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How Does a Thyristor Work?


“SCR,” which stands for “Silicon Controlled Rectifier” is a high-power semiconductor device with a four-layer structure consisting of three PN junctions. It is typically formed by connecting two thyristors in reverse parallel. Its functionality extends beyond rectification; it can also be used as a non-contact switch to rapidly turn circuits on or off, perform DC to AC inversion, convert one frequency of AC to another, and more. Like other semiconductor devices, SCR offers advantages such as small size, high efficiency, good stability, and reliable operation. Its introduction allowed semiconductor technology to transition from low-power to high-power applications, making it a widely adopted component in industrial, agricultural, transportation, military research, as well as commercial and consumer electronics sectors.


Working Process

During the operation of a thyristor (SCR), its anode (A) and cathode (K) are connected to the power source and load, forming the main circuit of the thyristor. The gate (G) of the thyristor is connected to the control device, creating the control circuit of the thyristor.

The structure and symbol of a bidirectional thyristor are different. It belongs to an NPNP four-layer device with three terminals labeled T1, T2, and G. Since this device can conduct in both directions, the two terminals other than the gate (G) are collectively referred to as the main terminals and are denoted as T1 and T2 without distinguishing between anode or cathode. Its characteristic is that when the voltage between the gate (G) and T2 with respect to T1 is positive, T2 acts as the anode, and T1 as the cathode. Conversely, when the voltage between the gate (G) and T2 with respect to T1 is negative, T1 becomes the anode, and T2 becomes the cathode. The bidirectional thyristor’s voltage-current (V-I) characteristics are symmetrical in both forward and reverse directions, allowing it to conduct in either direction.

Labels for the anode, cathode, gate, and p-n-p-n layers
Picture including the anode, cathode, gate, and layers of p-n-p-n semiconducting material.

Analyzing the working process of a thyristor from its internal structure: A thyristor is a four-layer, three-terminal device with three PN junctions (J1, J2, J3). It can be split into two parts, dividing the NP section in the middle, forming a PNP-type transistor and an NPN-type transistor combined in one device. To make the thyristor conduct when subjected to a positive forward anode voltage, it is necessary to neutralize the blocking effect of the reverse-biased PN junction J2. The collector current of each transistor is also the base current of the other transistor. Therefore, in this composite transistor circuit, when there is sufficient gate current (IGT) flowing in, it creates strong positive feedback, causing both transistors to saturate and conduct, resulting in thyristor conduction.

In the operation of a thyristor, with the collector currents of the PNP transistor and NPN transistor denoted as Ic1 and Ic2, respectively, and their emitter currents as Ia and Ik, along with current gain factors a1 = Ic1/Ia and a2 = Ic2/Ik, and a reverse leakage current Ic0 flowing through junction J2, the anode current of the thyristor can be expressed as the sum of the collector currents and leakage current:

Ia = Ic1 + Ic2 + Ic0 or Ia = a1Ia + a2Ik + Ic0

If the gate current is Ig, then the cathode current of the thyristor is given by Ik = Ia + Ig. Thus, the anode current of the thyristor can be determined as follows:

I = (Ic0 + Iga2) / (1 – (a1 + a2)) (1—1)

The current gain factors a1 and a2 for silicon PNP and NPN transistors change significantly with variations in their emitter currents.

When the thyristor is subjected to a positive forward anode voltage with no gate voltage (Ig = 0), the term (a1 + a2) is small, so the anode current Ia of the thyristor is approximately equal to Ic0. In this condition, the thyristor is in the forward blocking state. However, when the thyristor is under a positive forward anode voltage and a gate current Ig flows into the gate, the increased gate current Ig enhances the current gain factor a2 due to its passage through the NPN transistor’s emitter junction. This results in a significant increase in the collector current Ic2 flowing through the emitter junction of the PNP transistor and an increase in the current gain factor a1 for the PNP transistor. This leads to a higher collector current Ic1 flowing through the emitter junction of the NPN transistor. This strong positive feedback process happens rapidly.

When a1 and a2 increase with the emitter current and (a1 + a2) becomes approximately equal to 1, the denominator (1 – (a1 + a2)) in equation (1—1) becomes close to 0. Consequently, the anode current Ia of the thyristor increases significantly. At this point, the current flowing through the thyristor is determined solely by the voltage in the main circuit and circuit resistance. The thyristor is now in the forward conduction state.

In equation (1—1), once the thyristor is in conduction, even if the gate current Ig becomes zero, the thyristor can maintain its original anode current Ia and continue to conduct. After conduction is established, the gate no longer has an effect.

If, after conduction, the power supply voltage is gradually reduced or the circuit resistance is increased, causing the anode current Ia to decrease to a holding current IH, a1 and a2 decrease rapidly. When (1 – (a1 + a2)) becomes approximately equal to 0, the thyristor returns to the blocking state.

The description diagram of the thyristor principle
The description diagram of the thyristor principle

Silicon Controlled Rectifier (SCR) Operating Process

A thyristor, also known as a silicon-controlled rectifier (SCR), has three terminals – anode (A), cathode (K), and gate (G). Its core is a four-layer structure composed of P-type and N-type semiconductors, with a total of three PN junctions. This structural configuration is fundamentally different from that of a silicon rectifier diode, which has only one PN junction. The four-layer structure of the thyristor, coupled with the introduction of the control gate, forms the basis for its excellent “small control over large” control characteristics

Thyristor Principle
Thyristor Principle

When it comes to the application of thyristors, also known as silicon-controlled rectifiers (SCRs), it’s possible to control a substantial anode current or voltage with just a small current or voltage applied to the control gate. Currently, LJ-MD can manufacture thyristor devices with current capacities ranging from several hundred amperes to over a thousand amperes. Typically, thyristors with a current rating below 5 amperes are referred to as low-power thyristors, while those rated at 50 amperes or higher are considered high-power thyristors.

