3. TURN-ON AND TURN-OFF METHODS OF THYRISTORS

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

3. Turn-On and Turn-Off Methods of Thyristors

In power electronics, controlling the turning on and turning off of thyristors like the SCR (Silicon Controlled Rectifier) is crucial for their efficient operation. Various methods are used to trigger the SCR into conduction (turn-on) and to stop the conduction (turn-off). These methods help in controlling the performance of the SCR in high-power applications like motor control, inverters, and power supplies.

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3.1 SCR Turn-On Methods:

3.1.1 High Voltage Thermal Triggering:

• Method: This method relies on the application of a high voltage across the SCR terminals. When a large voltage is applied, it causes thermal breakdown in the junctions of the SCR, which leads to conduction.
• Explanation: A voltage exceeding the breakover voltage causes the SCR to switch on. The junction heats up due to the excessive voltage, and the thermal energy generated triggers the SCR into conduction.

3.1.2 Illumination Triggering:

• Method: In this method, the SCR is triggered into conduction by light.
• Explanation: Special types of SCRs, such as light-triggered SCRs (LASCR), use a photodiode or similar light-sensitive materials in their construction. When the SCR is exposed to light (usually from a laser or light source), the energy from the light causes a photocurrent, which triggers the SCR to turn on.
• Applications: Used in optical control circuits and for remote switching applications.

3.1.3 dv/dt Triggering:

• Method: This method involves the rate of change of voltage (dv/dt) across the SCR.
• Explanation: If the voltage across the SCR changes very rapidly (high dv/dt), the device can be triggered into conduction unintentionally, even without a gate pulse. This is known as dv/dt triggering.
  • How it works: When a high dv/dt is applied, the internal capacitances and parasitic inductances of the SCR cause charge injection into the base region, leading to unwanted turn-on.
• Control: To prevent dv/dt triggering, snubber circuits (resistor-capacitor networks) are often used across the SCR to control the rate of voltage change.

3.1.4 Gate Triggering:

Gate triggering is one of the most common and controlled methods of turning on an SCR. The gate terminal is used to apply a small voltage pulse, which triggers the SCR into conduction.

3.1.4.1 Resistance Triggering Circuit:

• Method: In this method, a resistor is placed in series with the gate to limit the gate current.
• Explanation: A pulse is applied to the gate through the resistor. The resistor controls the amount of current that flows into the gate, thereby controlling the triggering of the SCR.
• Key Point: This method provides a simple and reliable way of triggering the SCR. The value of the resistor is chosen to ensure the proper amount of current flows into the gate.

3.1.4.2 Resistance-Capacitance Triggering Circuit:

• Method: This method uses both resistor and capacitor in the gate circuit.
• Explanation: A capacitor is connected in series with a resistor and the gate terminal. When a pulse is applied, the capacitor charges up and provides a rapid rise in voltage to trigger the SCR.
• Key Point: This method provides faster triggering compared to resistance-only triggering due to the rapid voltage buildup across the capacitor.

3.1.5 SCR Triggering using UJT (Uni-Junction Transistor) and PUT (Programmable Unidirectional Thyristor):

• UJT Triggering:
  • Method: A UJT is used to generate a pulse that is then applied to the gate of the SCR.
  • Working: The UJT creates a pulse with a characteristic triggering voltage that can turn the SCR on.
  • Advantages: UJTs are simple and effective for generating triggering pulses for SCRs.
• PUT Triggering:
  • Method: The PUT (Programmable Unidirectional Thyristor) is another device that can be used to trigger an SCR. A gate pulse from the PUT controls the triggering of the SCR.
  • Working: The PUT is used to generate controlled pulses that can be used to trigger the SCR.

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3.2 SCR Turn-Off Methods:

After the SCR is turned on, it remains in the conducting state until the current through it falls below a certain threshold (holding current). The turn-off methods are used to stop the current flow and turn the SCR off.

3.2.1 Class A – Series Resonant Commutation Circuit:

• Method: A resonant circuit is used to turn off the SCR. This circuit includes a series combination of an inductor and capacitor, which forms a resonant LC circuit.
• Explanation: In a resonant circuit, the current is reduced to zero in the SCR by creating a high voltage that forces the SCR to turn off.
• Application: Used in AC switching circuits and in inverters where control of SCR turn-off is crucial.

3.2.2 Class B – Shunt Resonant Commutation Circuit:

• Method: Similar to the Class A circuit, but the resonant circuit is placed in a shunt configuration (parallel) with the SCR.
• Explanation: The circuit is designed so that the energy stored in the capacitor discharges, reducing the current flowing through the SCR to zero.
• Application: Commonly used in AC circuit breakers and switching regulators.

3.2.3 Class C – Complimentary Symmetry Commutation Circuit:

• Method: In this circuit, a combination of complimentary transistors (NPN and PNP) is used to help the SCR turn off.
• Explanation: The complementary symmetry circuit helps to balance the operation of the transistors, ensuring that the SCR current goes to zero and turns off.
• Application: Typically used in high-frequency switching circuits and pulse-width modulation (PWM) applications.

3.2.4 Class D – Auxiliary Commutation:

• Method: An auxiliary circuit is used to force the SCR to turn off. This can be achieved by a secondary energy source such as a capacitor or external switch.
• Explanation: The auxiliary commutation technique injects a pulse into the SCR to help interrupt the current and force it to turn off.
• Application: Common in motor control circuits and power supplies.

3.2.5 Class E – External Pulse Commutation:

• Method: A pulse is applied externally to the SCR to force it to turn off.
• Explanation: This method uses a positive or negative pulse to the gate to interrupt the conducting state and turn off the SCR.
• Application: Used in high-speed switching circuits where precise control of SCR turn-off is required.

3.2.6 Class F – Line or Natural Commutation:

• Method: This is the natural turn-off mechanism that occurs when the AC supply current goes to zero.
• Explanation: In AC circuits, the SCR naturally turns off when the current falls to zero during the zero-crossing point of the AC waveform.
• Application: Common in AC power control circuits like light dimmers and motor controllers.

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Conclusion:

The turn-on and turn-off methods for SCRs and other thyristors are crucial for their application in various power electronic systems. The methods vary depending on the need for precision, control, and speed in switching. Gate triggering and illumination triggering are common for turn-on, while commutation circuits (Class A to Class F) are employed to turn off the SCR effectively, depending on the application requirements.

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Important Questions to Practice:

1. Explain the different methods used for triggering an SCR.
2. Discuss the gate triggering method with the use of resistor and capacitor circuits.
3. What is the principle of dv/dt triggering, and how is it controlled?
4. Describe the Class A and Class B commutation circuits for turning off an SCR.
5. What is the difference between Class C and Class D commutation circuits?
6. Explain the working of an auxiliary commutation circuit (Class D).
7. How does the natural commutation (Class F) work in an AC circuit for turning off an SCR?
8. What are the applications of illumination triggering and high voltage thermal triggering?

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