The relation between the collector current and emitter current is shown below in this IC = β IB
Where
- IC = collector current
- IE = emitter current
- ICBO = comman base leakage current of collector-base junction of a transistor
- α = common base current gain
- IB = base current
- β = common emitter current gain.
- Ia = Anode Current
In the off-state of a transistor, collector current IC is related to emitter current IE as
IC = αIE + ICBO
For Transistor Q1, IC = IC1 , α = α1,ICBO = ICBO1 and IE = Ia
IC1 = α1Ia + ICBO1 ---------Eq(1)
Similarly Transistor Q2, IC = IC2 , α = α2, ICBO = ICBO2 and IE = Ik
IC2 =α2Ik + ICBO2 ---------Eq(2)
The sum of two collector currents given by Eqn1 and Eqn2 is equal to external circuit current Ia entering at anode terminal A.
Ia = IC1 + IC2
Ia = α1Ia + ICBO1 +α2Ik + ICBO2 -----Eq(3)
When gate current is applied,then Ik = Ia + Ig . Substituting this value of Ik in eqn3
Ia = α1Ia + ICBO1 + α2(Ia + Ig) + ICBO2
Ia = [α2Ig + ICBO1 + ICBO2]/[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 Ig = 0 and with thyristor forward biased, (α1 + α2) is very low as per Eq(4) and forward leakage current some what more than (lCBO1+ICBO2) 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),Ia 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:
- forward blocking mode
- forward conduction mode
- 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.