During the operation of a thyristor T, its anode (A) and cathode (K) are connected to the power source and the load, forming the main circuit of the device. Meanwhile, the gate (G) and cathode (K) are linked to the control circuit that manages the thyristor’s behavior. This control circuit plays a crucial role in initiating and regulating the conduction of the thyristor.
Internally, the thyristor is a four-layer, three-terminal semiconductor device, consisting of three PN junctions labeled J1, J2, and J3. The middle layer can be divided into two parts, effectively creating a composite structure of a PNP transistor and an NPN transistor. This configuration allows for complex interactions between the internal components during operation.
An image illustrating this structure is provided below:
When the thyristor is subjected to a forward voltage at the anode, it will only conduct if the reverse-biased junction J2 does not block the current. The collector current of each transistor acts as the base current for the other, leading to a positive feedback loop when sufficient gate current (Ig) is applied. This creates a rapid saturation of both transistors, enabling the thyristor to switch into a conducting state.
Let’s define the collector currents of the PNP and NPN transistors as Ic1 and Ic2, respectively. Their emitter currents are denoted as Ia (anode current) and Ik (cathode current). The current gain factors are given by a1 = Ic1/Ia and a2 = Ic2/Ik. Additionally, the reverse leakage current across junction J2 is represented as Ic0.
The total anode current of the thyristor can be expressed as:
Ia = Ic1 + Ic2 + Ic0 = a1Ia + a2Ik + Ic0
If the gate current is Ig, then the cathode current is Ik = Ia + Ig.
Substituting this into the equation gives:
Ia = (Ic0 + Ig a2) / (1 - (a1 + a2)) — Equation (1-1)
This equation shows how the anode current depends on the current gains and the gate current. When the gate is not activated (Ig = 0), the sum (a1 + a2) remains small, so the anode current Ia ≈ Ic0, and the thyristor remains in a forward-blocking state.
However, when a gate current Ig is applied, it increases the current gain a2 of the NPN transistor, which in turn increases the collector current Ic2. This causes the PNP transistor's gain a1 to increase as well, resulting in a strong positive feedback loop. As a1 and a2 approach 1, the denominator (1 - (a1 + a2)) approaches zero, causing a sharp rise in the anode current Ia. At this point, the thyristor enters a fully conducting state, and the current is determined by the main circuit’s voltage and resistance.
Once turned on, the thyristor continues to conduct even if the gate current is removed, because the internal feedback mechanism sustains the conduction. However, if the anode current drops below the holding current IH due to reduced voltage or increased resistance, the current gains a1 and a2 decrease rapidly, and the thyristor returns to the blocking state.
This detailed analysis highlights the importance of the gate signal in triggering the thyristor and the role of internal transistor action in maintaining conduction once it has been initiated.
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