This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor. A thyristor can be considered as two complementary transistors. One being pnp and the other npn. The two-transistor model is shown in figure 4.3 below.
Figure 4.3 Two-Transistor Model of Thyristor
The collector current IC of a transistor is related to the emitter current IE and the leakage current of the collector base junction ICBO as
The emitter current of transistor Q1 is the anode current IA of the thyristor and collector current IC1 is given by
where a1 and ICBO1 are the current gain and leakage current respectively for transistor Q1.
Similarly, the collector current for transistor Q2 is IC2 where
where a2 and ICBO2 are the current gain and leakage current respectively for transistor Q2.
Combining the two collector currents IC1 and IC2 yields
When a gate current IG is applied to the thyristor
Solving for anode current IA in equation 5 yields
The current gain a1 varies with emitter current IE1 which is equal to IA; and a2 varies with emitter current IE2 which is equal to Ik.
A typical variation of current gain a with emitter current IE is shown in figure 4.4.
Figure 4.4 Typical Variation of Current Gain With Emitter Current
If the gate current IG is increased from zero to some positive value, this will increase the anode current IA as shown by equation 6. An increase of IA which is an increase of IE1 would increase a1 as shown in figure 4.4 and also a2 since . The increase in values of both a1 and a2 would further increase the value of anode current IA which is a regenerative or positive feedback effect.
If a1 and a2 approach unity, the denominator of equation 6 approaches zero and a large value of anode current is produced causing the thyristor to turn on as a result of the application of a small gate current.
The capacitance of the pn junctions are shown in figure 4.5 below.
Figure 4.5 Two-transistor Transient Model of Thyristor
Under transient conditions, the capacitances of the pn junctions influence the characteristics of the thyristor.
If a thyristor is in the blocking state and a rapidly rising voltage is applied to the device, high currents would flow through the junction capacitors. The current through capacitor Cj2 can be expressed as
Cj2 = capacitance of junction j2
Vj2 = voltage of junction j2
qj2 = charge in junction j2
If the rate of rise of voltage dv/dt is large, then ij2 would be large, which would result in increased leakage currents ICBO1 and ICBO2. High enough values of ICBO1 and ICBO2 may cause a1 and a2 to approach unity, resulting in undesirable turn on of the thyristor.
It must be noted that a large current through the junction capacitors may cause damage to the device.