11.4 Thyristors and triacs
11 min read•august 20, 2024
Thyristors and triacs are crucial components in power electronics, enabling control of high voltages and currents. These devices act as switches, allowing precise regulation of power flow in various applications.
Thyristors have a four-layer with three terminals: anode, cathode, and gate. Triacs, on the other hand, are bidirectional thyristors with two main terminals and a gate, capable of conducting current in both directions.
Structure of thyristors
- Thyristors are four-layer semiconductor devices consisting of alternating P-type and N-type layers, forming a PNPN structure
- The unique PNPN structure enables thyristors to function as bistable switches, allowing them to control and switch high voltages and currents in power electronics applications
PNPN layers in thyristors
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- The four layers in a are arranged in a PNPN configuration, with two P-type layers sandwiching two N-type layers
- The outer P-type layer is heavily doped and called the anode (A)
- The outer N-type layer is heavily doped and called the cathode (K)
- The inner P-type and N-type layers are lightly doped
- The PNPN structure forms three P-N junctions: J1 (anode-P1), J2 (P1-N1), and J3 (N1-cathode)
- J1 and J3 are forward-biased, while J2 is reverse-biased during forward blocking mode
Anode, cathode, and gate terminals
- Thyristors have three terminals: anode (A), cathode (K), and gate (G)
- The anode is connected to the outer P-type layer and is the positive terminal
- The cathode is connected to the outer N-type layer and is the negative terminal
- The gate is connected to the inner P-type layer (P1) and is used for the thyristor
- The gate terminal allows control over the thyristor's switching behavior by injecting a small current to turn the device on
Operating principles of thyristors
- Thyristors operate as bistable switches, exhibiting two stable states: forward blocking (off) and forward conduction (on)
- The unique PNPN structure and the presence of a gate terminal enable thyristors to control high voltages and currents with a small gate trigger signal
Forward and reverse blocking states
- In the , the anode is positively biased with respect to the cathode, but the thyristor remains off
- J1 and J3 are forward-biased, while J2 is reverse-biased, preventing current flow
- The thyristor can withstand a high forward voltage (up to its rated forward ) without conducting
- In the , the anode is negatively biased with respect to the cathode
- J1 and J3 are reverse-biased, while J2 is forward-biased
- The thyristor can withstand a high reverse voltage (up to its rated reverse breakdown voltage) without conducting
Forward conduction mode
- When a sufficient is applied to the gate terminal, the thyristor switches from the forward blocking state to the
- The gate trigger current causes J2 to become forward-biased, allowing current to flow from the anode to the cathode
- In the forward conduction mode, the thyristor acts as a low-resistance path, with a small across the device
- The thyristor continues to conduct even if the gate trigger is removed, as long as the anode current remains above the
Latching and holding currents
- current (IL) is the minimum anode current required to switch the thyristor from the forward blocking state to the forward conduction state
- Once the anode current exceeds IL, the thyristor latches on and continues to conduct
- Holding current (IH) is the minimum anode current required to maintain the thyristor in the forward conduction state
- If the anode current falls below IH, the thyristor switches back to the forward blocking state
- The latching and holding currents are important parameters that determine the thyristor's switching behavior and must be considered in the design of thyristor-based circuits
Thyristor triggering methods
- Thyristors can be triggered from the forward blocking state to the forward conduction state using various methods
- The choice of triggering method depends on the application requirements, such as isolation, speed, and control flexibility
Gate triggering of thyristors
- Gate triggering is the most common method, where a small current pulse is applied to the gate terminal to turn the thyristor on
- The gate trigger current must exceed the minimum gate trigger current (IGT) to initiate conduction
- The gate trigger pulse width should be sufficient to ensure reliable triggering
- Gate triggering provides precise control over the thyristor's turn-on instant and allows for easy implementation of and pulse-width modulation techniques
dv/dt triggering of thyristors
- Thyristors can also be triggered by a rapid rise in the anode-to-cathode voltage, known as
- If the rate of voltage rise (dv/dt) exceeds a critical value, the thyristor may turn on unintentionally
- dv/dt triggering is usually undesirable and can lead to circuit malfunction or damage
- To prevent dv/dt triggering, or series inductors are used to limit the voltage rise rate across the thyristor
Light-triggered thyristors
- Some thyristors, called or LTTs, can be triggered by exposing the device to light
- LTTs have a transparent window that allows light to reach the inner P-type layer (P1)
- When light of sufficient intensity and wavelength falls on P1, it generates electron-hole pairs, triggering the thyristor
- Light triggering provides electrical isolation between the control circuit and the power circuit, making it useful in high-voltage applications (high-voltage DC transmission systems)
Thyristor characteristics
- Understanding the electrical characteristics and limitations of thyristors is crucial for their proper application in power electronic circuits
- Key thyristor characteristics include current-voltage relationship, switching speed, power dissipation, and thermal management
Current-voltage characteristics
- The current-voltage (I-V) characteristic of a thyristor exhibits three distinct regions: forward blocking, forward conduction, and reverse blocking
- In the forward blocking region, the thyristor acts as an open switch, withstanding high forward voltage without conducting
