Exploring the Two Transistor Model: Understanding the Inner Workings of a Thyristor

The two transistor model of a thyristor is a simplified representation of the thyristor that can be used for analysis and simulation. The model consists of two transistors, namely, the pnp transistor and the npn transistor. The pnp transistor represents the anode-to-cathode path of the thyristor, while the npn transistor represents the gate-to-cathode path of the thyristor. The base of the pnp transistor is connected to the collector of the npn transistor, and the emitter of the npn transistor is connected to the base of the pnp transistors.

The relation between the collector current and emitter current is shown below in this  I_C = β I_B

Where

•  I_C = collector current
• I_E = emitter current
•  I_CBO = comman base leakage current of collector-base junction of a transistor 
• α = common base current gain
• I_B = base current 
•  β = common emitter current gain.
• I_a = Anode Current 

In the off-state of a transistor, collector current I_C is related to emitter current I_E as

I_C = αI_E + I_CBO

For Transistor Q₁, I_C = I_C1 , α = α₁,I_CBO = I_CBO1 and I_E = I_a

I_C1 = α₁I_a + I_CBO1 ---------Eq(1)

Similarly Transistor Q₂, I_C = I_C2 , α = α₂, I_CBO = I_CBO2 and I_E = I_k

I_C2 =α₂I_k + I_CBO2 ---------Eq(2)

The sum of two collector currents given by Eqn1 and Eqn2 is equal to external circuit current I_a entering at anode terminal A.

I_a = I_C1 + I_C2

I_a = α₁I_a + I_CBO1 +α₂I_k + I_CBO2 -----Eq(3)

When gate current is applied,then I_k = I_a + I_g . Substituting this value of I_k in eqn3

I_a = α₁I_a + I_CBO1 + α₂(I_a + I_g) + I_CBO2

I_a = [α₂I_g + I_CBO1 + I_CBO2]/[1-(α₁+α₂)]----Eq(4)

For a silicon transistor, current gain α is very low at low emitter current With an increase in emitter current, a builds up rapidly as shown in Fig.

With gate current I_g = 0 and with thyristor forward biased, (α₁ + α₂) is very low as per Eq(4) and forward leakage current some what more than (l_CBO1+I_CBO2) flows If, by some means, the emitter current of two component transistors can be increased so that α₁ + α₂ approaches unity, then as per Eq(4),I_a would tend to become infinity thereby turning on the device. Actually, external load limits the anode current to a safe value after the thyristor begins conduction. 

Operation of the Two-Transistor Model

The principle of thyristor operation can be explained with the use of its two-transistor model (or two-transistor analogy).The two-transistor model of a thyristor operates in three modes:

1. forward blocking mode
2. forward conduction mode
3. reverse blocking mode.

1.Forward Blocking Mode

In the forward blocking mode, the anode of the thyristor is connected to a positive voltage, and the cathode is connected to a negative voltage. The voltage across the thyristor is in the forward direction, but the thyristor is not conducting because there is no current flow through the gate. In this mode, both BJTs are in the cut-off state.

2.Forward Conduction Mode

In the forward conduction mode, a positive voltage is applied to the gate of the thyristor, which causes a small current to flow through the gate circuit. This current triggers the thyristor and causes it to turn on. Once the thyristor turns on, it conducts current from the anode to the cathode. In this mode, both BJTs are in the active state.

3.Reverse Blocking Mode

In the reverse blocking mode, the voltage across the thyristor is reversed. The anode is connected to a negative voltage, and the cathode is connected to a positive voltage. In this mode, both BJTs are in the reverse active state.

The operational differences between thyristor-family and transistor family of devices

Conclusion

The Two-Transistor Model of a Thyristor is a useful tool for understanding the behavior of a thyristor. It allows us to analyze the switching behavior of a thyristor and can be used to design and optimize power electronics circuits. The model is a simplified representation of the complex behavior of a thyristor, but it provides a good starting point for understanding the operation of this important device.