When Gate is Open
The thyristor in this condition may be considered as three diodes in series, with directions such as to prevent conduction in either direction.
When the device is forward biased i.e. anode positive and cathode negative then junctions J1 & J2 are forward biased and junction J2 is reverse biased. No current flows other than leakage current until the breakover voltage of junction J2 is exceeded. It should be noted that the forward and reverse breakover voltages are similar in magnitude due to in the the reverse blocking state, almost all the voltage appears at the anode PN junction.
Once breakover in the forward direction occurs, the center P slice is neutralized by the electrons from the cathode, and the device acts as a conducting diode having two junctions giving a forward bolt drop approximately double that of diode.
In order for Thyristor to attain the ON-state, the anode current must reach the latching level. To retain the ON-state, the anode current must not fall below the holding current. Latching current is typically double of the holding current but both are low, being much less than 1 percent of the full load rated value.
The reverse characteristics i.e. with anode negative & cathode positive are same as those of an ordinary PN junction diode.
The minimum value of forward anode current which will hold the thyristor in the ON state is called holding current.
In order for the thyristor to attain and retain the ON state, the anode current must reach its latching level. This level of current is called latching current.
It is the minimum anode current required to maintain the thyristor in the ON state immediately after a thyristor has been turned ON.
Note:- Between IH to IL , there is ON to OFF & OFF to ON transition.
Application of the Gate voltage
Thyristor, when forward biased can be switched on by injecting current into the gate terminal relative to the negative cathode, as shown below.
The action of Gate current is to inject holes into inner P-slice which together with the electrons from cathode N layer break over the centrl control junction, switching the thristor into ON state.
If IG is increased from zero, the break over point will be reached sooner but the characteristic curve will have the same shape. If IG is further increased, the breakover voltage will be acheived more earlier. once the anode current has exceeded the latching level, the gate current can cease, the thyristor remaining in the ON state, irrespective of the conditions in the gate circuit.
Effect of IG on Break Over Voltage (VBO)
It is seen from the VI characteristics curve that forward breakover voltage decreases with an increase in gate current. In fact gate current could be increased to a point where breakover voltage would be so low that the device would have characteristics that closely resemble those of an ordinary PN junction diode.