blocking mode, the thyristor 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 thyristor. Hence, the thyristor 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 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 thyristor is said to be in a forward conduction mode and hence acts as a closed switch. This method of conducting the thyristor 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 thyristor conduction. However, if the anode current decreases to a value called ad holding current, the thyristor 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.
What is Thyristors in Power Electronics ?
Thyristors in Power Electronics are used as power semiconductor devices which are used as on/off switches in power control circuits. A power semiconductor device is a semiconductor device used as a switch or rectifier in power electronics for example in a switch-mode power supply. A thyristor is the most important type of power semiconductor device. They are extensively used in power electronic circuits. They are operated as bi-stable switches from non-conducting to conducting state. Thyristors are high speed switches that can be used to replace electromechanical relays in many circuits as they have no moving parts, no contact arcing, or suffer from corrosion or dirt. But in addition to simply switching large currents “ON” and “OFF”, thyristors can be made to control the mean value of an AC load current without dissipating large amounts of power.
Table of Content
- What is a Thyristor?
- Properties of Thyristors
- Construction of Thyristor
- Working of Thyristor
- V-I characteristics of thyristor
- Types of Thyristors in Power Electronics
- Examples of Thyristors in Power Electronics