Depth analysis of the difference between thyristor and thyristor

Thyristor

A Thyristor, also known as a Silicon Controlled Rectifier (SCR), is a high-power semiconductor device widely used in electrical and electronic systems. It offers advantages such as compact size, high efficiency, and long operational life. In automatic control systems, thyristors serve as high-power drivers, allowing low-power signals to control high-power devices. They are extensively used in AC and DC motor speed control systems, power regulation circuits, and servo mechanisms.

Thyristors can be categorized into two main types: unidirectional thyristors and bidirectional thyristors. The latter is commonly referred to as a triac or TRIAC. A triac is essentially formed by connecting two unidirectional thyristors in reverse, enabling it to conduct current in both directions. Its on/off state is controlled via the gate terminal. Applying a positive or negative pulse to the gate allows it to switch between forward and reverse conduction. This makes triacs ideal for AC non-contact switching applications due to their simple control circuitry and absence of reverse voltage blocking capability.

Working Principle

Original Structure

A standard thyristor consists of a four-layer (P1N1P2N2) structure with three terminals: anode, cathode, and gate. When analyzing its operation, it can be viewed as a combination of a PNP transistor and an NPN transistor. The equivalent circuit diagram illustrates this internal structure. A triac, on the other hand, is a bidirectional thyristor that can control alternating current without physical contacts. It allows small currents to control large ones, offering benefits like no sparking, fast response, long life, and high reliability. From the outside, a triac resembles a regular thyristor but has three terminals, with one called the gate (G), and the other two referred to as main terminals T1 and T2.

The symbol of a triac differs from that of a standard thyristor, as it is represented by two thyristors connected in reverse. In China, triacs are often labeled as "3CTS" or "KS," while foreign manufacturers use "TRIAC." The pin arrangement may vary among different manufacturers, but most follow the order of T1, T2, and G from left to right. The orientation usually has the marked side facing up.

Thyristor Overview

The term "thyristor" is a general name for a family of semiconductor devices, including SCRs and triacs. First developed by General Electric in 1957 and commercialized in 1958, thyristors have since become essential in power electronics. They consist of a PNPN four-layer structure with three electrodes: anode, cathode, and gate. These devices are capable of operating under high voltage and current conditions, and their conduction can be controlled, making them suitable for applications such as controllable rectification, AC voltage regulation, non-contact switches, inverters, and more.

Working Conditions

During operation, the anode (A) and cathode (K) of a thyristor are connected to the power supply and load, forming the main circuit. The gate (G) and cathode (K) form the control circuit. Thyristors are semi-controlled devices, meaning they can be turned on by a gate signal but cannot be turned off directly—only when the current drops below a certain threshold. The key working conditions are:

1. When a reverse anode voltage is applied, the thyristor remains in the reverse-blocking state regardless of the gate voltage.
2. If a forward anode voltage is applied along with a forward gate voltage, the thyristor turns on and conducts current.
3. Once turned on, the gate loses control, and the thyristor continues to conduct as long as there is sufficient anode voltage.
4. To turn off the thyristor, the main circuit current must drop near zero, or a reverse voltage must be applied.

Difference Between Thyristor and Triac

While both thyristors and triacs are semiconductor switching devices, they differ in functionality and application. Thyristors, or SCRs, are unidirectional, conducting only in one direction once triggered. Triacs, however, are bidirectional and can conduct in both directions, making them ideal for AC control. The symbols and pin configurations also differ, with triacs typically having T1, T2, and G terminals.

Unidirectional thyristors have three PN junctions, with the anode and cathode at the outer layers and the gate at the middle. They remain off unless both the anode and gate are forward-biased. Once turned on, they stay on until the current drops below a threshold. Triacs, on the other hand, respond to gate pulses of varying magnitude or timing, allowing precise control over AC loads. Their ability to switch based on the polarity of the gate signal makes them suitable for AC applications where full-wave control is needed.

Thyristors come in various types, including micro-trigger, high-sensitivity, and standard models, and are available in different package styles. They are widely used in industrial and consumer electronics for applications such as speed control, dimming, voltage regulation, and non-contact switching. With continuous advancements in technology, thyristor products have become more efficient, reliable, and versatile, finding applications in everything from household appliances to industrial automation systems.