Thyristors: Types, Working Principles, Characteristics & Applications

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

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.

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.

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.

Thyristors: Types, Working Principles, Characteristics & Applications

Thyristors: Types, Working Principles, Characteristics & Applications

Overview

What is a Thyristor?

Thyristor Family

Basic Construction

Shockley Diode

Two-Transistor Model

Working Principle

Operating Regions and Key Characteristics

Silicon Controlled Rectifier (SCR)

SCR Equivalent Circuit

Turning SCR ON (Gate Triggering)

SCR Characteristic Curves

SCR Rectifiers

SCR Rectifier Waveforms

SCR Half-Wave Rectifier: Average & RMS Output Voltages

SCR Full-Wave Rectifier - Average Output Voltage

TRIAC (Triode for Alternating Current)

Bilateral Operation of a TRIAC

DIAC (Diode for Alternating Current)

Gate Turn-Off Thyristor (GTO)

Working Principle and Characteristics of GTO

Light Activated SCR (LASCR)

Comparison of Thyristor Family Devices

Thyristors vs. Power FETs

Thyristor Protection

Snubber Circuit for \(dv/dt\) Protection

Conclusion