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 thyristor. Meanwhile, the gate (G) and cathode (K) are linked to the control device, which is responsible for regulating the thyristor’s behavior. This control circuit plays a crucial role in initiating and managing the conduction process.
Internally, the thyristor is a four-layer, three-terminal semiconductor device with three PN junctions: J1, J2, and J3. The middle layers form a composite structure that resembles a PNP transistor and an NPN transistor connected in a way that allows for mutual feedback. This configuration enables the thyristor to switch between conducting and non-conducting states based on the applied voltages and currents.
When a forward voltage is applied to the anode, the thyristor will not conduct unless the reverse-biased junction J2 is prevented from blocking. This is achieved by injecting a gate current (Ig), which triggers a positive feedback mechanism between the two transistors. As the gate current increases, it enhances the base current of one transistor, which in turn increases the collector current of the other, creating a chain reaction that leads to saturation.
Let’s define the collector currents of the PNP and NPN transistors as Ic1 and Ic2, respectively. The emitter currents would be the anode current (Ia) and the cathode current (Ik). The current gain factors are given by a1 = Ic1/Ia and a2 = Ic2/Ik. Additionally, there is a leakage current Ic0 through the J2 junction.
The total anode current can be expressed as:
Ia = Ic1 + Ic2 + Ic0 = a1 * Ia + a2 * Ik + Ic0
If the gate current is Ig, then the cathode current becomes Ik = Ia + Ig. Substituting this into the equation gives:
Ia = (Ic0 + Ig * a2) / (1 - (a1 + a2))
This formula highlights how the gate current influences the anode current. When no gate current is applied (Ig = 0), the denominator (1 - (a1 + a2)) remains large, keeping the anode current low and the thyristor in a blocking state. However, when a sufficient gate current is applied, the values of a1 and a2 increase, causing the denominator to approach zero, which dramatically increases the anode current.
Once the thyristor is turned on, the gate loses its influence. The anode current is then determined solely by the main circuit’s voltage and resistance. The thyristor remains in the conducting state until the anode current drops below the holding current (IH), either due to a reduction in supply voltage or an increase in loop resistance. At this point, the current gains (a1 and a2) decrease, and the thyristor returns to the blocking state.
Understanding these dynamics is essential for effectively using thyristors in power control applications, such as AC dimming, motor speed control, and power supplies.
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