English
Menu

What is Thyristor?

A semiconductor switch with four alternating p- and n-type layers (pnpn). It starts conducting when a current is fed into its gate while it is forward biased, and then keeps conducting even after the gate signal is removed, until the current falls below the holding current. It is used for phase control of AC and for high-voltage, high-current power conversion, and gate turn-off (GTO) thyristors can also be turned off from the gate. Latch-up in CMOS is the turn-on of a parasitic thyristor structure formed inside the circuit.

Japanese name: サイリスタ

Thyristor appears in 12 practice questions across 8 topics, including Power Electronics Circuits, Power semiconductors, Reliability.

Field: Power Electronics Circuits

Try it: questions about Thyristor

Which is a characteristic of a thyristor?

  1. AOnce turned on by a gate signal, it stays on until the current falls nearly to zero
  2. BIt can be turned off freely at any time by a gate signal
  3. CIt is a two-terminal device
Show answer and explanation

Answer: A. Once turned on by a gate signal, it stays on until the current falls nearly to zero

A thyristor is a four-layer PNPN switch; a gate pulse triggers positive feedback between its PNP and NPN transistor sections, turning it on and keeping it on without further gate signal (latching). Turning it off requires dropping the current below the holding current, which happens naturally at each AC zero crossing. Handling several kV and kA, thyristors serve in HVDC transmission and high-power rectification; GTOs can also be turned off by the gate.

In phase control with an AC supply and thyristors, which of the following correctly describes how the average output voltage changes when the firing angle (control angle) α is increased?

  1. AThe average output voltage stays constant regardless of the firing angle
  2. BThe larger the firing angle, the higher the average output voltage
  3. CThe larger the firing angle, the lower the average output voltage
Show answer and explanation

Answer: C. The larger the firing angle, the lower the average output voltage

A thyristor starts conducting when a signal is applied to its gate while it is forward biased. Phase control adjusts the part of each half cycle during which current flows by changing the angle α from the zero crossing of the supply voltage to the moment of firing. Increasing α shortens the conduction interval and lowers the average output voltage. The method has long been used in lamp dimmers and in speed control of DC motors.

Thyristor in the explanations

Questions that mention Thyristor (12)

  1. Which is a characteristic of a thyristor?
  2. In phase control with an AC supply and thyristors, which of the following correctly describes how the average output voltage changes when the firing angle (control angle) α is increased?
  3. A 200 V RMS single-phase AC supply feeds a resistive load through a single-phase full-wave rectifier using thyristors, with phase control. Which of the following is closest to the average output voltage at a firing angle of 60°?
  4. In a thyristor phase-control circuit connected to an AC supply, which of the following correctly explains why the thyristor turns off without any action to turn it off from the gate?
  5. What is the main role of power semiconductors?
  6. Why are latch-up tests on automotive ICs required to be performed at the product's maximum operating temperature, which is more severe than room temperature?
  7. Which method irradiates FZ silicon with neutrons to convert some silicon into phosphorus, making it uniformly n-type?
  8. What is an advantage of a wafer with a lightly doped p-type epitaxial layer grown on a p+ substrate (p/p+ epitaxial wafer)?
  9. In CMOS well formation, what is a well called in which high-energy implantation makes the dopant concentration higher deep below the surface?
  10. Which correctly describes latch-up in CMOS circuits?
  11. Which phenomenon degrades transistor characteristics when carriers accelerated by the high electric field near the drain are injected into the gate dielectric?
  12. Which category of semiconductor products, such as diodes, single transistors and power MOSFETs, performs a single function on its own?

Books on Power Electronics Circuits Ad

As an Amazon Associate I earn from qualifying purchases.

Related terms

Latch-up CMOS Power semiconductor device Rectifier circuit Current gain (hFE) AEC-Q100 ESD (electrostatic discharge) Oxygen precipitates (BMD) Czochralski (CZ) method Gettering

Back to the glossary