Applications of Thyristors

Thyristors have the ability to precisely control current flow and are used in a wide range of applications across various industries. Here we explore some of the various applications where these electronic components play a central role:


The most fundamental use of a thyristor, or silicon-controlled rectifier (SCR), is controllable rectification. Diode rectifier circuits are considered uncontrollable rectifier circuits. By replacing the diode with a thyristor, a controllable rectifier circuit can be created. Let’s consider the simplest example of a single-phase half-wave controllable rectifier circuit. During the positive half-cycle of the sinusoidal AC voltage U2, if there is no input trigger pulse Ug applied to the control gate of the thyristor VS, the thyristor will not conduct. It only conducts when U2 is in the positive half-cycle and a trigger pulse Ug is applied to the control gate. Examining the waveform, it is evident that voltage UL is only output on the load RL when the trigger pulse Ug arrives. If Ug arrives earlier, the thyristor conducts earlier, and if Ug arrives later, the thyristor conducts later. By varying the timing of the trigger pulse Ug on the control gate, the average voltage UL (the area of the shaded region) on the load can be adjusted. In electrical engineering, half of an AC cycle is often defined as 180°, known as the electrical angle. Thus, in each positive half-cycle of U2, the electrical angle that the thyristor experiences from zero to the moment when the trigger pulse arrives is called the firing angle α; the conduction angle θ is the electrical angle during which the thyristor conducts in each positive half-cycle. It is clear that α and θ are used to represent the range of conduction or blocking of the thyristor over half a cycle of positive voltage. By changing the control angle α or the conduction angle θ, the average DC voltage UL on the load is altered, achieving controllable rectification.

Non-Contact Switching

A key application of the thyristor is serving as a non-contact switch. In automation equipment, replacing conventional relays with non-contact switches using thyristors has gained gradual adoption. These switches are characterized by their noiseless operation and long lifespan.

Switching and Voltage Regulation

Thyristors are also used for switching and voltage regulation in AC circuits. Due to the variation in their triggering times, thyristors control only a portion of the AC cycle with their current and only a portion of the full voltage with their voltage. This characteristic allows them to regulate the output voltage effectively.

Product Characteristics

1. Bi-Directional Conduction

In a bidirectional thyristor, regardless of the polarity of the applied voltage between the first anode (A1) and the second anode (A2), as long as a triggering voltage of opposite polarity is applied between the control gate (G) and the first anode (A1), the thyristor can be triggered into a low-resistance conducting state. During conduction, the voltage drop between A1 and A2 is typically around 1V.

2. Latch-On Behavior

Once a bidirectional thyristor is triggered into conduction, it can maintain this conduction state even if the triggering voltage is removed. It continues conducting until the current between the first anode (A1) and the second anode (A2) decreases to a level below the holding current, or if the voltage polarity between A1 and A2 changes without the presence of a triggering voltage. In such cases, the bidirectional thyristor will turn off and can only be turned on again by reapplying a triggering voltage.


Classification of Thyristors

Thyristors can be categorized into two main types:

1. Unidirectional Thyristors

Unidirectional thyristors are also known as one-way thyristors. They allow current flow in only one direction, from the anode to the cathode. These thyristors are commonly used in rectification applications.

2. Bidirectional Thyristors

Bidirectional thyristors, also known as triacs, are a type of three-terminal bidirectional thyristor. In their structure, two unidirectional thyristors are connected in reverse. This configuration allows bidirectional conduction, meaning current can flow in both directions between the main terminals. The on-off state of a bidirectional thyristor is determined by the control gate (G). Applying a positive (or negative) pulse to the control gate can trigger conduction in the forward (or reverse) direction. The advantages of these devices include a simple control circuit and no reverse voltage handling issues, making them particularly suitable for use as AC non-contact switches.

Main Parameters of Thyristors

It represents the average value of the current that can continuously flow through the device when it is conducting at the rated forward average current under standard cooling conditions (usually at +40°C) with a power supply frequency that is typically the power grid frequency (e.g., 50Hz or 60Hz).

This is the maximum reverse voltage that the thyristor can withstand in its off-state without turning on unintentionally. Exceeding the VDRM rating can lead to unintended conduction and damage to the device. It’s an important parameter to consider when selecting and using thyristors in electronic circuits.

When a diode or thyristor turns off, there is a short-duration period during which a small reverse current may flow before the device fully blocks reverse current. IRRM measures the peak value of this reverse current during this transition period.

It is the minimum forward current required to maintain the thyristor in the conducting state when the control gate is turned off.

It is the minimum direct current through the control gate required to turn the thyristor fully on when a DC voltage of 6V is applied between the anode and cathode.

This refers to the minimum DC voltage applied to the control gate that causes the thyristor to transition from the blocking state to the conducting state.


Ready to Transform Your Power Control?

For all your thyristor inquiries and purchases, look no further than LJ-MD. Visit our website, diodethyristor.com, to discover our wide range of high-quality stud type thyristors and related products. Our experienced team is ready to assist you in selecting the ideal components to meet your manufacturing requirements. Contact us today to embark on a journey toward improved efficiency, precision, and success in your industry.

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