- In the forward conduction region, the thyristor acts as a closed switch, allowing current to flow with a small on-state voltage drop
- In the reverse blocking region, the thyristor blocks reverse voltage up to its rated reverse breakdown voltage
- The I-V characteristic also shows important parameters such as the forward breakover voltage (VBO), latching current (IL), and holding current (IH)
Switching characteristics and speed
- Thyristor switching characteristics determine its suitability for different applications
- Turn-on time (ton) is the time required for the thyristor to switch from the forward blocking state to the forward conduction state after receiving a gate trigger
- (toff) is the time required for the thyristor to regain its forward blocking capability after the anode current falls below the holding current
- Thyristors have relatively slow switching speeds compared to other power semiconductor devices (MOSFETs and IGBTs), limiting their use in high-frequency applications
- Typical turn-on times range from a few microseconds to tens of microseconds
- Turn-off times can be several hundred microseconds to a few milliseconds
Power dissipation and thermal considerations
- Power dissipation in thyristors occurs due to the on-state voltage drop and switching losses
- On-state power dissipation is proportional to the on-state voltage drop and the average current through the device
- Switching losses occur during the turn-on and turn-off transitions and depend on the switching frequency and speed
- Proper thermal management is essential to prevent overheating and ensure reliable operation of thyristors
- Thyristors are mounted on heat sinks to facilitate heat dissipation
- The maximum allowable junction temperature (Tj_max) and thermal resistance (Rth) are key parameters in designing the thermal management system
Thyristor commutation techniques
- Commutation is the process of turning off a conducting thyristor by reducing the anode current below the holding current
- Effective commutation is essential for controlling thyristors in various power electronic applications, such as rectifiers, inverters, and cycloconverters
Natural commutation of thyristors
- occurs when the anode current naturally falls below the holding current due to the circuit conditions
- In AC circuits, natural commutation happens when the supply voltage reverses polarity, causing the anode current to decrease to zero
- Examples of natural commutation include line-commutated converters and phase-controlled rectifiers
- Natural commutation is simple and does not require additional components, but it relies on the AC supply frequency and limits the control flexibility
Forced commutation methods
- involves externally forcing the anode current to fall below the holding current, turning off the thyristor
- Various forced commutation techniques exist, such as self-commutation, impulse commutation, and resonant pulse commutation
- Self-commutation uses auxiliary thyristors and capacitors to create a reverse voltage across the main thyristor, turning it off
- Impulse commutation employs a high-frequency pulse transformer to apply a reverse voltage pulse to the thyristor
- Resonant pulse commutation utilizes an LC resonant circuit to generate a high-frequency oscillatory current that reduces the anode current to zero
- Forced commutation allows for more flexible control and higher operating frequencies compared to natural commutation
Commutation circuits and components
- are designed to provide the necessary conditions for turning off thyristors
- Commutation components include capacitors, inductors, diodes, and auxiliary thyristors
- Commutation capacitors store energy and provide reverse voltage for turning off the main thyristor
- Commutation inductors limit the rate of current change (di/dt) and help shape the commutation waveforms
- Commutation diodes and auxiliary thyristors are used to control the flow of commutation current and voltage
- The choice of commutation circuit depends on factors such as the power level, operating frequency, and desired control characteristics
Thyristor protection
- Thyristors are susceptible to damage from excessive current, voltage, and temperature stress
- Proper protection measures are essential to ensure the reliable and safe operation of thyristor-based power electronic systems
di/dt and dv/dt protection
- High rates of current change (di/dt) during turn-on can cause localized heating and damage to the thyristor
- Series inductors or saturable reactors are used to limit the di/dt and prevent excessive current spikes
- Rapid voltage changes (dv/dt) across the thyristor can cause unintentional triggering and lead to device failure
- Snubber circuits, consisting of resistors and capacitors, are used to suppress voltage transients and limit the dv/dt across the thyristor
Overvoltage and overcurrent protection
- Thyristors must be protected against overvoltage conditions that exceed their rated blocking voltage
- Metal oxide varistors (MOVs) or surge arresters are connected in parallel with the thyristor to clamp the voltage and absorb surge energy
- Overcurrent protection is necessary to prevent damage from short-circuit currents or overloads
- Fast-acting fuses or electronic overcurrent relays are used to detect and interrupt fault currents quickly
- Current-limiting reactors or series impedances can also be employed to limit the fault current levels
Snubber circuits for thyristors
- Snubber circuits are used to protect thyristors from voltage and current stresses during switching transitions
- RC snubbers, consisting of a resistor and a capacitor in series, are connected in parallel with the thyristor
- The capacitor absorbs the energy stored in the circuit inductance during turn-off, limiting the voltage rise rate (dv/dt)
- The resistor dissipates the stored energy and damps the oscillations caused by the capacitor and stray inductances
- Properly designed snubber circuits improve the switching performance, reduce electromagnetic interference (EMI), and enhance the reliability of thyristor-based systems
Thyristor applications
- Thyristors find extensive use in various power electronic applications due to their high power handling capability and robustness
