2. THYRISTOR FAMILY DEVICES

For 4th Semester Polytechnic EE Students
Written by Garima Kanwar | Blog: Rajasthan Polytechnic

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Course Code EE 4001
Course Title Fundamentals of Power Electronics

2. Thyristor Family Devices

Thyristors are semiconductor devices that act as switches, controlling large amounts of power. They are widely used in power electronics for applications like motor control, power switching, and rectifiers. Let’s dive deeper into the SCR (Silicon Controlled Rectifier) and other devices that belong to the Thyristor family.

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2.1 SCR (Silicon Controlled Rectifier)

2.1.1 Construction of SCR:

• An SCR is a four-layer, three-junction device (PNPN structure) with four terminals:

  1. Anode (A): The positive terminal connected to the external circuit.
  2. Cathode (K): The negative terminal connected to the external circuit.
  3. Gate (G): The control terminal used to trigger the SCR into conduction.
  4. Anode-Cathode Junction: This forms the main current-carrying path between the anode and cathode.

  Structure Diagram:

  
     Anode (A) | P-type | N-type | P-type | N-type | Cathode (K)
                |  (P)  |   (N)  |  (P)   |  (N)   |  
           (Gate G)
  

2.1.2 Two Transistor Analogy:

• An SCR can be understood using a two-transistor analogy. It consists of two transistors, Q1 (PNP) and Q2 (NPN), connected in feedback configuration:

  • Q1 conducts when the anode is positive relative to the cathode.
  • Q2 is responsible for the feedback loop to keep the SCR in its conducting state once triggered.

  Triggering Action:

  • When the gate is triggered, a small current flows into the gate and starts the conduction process by turning on Q1, which in turn, activates Q2.
  • This feedback loop causes the SCR to remain on even after the gate current is removed, until the anode current is reduced to zero (i.e., the SCR is turned off by completely interrupting the current flow).

2.1.3 Types, Working, and Characteristics of SCR:

• Types of SCR:

  • Standard SCR: Used for general-purpose switching.
  • Light-triggered SCR (LASCR): Triggered by light instead of an electrical signal.

• Working Principle:

  • An SCR remains off until a small current is injected into the gate terminal, causing it to turn on (conduct current between anode and cathode).
  • Once the SCR is on, it remains in the conducting state even if the gate current is removed.
  • It will only turn off when the current through it falls below a certain threshold (called holding current).

• V-I Characteristics:

  • Forward Blocking Region: The SCR is off, and no current flows between the anode and cathode when forward biased.
  • Forward Conducting Region: Once the gate current is triggered, the SCR turns on, and current flows freely.
  • Reverse Blocking Region: The SCR does not conduct when reverse biased, similar to a diode.

  Graph:

  • X-axis: Voltage across SCR (V)
  • Y-axis: Current through SCR (I)

2.1.4 SCR Mounting and Cooling:

• Mounting: SCRs are typically mounted on heat sinks or ceramic packages to dissipate heat generated during operation.
• Cooling: Since SCRs handle high power, effective cooling is required. Heat sinks, fans, and sometimes liquid cooling are used.

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2.2 Types of Thyristors:

The Thyristor family consists of various devices that have different characteristics and applications. Let’s go through the common types:

2.2.1 SCR (Silicon Controlled Rectifier)

• Symbol:

  
       Anode (A)  |   Gate (G)   |   Cathode (K)
  

• Construction: Four layers of semiconductor (PNPN).

• Operating Principle: Used as a switch for controlling high power. It conducts when triggered and stays on until the current drops to zero.

• V-I Characteristics: Similar to the characteristics explained above for SCR.

2.2.2 SCS (Silicon Controlled Switch)

• Symbol: Similar to SCR with an extra gate terminal.
• Construction: Similar to SCR but with an additional terminal for controlling the switching behavior.
• Operating Principle: Used for switching and controlling power with more flexibility than an SCR.
• V-I Characteristics: Shows the ability to turn on and off in both positive and negative directions.

2.2.3 GTO (Gate Turn-Off Thyristor)

• Symbol: Similar to SCR with a gate terminal for turning off.
• Construction: Similar to SCR but with the ability to be turned off using the gate terminal.
• Operating Principle: GTO can be turned on by the gate but can also be turned off by applying a negative pulse to the gate.
• V-I Characteristics: Features both turn-on and turn-off control via the gate.

2.2.4 UJT (Uni-Junction Transistor)

• Symbol: A single junction transistor with three terminals.
• Construction: Made of a single PN junction with two terminals (emitter and base).
• Operating Principle: UJT is used for triggering other thyristors and for oscillators in power electronic circuits.
• V-I Characteristics: Shows a negative resistance region where the voltage decreases with increasing current.

2.2.5 PUT (Programmable Unidirectional Thyristor)

• Symbol: Similar to a diode with an extra gate terminal.
• Construction: A modified thyristor with a gate control for both turning on and off.
• Operating Principle: Can be triggered to conduct with the gate and controlled to stop conduction with a negative pulse.
• V-I Characteristics: A unidirectional characteristic with gate control.

2.2.6 DIAC (Diode for Alternating Current)

• Symbol: A bi-directional symbol similar to a diode.
• Construction: A two-terminal device with no gate terminal. It has a breakover voltage for triggering.
• Operating Principle: It conducts in both directions after a certain voltage is applied, used for triggering TRIACs.
• V-I Characteristics: Conducts after a breakover voltage in both directions.

2.2.7 TRIAC (Triode for Alternating Current)

• Symbol: Similar to SCR but with bi-directional operation.
• Construction: A combination of two SCRs connected in parallel but with opposite polarity.
• Operating Principle: It can be triggered by the gate in both positive and negative half-cycles of AC and remains on until the current drops to zero.
• V-I Characteristics: Shows both positive and negative conduction, allowing it to work in AC circuits.

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2.3 Protection Circuits:

Thyristors are used in high-power applications and require protection circuits to safeguard them from voltage or current surges.

2.3.1 Over-voltage Protection:

• Purpose: Protects the thyristor from voltage spikes above its maximum rating.
• Method: Use of a voltage clamping device, like a crowbar circuit or snubber.

2.3.2 Over-current Protection:

• Purpose: Prevents excessive current that could damage the thyristor.
• Method: A current-sensing circuit and fuse or circuit breaker can be used.

2.3.3 Snubber Circuit:

• Purpose: Protects thyristors from high-voltage transients and to reduce the effect of voltage spikes.
• Method: A resistor-capacitor (RC) network is placed across the thyristor to absorb the surge.

2.3.4 Crowbar Circuit:

• Purpose: Protects the thyristor by short-circuiting the power supply in the event of an over-voltage condition.
• Method: Uses a thyristor or SCR to short-circuit the system in case of over-voltage, thus protecting the device.

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Conclusion:

The Thyristor family consists of versatile semiconductor devices used in high-power applications for switching and control. The SCR is the most common member of this family, while others like TRIAC, DIAC, and GTO provide specific control over power switching. Proper protection circuits, including snubber and crowbar circuits, are essential for maintaining the longevity and reliability of these devices.

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Important Questions to Practice:

1. Explain the construction and working principle of an SCR.
2. What is the two-transistor analogy of an SCR?
3. Discuss the V-I characteristics of an SCR and explain its operating regions.
4. Differentiate between SCR, GTO, and TRIAC.
5. What is the function of a snubber circuit? How does it protect SCRs?
6. Explain the working and application of a TRIAC.
7. Discuss the principle of operation of a GTO and how it differs from an SCR.
8. What are the protection circuits used for thyristors? Discuss each in detail.

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