Thyristors and SCRs

Introduction

Thyristors are high-power semiconductor switching and rectification devices that can control large currents and voltages with minimal input power. They are fundamental components in power electronics, enabling efficient control of electrical power in applications ranging from industrial machinery to consumer appliances. The Silicon Controlled Rectifier (SCR), also called a thyristor, is one of the most important power semiconductor devices in modern electronics.

Historical Development

Thyristors were invented in 1956 by General Electric. The development of thyristors revolutionized power electronics by enabling:

• Efficient motor speed control
• Power conversion and regulation
• Power factor correction
• Industrial automation
• Renewable energy systems

Basic Structure and Operating Principle

Device Structure

The Silicon Controlled Rectifier (SCR) is a four-layer semiconductor device with the structure: p-n-p-n (pnpn).

Terminal definitions:

• Anode (A): Positive terminal under normal operation
• Cathode (K): Negative terminal under normal operation
• Gate (G): Control terminal for triggering the device

Internal layers:

• P-layer (top) - Anode connection
• N-layer (upper)
• P-layer (lower)
• N-layer (bottom) - Cathode connection

Equivalent Circuit Model

The SCR can be understood as two transistors (one PNP and one NPN) in a positive feedback configuration:

• Upper transistor (PNP) receives base drive from lower transistor
• Lower transistor (NPN) receives base drive from upper transistor
• Once triggered, positive feedback holds the device ON
• Device remains ON until anode current drops below holding current

Operating Physics

Forward Blocking State (device OFF):

• Anode positive, Cathode negative (forward bias)
• No gate current
• Middle p-n junction reverse biased
• Device acts as open switch
• Minimal leakage current flows (microamps range)

Transition to Conducting State (triggering):

• Gate current applied (positive voltage on gate)
• Gate-cathode junction becomes forward biased
• Injects carriers into middle p-n junction
• Lower transistor conducts
• Collector current of lower transistor drives base of upper transistor
• Upper transistor conducts
• Collector current of upper transistor drives base of lower transistor
• Regenerative process: device latches ON

Forward Conducting State (device ON):

• Gate current no longer needed to maintain conduction
• Device exhibits positive feedback
• Anode to cathode voltage drop: 0.8-1.5V (very low)
• Anode current can be very large (limited only by external circuit)
• Device remains ON until anode current drops below holding current threshold

Reverse Blocking State:

• Cathode positive, Anode negative
• Device blocks current in reverse direction
• Acts as open circuit
• Typical reverse voltage rating: 100V to 2400V

Operating Modes and I-V Characteristics

Mode 1: Forward Blocking (Off State)

• Forward voltage applied (Anode positive)
• No gate current or insufficient gate current
• Device blocks current flow
• Leakage current: 0.1-10 mA (device dependent)
• Holds voltage across device (can be hundreds of volts)
• Virtually all supply voltage appears across device

Mode 2: Forward Conducting (On State)

• Forward voltage applied
• Sufficient gate current triggers device
• Device enters latched state
• Conducts large current
• Voltage drop across device: 0.8-1.5V (very low)
• Gate current can be removed; device remains ON
• Power dissipation in device: P = Vak × Iak

Mode 3: Reverse Blocking (Reverse Bias Off)

• Reverse voltage applied (Anode negative)
• Device blocks in reverse direction
• Acts as open circuit
• Very high impedance
• Minimal leakage current

Mode 4: Reverse Conducting (Less Common)

• Some specialized SCRs can conduct in reverse
• Rarely used
• Not available in standard SCRs

Key Electrical Parameters

Anode-Cathode Parameters

Forward Voltage Drop (Vak or Vf)

Definition: Voltage across device when conducting at rated current

Typical values:

• Low power SCRs (< 10A): 0.8-1.2V
• Medium power SCRs (10-100A): 1.0-1.5V
• High power SCRs (> 100A): 1.2-2.0V

Temperature dependence:

• Increases with temperature at negative temperature coefficient
• Approximately -2mV per °C
• Important in high-current and high-temperature applications

Holding Current (Ih)

Definition: Minimum anode current required to maintain conduction after gate trigger is removed

