Thyristor

A thyristor is an electronic semiconductor component that is often used in electronics to control and regulate high currents and voltages. It can be described as an electronic switch that conducts or blocks under certain conditions. In power electronics, it is particularly suitable for applications in which high electrical power must be safely switched.

A thyristor consists of four alternately doped layers of silicon (p-n-p-n structure). Each layer performs a specific function in the current flow behavior. The structure is similar in parts to that of a transistor, and the thyristor can be represented in an equivalent circuit as a combination of a PNP and an NPN transistor.

1. structure and function

The four semiconductor layers form three pn junctions and thus generate complex control behavior. The component has three connections:

• Anode (A)
• Cathode (K)
• Gate (G)

The gate, which can be compared in its function to the base of a transistor, is used to ignite the thyristor. The thyristor remains in a blocked state as long as no control current flows through the gate. Only when a short pulse is applied to the gate does the component ignite – the current now flows through the thyristor from the anode to the cathode. This state remains, even when the gate pulse ends, as long as a minimum current, the so-called holding current, flows through the component.

The thyristor therefore exhibits bistable behavior: It is either fully conducting or fully blocking. It is also sometimes referred to as a silicon controlled rectifier (SCR), which means “silicon-controlled rectifier”. This term illustrates the close relationship to the diode, with the difference that the thyristor can be controlled by a gate pulse.

2. comparison with diodes and transistors

Compared to the diode, which always conducts when it is operated in the forward direction, the thyristor is a controllable component. Conduction only begins after a specific pulse at the gate. In contrast to the transistor, which operates continuously through a base current control, the thyristor maintains its conductive state independently – until the current falls below the holding current or is interrupted by external measures.

The internal feedback between the layers ensures this self-retaining behavior, which can be illustrated by the transistors (PNP and NPN) shown in the equivalent circuit.

3. other thyristor types: Triac and more

In addition to the classic thyristor, there are other types that are more suitable depending on the application:

• Triac: A component that functions like two thyristors coupled in opposition and is referred to as a triac. It is often used in alternating current applications, for example in light dimmers or motor controls.
• Diac: Serves as an ignition element for the triac.
• GTO (Gate Turn-Off Thyristor): Can also be switched off again via the gate.
• IGCT (Integrated Gate-Commutated Thyristor): High-performance thyristor for energy transmission.

4. applications in power electronics

Thyristors are used in many areas where high voltages and currents have to be switched:

• Phase angle controls
• Motor controls
• Power converters and rectifiers
• Heating controls
• Dimmers and controls for household appliances
• Welding equipment

The ability to switch high power with low control effort makes them particularly suitable for industrial applications and energy supply.

5. advantages and disadvantages

Advantages:

• High performance with low power loss
• Robust and durable
• Simple activation with a short pulse
• High voltage and current resistance

Disadvantages:

• No direct switch-off via the gate possible (except for special types)
• Prone to unintentional ignition with steep voltage rises
• Less flexible than modern transistors

Conclusion

The thyristor is a central component of modern power electronics. It combines the properties of diodes and transistors and enables reliable switching of large currents thanks to its special structure of four semiconductor layers. Special types such as the triac or GTO extend its range of applications. Despite new technologies, the thyristor remains a proven and versatile component in many areas.