Static V- I Characteristics
A thyristor works in three basic modes
a.Reverse blocking mode
b.Forwards blocking mode and
c.Forward conducting mode
In reverse blocking mode cathode is made positive with respect to anode and gate circuit remains open as shown in figure in this mode junctions J1 and J3 are seen to be reverse biased and junction J2 is forward biased. A very small current flows.
Figure(a) reverse blocking mode (b)forwards blocking mode (c)forward conducting mode
An avalanche breakdown occurs at junction J1 and J3. When reverse voltage is increased and reverse current increase rapidly as shown in figur.
In forward blocking mode anode is positive with respect to cathode and gate is opened. Since the junctions J1 and J3 are forward biased whereas junction J2 is reverse biased. Hence a small leakage current flow called forward leakage current. When forward voltage is increased junction J2 will have avalanche breakdown at a voltage named forward break over voltage vBo.
In forward conduction mode anode is made positive with respect to cathode and gate is also made positive with respect to cathode. As positive gate voltage is applied between gate and cathode at a particular forward anode voltage junctions J2 comes in forwards bias from reverse bias condition and a forwards anode current flow in the circuit with very small voltage drop across the thyristor.