Thyristors: Types, Working Principles, Characteristics & Applications

Overview

  • Introduction to Thyristors
  • Thyristor Family and Types
  • Construction and Operation
  • Key Characteristics
  • Applications
  • Protection Mechanisms
  • Conclusion

What is a Thyristor?

  • A semiconductor switching device with three or more PN junctions.
  • Operates as a bistable switch: ON or OFF.
  • Requires no continuous gate bias to maintain conduction.
  • Handles high voltages (>1000 V) and currents (>1000 A).
  • Derived from Greek: Thyra ("door"), symbolizing current flow control.

Thyristor Family

  • Shockley Diode (PNPN Diode)
  • Silicon Controlled Rectifier (SCR)
  • TRIAC (Triode for Alternating Current)
  • DIAC (Diode for Alternating Current)
  • Gate Turn-Off Thyristor (GTO)
  • Light-Activated SCR (LASCR)

Note: SCR and TRIAC are widely used in power control.

Basic Construction

  • Four-layer PNPN structure.
  • Terminals:
    • Shockley Diode: Anode (A), Cathode (K).
    • SCR: Anode (A), Cathode (K), Gate (G).
    • TRIAC: Terminal 1 (A\(_1\)), Terminal 2 (A\(_2\)), Gate (G).
Diagram illustrating the PNPN layer structure and terminals of a thyristor
Thyristor PNPN structure

Shockley Diode

  • Four-layer PNPN diode (Silicon Unilateral Switch).
  • Two terminals: Anode, Cathode.
  • Turns ON when forward voltage exceeds breakover (\(V_{BR(F)}\)).
  • Exhibits latching behavior.

Two-Transistor Model

  • Represented as two transistors:
    • \(Q_1\): PNP transistor.
    • \(Q_2\): NPN transistor.
  • Base of \(Q_1\) connected to collector of \(Q_2\), and vice versa.
  • Provides regenerative feedback for latching.
Two-transistor equivalent model of a thyristor with regenerative feedback
Two-transistor model of a thyristor

Working Principle

  • OFF state: High resistance, minimal leakage current.
  • Triggered ON by:
    • Forward voltage exceeding breakover (\(V_{BR(F)}\)).
    • Gate pulse (for SCR, TRIAC, GTO).
  • ON state: Low resistance, latches until current drops below holding current (\(I_H\)).
Graph showing thyristor switching characteristics during turn-on process
Thyristor switching characteristics
IV curve of thyristor showing forward and reverse blocking regions
Thyristor IV characteristics

Operating Regions and Key Characteristics

Region Condition Behavior
Off (Forward Blocking) \(I_A < I_S\) High resistance, OFF
On (Forward Conduction) \(I_A > I_S\) Low resistance, ON
Turn-Off \(I_A < I_H\) Returns to OFF state
  • Latching Behavior: Stays ON without continuous gate signal.
  • Switching Current (\(I_S\)): Current at which device turns ON.
  • Holding Current (\(I_H\)): Minimum current to maintain ON state.
  • Unidirectional (SCR, Shockley Diode) or Bidirectional (TRIAC, DIAC).
  • High \(di/dt\) and \(dv/dt\) ratings critical for performance.

Silicon Controlled Rectifier (SCR)

  • Three-terminal device: Anode, Cathode, Gate.
  • Turns ON via gate pulse or exceeding \(V_{BR(F)}\).
  • Gate triggering lowers \(V_{BR(F)}\) for controlled switching.
  • Used in rectification and power control.
Schematic symbol and circuit configuration of an SCR
SCR schematic and circuit

SCR Equivalent Circuit

Equivalent circuit of SCR using two-transistor model with gate control
SCR two-transistor equivalent circuit
  • Without Gate: behaves like Shockley Diode.
  • With Gate Pulse:
    • Initiates regenerative feedback.
    • SCR latches ON, stays ON even after gate is removed.

