The v-i characteristics of a thyristor or static cathode characteristics is a curve plotted between the anode current and anode-cathode voltage for various values of gate currents. It explains the working of thyristor for different modes of operation. There are three modes of operation of thyristor which are mentioned below.
- Reverse Blocking Mode
- Forward Blocking Mode (OFF Mode)
- Forward Conduction Mode (ON Mode)
Reverse Blocking Mode
The reverse blocking mode of thyristor refers to the condition in which thyristor is blocking current flow in reverse direction i.e it block current to pass from the anode to the cathode.
In reverse blocking mode of thyristor, voltage is applied across the anode and cathode terminals such that cathode is connected to positive terminal and anode is connected to negative terminal. This arrangement is similar to two diodes connected in series with reverse voltage applied across them. This reverse bias voltage prevents the transistor from turning on or conducting current in forward bias.
During reverse blocking mode, the junction J1 and J3 are reverse biased, J2 is forward biased and thyristor is in high-impedance state i.e. thyristor functions as open circuit. If the reverse voltage exceeds a certain critical value known as reverse breakdown voltage (VBR) or reverse avalanche voltage, the thyristor may breakdown at the junctions J1 and J3 and reverse current starts to flow through it which can damage the thyristor if not properly protected.
To turn-off the thyristor in reverse blocking mode, a negative voltage pulse is applied to the gate terminal. This pulse should have enough amplitude and duration to trigger the thyristor into the forward blocking state.
Forward Blocking Mode (OFF Mode)
The term forward blocking mode typically refers to the behavior of the thyristor when it is in non-conducting state with the forward voltage across it terminals. In this mode, thyristor do not conduct current even when the forward voltage is applied across the terminals.
During forward blocking mode, the junctions j1 and j3 are forward biased, j2 is reverse biased and thyristor is in high-impedance state i.e. thyristor functions as open circuit. A small current called forward leakage current flows initially in this mode. Also, If the forward voltage exceeds a certain critical value known as forward breakdown voltage (VBF) or forward avalanche voltage or forward break-over voltage (VBO), thyristor may breakdown at the junction j2 and forward current starts to flow through it.
Forward Conduction Mode (ON Mode)
When a voltage greater than forward breakdown voltage (VBF) is applied across the anode and cathode of thyristor or a positive pulse is applied across the gate and cathode terminals, the thyristor is triggered and entered into the conduction mode i.e. it behaves as a closed circuit.
During forward conduction mode, basically enough forward anode to cathode voltage is given along with the positive gate voltage. The junction J1 and J3 are forward biased with forward anode to cathode voltage and junction J2 is forward biased with positive gate voltage. Hence, thyristor is triggered into forward conduction mode.
Once triggered, the anode cathode voltage must be enough to maintain the thyristor in conduction mode. The thyristor will continuously conduct until the anode to cathode voltage is above the certain threshold value called forward voltage drop (VF).
As thyristor enters conduction mode, it become low resistance path for current to flow from anode to cathode. The thyristor forward conduction mode is characterized by a low voltage drop across the device making it an efficient choice for application requiring high current rectification and voltage control.
Conclusion
Hope, you have clearly understand about the v-i characteristics of a thyristor in different modes of the operations. Make sure to leave your opinion and suggestion about this post in the comment section below.