Basic operating principle of a thyristor
Basic Operating Principle of a Thyristor:
Fig: Two transistor analogy of a thyristor construction. (a) Schematic Construction, (b) Schematic division in component transistor (c) Equivalent circuit in terms of two transistors.
Thyristors have three states:
- Reverse blocking mode— Voltage is applied in the direction that would be blocked by a diode
- Forward blocking mode— Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
- Forward conducting mode— The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"
Let us consider the behavior of this p n p n device with forward voltage applied, i.e anode positive with respect to the cathode and the gate terminal open. With this voltage polarity J1 & J3 are forward biased while J2 reverse biased.Under this condition
Where ∝1 & ∝2 are current gains of Q1 & Q2 respectively while Ico1 & Ico2 are reverse saturation currents of the CB junctions of Q1 & Q2 respectively.
- We have after deduction,
Where is the total reverse leakage current of J2
- As long as VAK is small Ico is very low and both ∝1 & ∝2 are much lower than unity. Therefore, total anode current IA is only slightly greater than Ico. However, as VAK is increased up to the avalanche break down voltage of J2, Ico starts increasing rapidly due to avalanche multiplication process. As Ico increases both ∝1 & ∝2 increase and ∝1 ∝2 approaches unity. Large anode current starts flowing, restricted only by the external load resistance.
- Voltage drop in the external resistance causes a collapse of voltage across the thyristor. The CB junctions of both Q1 & Q2 become forward biased and the total voltage drop across the device settles down to approximately equivalent to a diode drop. The thyristor is said to be in “ON” state.
- When a reverse voltage is applied across a thyristor (i.e, cathode positive with respect to anose.) junction J1 and J3 are reverse biased while J2 is forward biased. Of these, the junction J3 has a very low reverse break down voltage since both the n and p regions on either side of this junction are heavily doped.
Therefore, the applied reverse voltage is almost entirely supported by junction J1. The maximum value of the reverse voltage is restricted by
The maximum field strength at junction J1 (avalanche break down)
Punch through of the lightly doped n- layer.