Turning SCR ON (Gate Triggering)

Circuit diagram illustrating SCR gate triggering mechanism
SCR gate triggering circuit

SCR Characteristic Curves

Characteristic curves of SCR showing various operating regions
SCR characteristic curves

SCR Rectifiers

  • Provide controlled rectification for AC to DC conversion.
  • Types:
    • Half-Wave: 1 SCR, controls positive half-cycle.
    • Full-Wave: 2 SCRs, center-tapped transformer.
    • Bridge: 2 SCRs + 2 diodes, no center-tap.
  • Output controlled by firing angle \(\theta\).
Circuit diagram of SCR full-wave rectifier with center-tapped transformer
SCR full-wave rectifier circuit

SCR Rectifier Waveforms

  • Half-Wave: Conducts from \(\theta\) to \(180^\circ\) in positive cycle.
  • Full-Wave/Bridge: Conducts in both half-cycles.
  • Average output voltage: \(V_{\mathrm{av}} = \frac{V_m}{\pi}(1 + \cos \theta)\) (full-wave).
Waveforms of SCR rectifier showing conduction angles
SCR rectifier output waveforms

SCR Half-Wave Rectifier: Average & RMS Output Voltages

Given: \(V = V_m \sin \omega t\), conduction from \(\theta\) to \(\pi\)

\[\begin{aligned} V_{\mathrm{av}} & = \frac{1}{2\pi} \int_\theta^\pi V_m \sin \omega t \, d(\omega t) \\ &= \frac{V_m}{2\pi}(1 + \cos \theta) \end{aligned}\]
Average output voltage:
  • \(\theta = 0^\circ \Rightarrow V_{\mathrm{av}} = \frac{V_m}{\pi}\)
  • \(\theta = 90^\circ \Rightarrow V_{\mathrm{av}} = \frac{V_m}{2\pi}\)

RMS output voltage:

\[\begin{aligned} V_{\mathrm{rms}} & = \sqrt{ \frac{1}{2\pi} \int_\theta^\pi (V_m \sin \omega t)^2 \, d(\omega t) } \\ & = \frac{V_m}{2} \left[ \frac{1}{\pi}(\pi - \theta + \sin 2\theta) \right]^{\frac{1}{2}} \end{aligned}\]

Special case:

  • At \(\theta = 0^\circ\), \(V_{\mathrm{rms}} = \frac{V_m}{2}\)

SCR Full-Wave Rectifier - Average Output Voltage

Given: \(V = V_m \sin \omega t\), SCRs conduct from \(\theta\) to \(\pi\) in each half

\[ V_{\mathrm{av}} = \frac{1}{\pi} \int_\theta^\pi V_m \sin \omega t \, d(\omega t) = \frac{V_m}{\pi}(1 + \cos \theta) \]
Average output voltage:
  • Double of SCR half-wave case since both half-cycles are utilized
  • Controlled output by adjusting firing angle

TRIAC (Triode for Alternating Current)

  • Bidirectional switching device for AC control.
  • Terminals: A\(_1\), A\(_2\), Gate.
  • Equivalent to two SCRs in anti-parallel.
  • Triggered by positive or negative gate pulses.
  • Controls power via phase control (firing angle \(\theta\)).
Schematic symbol and circuit configuration of a TRIAC
TRIAC schematic and circuit

Bilateral Operation of a TRIAC

Diagram showing bidirectional conduction paths in TRIAC operation
TRIAC bidirectional conduction
  • Remains OFF until breakover voltage is reached.
  • Leakage current flows in OFF state.
  • On conduction: passes large current, requires external resistor to limit surge.
  • Positive Half Cycle: \(\text{A}_1 > \text{A}_2\).
  • Negative Half Cycle: \(\text{A}_2 > \text{A}_1\).
  • Triggered by positive or negative gate pulse.
  • Bidirectional conduction allows phase control.
  • Gate signal adjusts firing angle \(\theta\).
  • Efficient control of AC power to the load.