Typical values: 10-100 mA depending on device

Significance:

• Below this current, device turns OFF
• Used to design turn-off circuits
• Critical for pulse-triggered operation

Leakage Current (Ileak or Iger)

Definition: Reverse current flowing through device when:

1. Forward biased but not triggered (blocking state)
2. Reverse biased

Typical values: 0.1 mA to 10 mA at room temperature

Temperature dependence:

• Doubles approximately every 10°C
• Very temperature sensitive
• Important design consideration for high-temperature circuits

Gate Parameters

Gate Trigger Voltage (VGT)

Definition: Gate voltage at which gate current causes anode current to reach 200mA at maximum rated anode voltage

Typical values: 0.5-2.0V depending on device type

Measurement conditions: Specific test circuit defined in datasheets

Gate Trigger Current (IGT)

Definition: Gate current required to trigger the SCR at rated anode voltage

Typical values: 5-50 mA for standard SCRs

Pulse vs. DC trigger:

• DC triggering: higher IGT value
• Pulse triggering (common): lower IGT value required

Gate-Cathode Forward Voltage (VGK)

Definition: Forward voltage drop between gate and cathode when gate current flows

Typical values: 0.5-1.0V

Similar to: A regular diode forward voltage

Turn-On Characteristics

Gate Pulse Width (tg)

Definition: Duration of gate current pulse

Requirements: Must be long enough to ensure device latches on

Typical values: 0.5 μs to 10 μs (pulse mode operation)

Anode Current Rise Time (tA)

Definition: Time for anode current to rise from 10% to 90% of final value

Depends on: Anode voltage and gate drive circuit

Typical values: 1-5 μs

Turn-On Time (ton)

Definition: Delay from gate trigger to device conduction

Components:

• Delay time: 0.5-2 μs
• Rise time: 1-5 μs
• Total turn-on: 1-10 μs

Turn-Off Characteristics

Turn-Off Time (toff)

Definition: Time for anode current to fall to zero after removal of gate current

Components:

• Fall time: 5-50 μs
• Tail current decay: 10-100 μs
• Total: 15-200 μs depending on device

Reverse Recovery Charge (Qr)

Definition: Charge that must be removed when transitioning from forward conducting to forward blocking

Importance: Affects switching losses and EMI generation

dV/dt Rating

Definition: Maximum rate of change of anode voltage the device can withstand without unwanted triggering

Typical values: 100-500 V/μs

Significance: Voltage spikes during turn-off can inadvertently trigger device

Mitigation: Snubber circuits limit dV/dt

di/dt Rating

Definition: Maximum rate of change of anode current the device can safely handle

Typical values: 100-1000 A/μs

Significance: Excessive di/dt causes uneven current distribution, hot spots, and device destruction

Mitigation: Series inductor or current-limiting circuits

Other Thyristor Devices

Triacs (Bidirectional Thyristors)

Structure: Two SCRs in reverse parallel, sharing a common gate

Characteristics:

• Conducts in both directions
• Used primarily for AC power control
• Three terminals: Main terminal 1 (MT1), Main terminal 2 (MT2), Gate (G)
• Can be triggered by positive or negative gate current
• Four quadrant operation (four combinations of polarity)

Applications:

• Light dimmers
• AC motor speed control
• Heating element control
• AC switching applications

Advantages:

• Controls AC power with single device
• Simpler circuits than back-to-back SCRs
• Lower cost for AC applications

Disadvantages:

• Lower surge current rating than equivalent SCR
• More complex control circuitry for good dv/dt immunity
• Inferior voltage drop characteristics

Diacs (Bidirectional Trigger Diodes)

Structure: Thyristor device with only two terminals (no gate)

Characteristics:

• Breaks down at specific voltage in both directions
• Conducts in both directions above breakover voltage
• Used as trigger device for Triacs

Breakover voltage: 30V typical (varies from 20-35V)

Applications:

• Triac gate triggering in AC applications
• Automatic trigger circuits
• Phase angle control circuits

Gate Turn-Off Thyristors (GTO)

Key difference from standard SCR: Can be turned OFF by negative gate current

Advantages:

