Industrial Electronics

Fundamentals of thyristors

17 June 2024
Source: vlabo/Adobe Stock

Thyristors play a crucial role in power electronics due to their ability to handle large currents, switch efficiently, offer solid-state reliability and be cost-effective. Their unique properties make them irreplaceable in many high-power applications.

What is a thyristor?

A thyristor is a solid-state semiconductor device with four layers of alternating P-type and N-type materials used for high-power applications. It functions like a bistable switch, meaning it can be either on or off. The following are its two main designs, differing in how they are triggered to turn on:

  1. Silicon-controlled rectifier (SCR):
    • This is the most widely used thyristor type.
    • It has four layers of alternating P- and N-type semiconductors, resulting in a three-terminal structure: anode, cathode and gate.
    • A small current pulse applied to the gate electrode triggers the SCR to turn on, allowing a much larger current to flow between the anode and cathode. Once turned on, the SCR remains conducting even if the gate signal is removed. It can only be turned off by interrupting the current flow through the device, typically by reducing the anode current to zero.
  2. Gate turn-off thyristor (GTO):
    • GTOs offer more control compared to SCRs.
    • Similar to SCRs, they have a four-layer structure and three terminals (anode, cathode and gate).
    • The key difference lies in the gate's ability to turn the GTO on and off. A positive gate current turns it on, while a negative gate current with sufficient magnitude forces it back to the off-state, even when a large current is flowing through the device. This allows for more precise control over the switching process.

Advantages of thyristors

They are essential in various applications due to these capabilities:

  • High power handling: Unlike transistors, thyristors can handle very large currents, sometimes reaching thousands of amps, making them ideal for applications dealing with significant electrical power.
  • Switching speed and efficiency: Thyristors can switch on and off very quickly, efficiently managing power flow. This is crucial for applications requiring rapid control of high currents.
  • Solid-state reliability: Compared to mechanical relays, thyristors have no moving parts, leading to increased reliability and reduced maintenance needs.
  • Compact design: For their power handling capabilities, thyristors are relatively compact devices, making them suitable for space-constrained circuits.
  • Cost-effective: Thyristors are generally less expensive than other options for high-power switching applications.

Applications of thyristors

Here are some specific examples of how these advantages come into play:

  • Power grid management: Thyristors are used in high-voltage direct current (HVDC) transmission systems to control the flow of electricity efficiently over long distances. They act as switches within the converters, enabling the conversion between AC and DC efficiently. This minimizes power losses during long-distance transmission, making HVDC a viable option for transporting electricity over vast distances. Furthermore, the gate control of thyristors allows for precise regulation of the power flow within the HVDC system. This is crucial for maintaining grid stability and ensuring balanced power delivery across different regions.
  • Motor speed control: In adjustable-speed drives for electric motors, thyristors regulate the power delivered to the motor, controlling its speed. They act as electronic switches within a circuit feeding the DC motor. These circuits are often configured as either half-wave or full-wave converters, depending on the desired level of control.
  • Light dimming: Light dimmers often use thyristors to control the average power supplied to lamps, resulting in adjustable brightness. In a light dimmer circuit, a thyristor acts as a variable resistor, electronically controlling the current delivered to the bulb. It doesn't simply turn the bulb on and off rapidly, but rather chops off a portion of the AC sine wave that supplies the bulb. Most commonly, a specific type of thyristor called a triac is used in light dimmers. Unlike SCRs which only control current in one direction, triacs can handle current flow in both positive and negative halves of the AC cycle. This makes them ideal for regulating AC power to light bulbs.
  • Electric trains: Electric trains receive DC power from an overhead supply line. However, traction motors, the workhorses that drive the train wheels, typically operate on AC power. This creates a need to convert the DC input to AC for efficient motor operation. Thyristor-based converters are used in electric trains to convert DC power from the overhead lines to AC power for the traction motors. The converter utilizes multiple thyristors acting as electronic switches. By controlling the on and off states of these switches very rapidly, they can manipulate the DC input to create a desired AC output waveform.

[Learn more about thyristors on GlobalSpec]

Conclusion

Thyristors are a valuable technology in various power control applications, particularly when robustness, cost-effectiveness and high-power handling are priorities. While offering good on/off control and basic dimming or speed regulation, thyristor control may not be as precise or sophisticated as some newer technologies. Therefore, for applications demanding more precise control, lower harmonic distortion or a smaller footprint, newer technologies like insulated gate bipolar transistors might be a better choice.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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