DIAC (Diode for Alternating Current)

  • Two-terminal, three-layer bidirectional device.
  • Terminals: A\(_1\), A\(_2\) (interchangeable) not labeled as anode/cathode.
  • Triggers at breakover voltage, no gate control.
  • Commonly used to trigger TRIACs in phase control.
  • In OFF state: High resistance until breakover voltage is reached.
  • Once breakover voltage is exceeded: Device switches ON and conducts.
  • Conduction continues until the current falls below holding current.
Schematic symbol and characteristic curve of a DIAC
DIAC symbol and characteristics

Gate Turn-Off Thyristor (GTO)

  • Power semiconductor device that can be turned off using gate signal.
  • Belongs to thyristor family, but differs from SCRs in turn-off capability.
  • Used in motor drives, traction, HVDC transmission.
  • PNPN structure with three terminals: Anode (A), Cathode (K), Gate (G).
  • Gate used for both turn-on and turn-off.
  • Turn-on: Positive gate pulse; Turn-off: Negative gate current.
Schematic symbol and structure of a GTO thyristor
GTO thyristor symbol and structure

Working Principle and Characteristics of GTO

  • Requires fast gate drive for effective switching.
  • Static: Forward/Reverse blocking, Forward conduction.
  • Dynamic: Turn-on time: 1-2 μs, Turn-off time: 10-30 μs.
  • Gate-controlled turn-off eliminates need for commutation circuits.
  • Efficient switching for high power.
  • Snubber circuits needed to handle dV/dt and dI/dt.
Characteristic curves of GTO thyristor showing turn-off capability
GTO characteristic curves

Light Activated SCR (LASCR)

  • A LASCR is a type of SCR that is triggered by light instead of gate current.
  • Similar to a conventional SCR, but with a photosensitive window allowing light to penetrate the semiconductor layers.
Structure and schematic symbol of a Light Activated SCR (LASCR)
LASCR structure and symbol
  • Working Principle:
    • Incident light generates electron-hole pairs.
    • These carriers trigger conduction, switching the device ON.
    • Device remains ON until current drops below the holding current.
  • Applications:
    • Optical triggering of high-voltage AC lines
    • Over-voltage protection
    • Photonic switches in HVDC systems

Comparison of Thyristor Family Devices

Device Control Terminal Turn-On Method Turn-Off Method Switching Speed Applications
SCR (Silicon Controlled Rectifier) Gate Gate pulse or forward breakover Natural or forced commutation Slow AC motor control, rectifiers
Triac Gate Gate pulse (positive or negative) Current goes below holding current Moderate Light dimmers, fan regulators
Diac None Breakover voltage Current goes below holding current Moderate Triggering device for Triac
GTO (Gate Turn-Off Thyristor) Gate Gate pulse Negative gate pulse Faster than SCR Choppers, inverters
IGCT (Integrated Gate-Commutated Thyristor) Gate Gate pulse Gate pulse withdrawal High High-power drives, FACTS
PUT (Programmable Unijunction Transistor) Gate Programmable voltage Current goes below holding current High Oscillators, timers
LASCR (Light Activated SCR) None or Gate Exposure to light (or gate pulse) Natural or forced commutation Moderate HVDC transmission, optical triggering

Thyristors vs. Power FETs

Feature Thyristors Power FETs
Switching Bistable (ON/OFF) Linear/ON/OFF
Current Rating Very High (>1000 A) Moderate to High
Control Signal Gate Pulse Voltage-Controlled
Bidirectional Available (TRIAC) Mostly Unidirectional
Switching Speed Slower Faster

Thyristor Protection

  • Vulnerabilities:
    • Overvoltage and overcurrent.
    • High \(di/dt\) (current rise rate) at turn-ON.
    • High \(dv/dt\) (voltage rise rate) causing false triggering.
  • Protection methods:
    • \(di/dt\): Series inductor to limit current rise.
    • \(dv/dt\): Snubber circuit (R-C) to control voltage rise.
Circuit diagram of thyristor protection mechanisms
Thyristor protection circuit

Snubber Circuit for \(dv/dt\) Protection

  • Components: Resistor (\(R\)) and Capacitor (\(C\)) in series, parallel to thyristor.
  • Operation:
    • \(C\) limits \(dv/dt\) by charging gradually.
    • \(R\) limits discharge current when thyristor turns ON.
  • Benefits: Prevents false triggering and controls \(di/dt\).
Snubber circuit diagram for dv/dt protection in thyristors
Snubber circuit for dv/dt protection

Conclusion

  • Thyristors are robust semiconductor switches for high-power applications.
  • Key types (SCR, TRIAC, DIAC, GTO) cater to diverse needs.
  • Enable precise control in rectification, AC switching, and power systems.
  • Protection mechanisms ensure reliability and longevity.
  • Critical in modern power electronics for efficiency and control.