• Can be switched off by gate signal
• Higher switching frequencies than SCR
• Better for inverter applications

Disadvantages:

• More complex gate drive circuits required
• Higher on-state voltage drop (2-3V)
• More expensive than SCRs
• Lower current density

Applications:

• Inverters (AC from DC)
• Motor drives
• Power conversion circuits
• Power factor correction

Integrated Gate Commutated Thyristor (IGCT)

Hybrid device combining GTO features with modern integration:

• Integrated gate driver
• Faster switching
• Lower losses
• Used in high-power applications

MOS-Controlled Thyristor (MCT)

Combines thyristor current capability with MOSFET control:

• Gate-controlled via MOSFET
• Faster switching than GTO
• Lower power dissipation
• Used in high-frequency power conversion

SCR Applications

1. Power Conversion and Rectification

Applications:

• Controlled rectification
• Converting AC to DC with adjustable voltage
• Power conversion efficiency control

Configurations:

• Single-phase half-wave rectifier
• Single-phase full-wave rectifier
• Three-phase bridge rectifier

Advantages:

• Efficiency: 95-98%
• Simple control
• Reliable operation

2. AC Motor Speed Control

Method: Phase angle control (leading edge or trailing edge triggering)

Advantages:

• Smooth speed control
• Energy efficient
• Simple control circuit

Disadvantages:

• Generates harmonic distortion
• EMI generation
• Heat dissipation at low speeds

Common applications:

• Light dimmers
• Fan speed controllers
• Industrial motor drives

3. DC Motor Speed Control

Method: Controlling average DC voltage by varying conduction angle

Advantages:

• Full-wave control available
• Good efficiency over speed range
• Suitable for industrial applications

4. Power Factor Correction

Using thyristors to switch capacitor banks:

• Improves power factor
• Reduces reactive power
• Reduces utility bills
• Decreases system losses

5. Lighting Control

Applications:

• Theater and stage lighting
• Architectural lighting
• Display lighting
• Street lighting control

Methods:

• Phase angle control (Triacs)
• On-off switching (SCRs)

6. Welding Power Supplies

Applications:

• Resistance welding
• Arc welding
• Energy spot welding

Advantages:

• Precise power control
• Consistent weld quality
• High efficiency

7. Power Protection

Applications:

• Overvoltage protection
• Current limiting
• Surge suppression
• Crowbar circuits for transient protection

8. Induction Heating

Applications:

• Industrial induction furnaces
• Induction cooking
• Hardening and annealing processes

Gate Drive Circuitry

Basic Gate Drive Requirements

Requirements:

1. Provide sufficient gate current (5-50 mA minimum)
2. Gate pulse duration adequate for device to latch
3. Pulse shape considerations for noise immunity
4. Isolation (if floating from main circuit)
5. Protection against excessive dv/dt
6. Heat dissipation considerations

Drive Signal Characteristics

Gate pulse width: 0.5-10 μs (typical)

Gate current: 10-100 mA (sufficient overdrive to 2-5× IGT)

Gate voltage: 5-15V typical

Peak power: 1-10 W per gate drive

Gate Protection

Zener diode: Limits maximum gate voltage (prevents gate overstress)

Series resistor: Limits peak gate current

Gate-cathode diode: Prevents negative voltage excursions

Optoisolator: Provides electrical isolation from control circuit

Design Considerations

Thermal Management

Heat dissipation calculation: $P_{dissipated} = V_{ak(avg)} × I_{a(avg)} + \text{switching losses}$

Heatsink requirements:

• Thermal resistance: θJA = (Tj - Ta) / P
• Cooling methods: natural convection, forced air, liquid cooling
• Thermal interface materials for contact resistance reduction

EMI and Snubber Design

EMI sources in thyristor circuits:

• dV/dt causing parasitic capacitive coupling
• di/dt causing EMI through inductances
• Harmonic current generation

Snubber circuit types:

1. RC Snubber: Resistor-capacitor across anode-cathode

   • Limits dV/dt
   • Provides safe turn-off path
   • Dissipates power as heat

2. Flywheel Diode: Reverse parallel diode across inductive load

   • Provides free-wheeling path for decaying current