- Some common applications include power control, voltage regulation, and high-power switching
Power control and switching
- Thyristors are widely used in power control and switching applications, such as motor drives, heating controls, and lighting systems
- Phase-controlled thyristor circuits allow smooth control of AC power by varying the firing angle of the thyristors
- Thyristor-based soft starters are used to gradually increase the voltage applied to motors during starting, reducing mechanical stress and current surges
- Thyristors are also employed in high-power switching applications, such as HVDC transmission systems and static VAR compensators (SVCs)
- In HVDC systems, thyristor valves are used to convert AC to DC and vice versa, enabling long-distance power transmission
- SVCs utilize thyristor-controlled reactors (TCRs) and thyristor-switched capacitors (TSCs) to provide dynamic reactive power compensation and voltage regulation
AC voltage regulation
- Thyristors are used in AC voltage regulators to control the output voltage by varying the conduction angle of the thyristors
- Single-phase and three-phase thyristor-controlled reactors (TCRs) are employed for stepless voltage regulation
- The TCR consists of a fixed reactor in series with a bidirectional thyristor switch, allowing continuous control of the reactor's effective impedance
- Thyristor-based AC voltage regulators are used in applications such as lighting dimming, industrial process control, and power quality improvement
Thyristor-controlled reactors and switches
- Thyristor-controlled reactors (TCRs) and thyristor-switched reactors (TSRs) are used for reactive power control and voltage regulation in power systems
- TCRs provide continuous control of the reactor's effective impedance by varying the thyristor firing angle
- TSRs offer stepwise control of the reactor by fully switching the thyristors on or off
- Thyristor-controlled switches (TCSs) and thyristor-switched capacitors (TSCs) are used for capacitor bank switching and reactive power compensation
- TCSs allow smooth and transient-free switching of capacitor banks by controlling the thyristor firing angle
- TSCs provide stepwise control of the capacitor banks by fully switching the thyristors on or off
- These thyristor-based devices are essential components in flexible AC transmission systems (FACTS) for enhancing power system stability, controllability, and power transfer capability
Introduction to triacs
- Triacs (triode alternating current switches) are bidirectional thyristors that can conduct current in both directions
- Triacs are widely used in AC power control applications due to their ability to control current flow in both positive and negative half-cycles of the AC waveform
Structure and terminals of triacs
- Triacs have a three-layer, five-terminal structure consisting of two main terminals (MT1 and MT2) and a gate terminal (G)
- The main terminals are connected to the outer P-type and N-type layers, allowing bidirectional current flow
- The gate terminal is used to control the triggering of the
- The internal structure of a triac is equivalent to two thyristors connected in antiparallel, sharing a common gate terminal
- This arrangement enables the triac to conduct in both directions when triggered by a gate signal of either polarity
Comparison of triacs vs thyristors
- Triacs and thyristors share some similarities but also have distinct differences
- Both devices are controlled switches that can be triggered by a gate signal to switch from a blocking state to a conducting state
- However, thyristors are unidirectional devices that conduct current only in one direction, while triacs are bidirectional and can conduct in both directions
- Triacs have a simpler gating circuit compared to thyristors, as they require only one gate signal for bidirectional control
- In contrast, thyristors need separate gating circuits for each direction of current flow
- Triacs have higher switching losses and lower current ratings compared to thyristors of similar size
- This is due to the triac's bidirectional conduction and the presence of multiple junctions in its structure
Triac operating modes
- Triacs can operate in different modes depending on the polarity of the applied voltage and the gate triggering conditions
- Understanding the operating modes is essential for designing triac-based control circuits and ensuring proper device functionality
Quadrant I and III operation
- Quadrant I operation occurs when the main terminal MT2 is positive with respect to MT1, and a positive triggers the triac
- In this mode, the triac conducts current from MT2 to MT1, similar to a forward-biased thyristor
- Quadrant III operation occurs when MT2 is negative with respect to MT1, and a negative gate current triggers the triac
- In this mode, the triac conducts current from MT1 to MT2, similar to a reverse-biased thyristor
- Quadrant I and III are the most common operating modes for triacs in AC power control applications
Quadrant II and IV operation
- Quadrant II operation occurs when MT2 is positive with respect to MT1, and a negative gate current triggers the triac
- In this mode, the triac conducts current from MT2 to MT1, but the gate triggering requirements are different from Quadrant I
Key Terms to Review (28)
Breakover voltage: Breakover voltage is the minimum voltage that must be applied across a thyristor or triac to switch it from its non-conducting state to its conducting state. This threshold is crucial because it determines the operational limits of these semiconductor devices, impacting how they control electrical power in various applications. Understanding breakover voltage helps in designing circuits that utilize thyristors and triacs effectively.
Commutation Circuits: Commutation circuits are specialized electrical circuits used to control the switching and operation of thyristors and triacs, allowing them to turn on and off in a controlled manner. These circuits play a crucial role in ensuring that the devices can switch states efficiently, minimizing power loss and maintaining performance in various applications, including motor control and power regulation.
Dv/dt triggering: dv/dt triggering refers to the method used to turn on thyristors and triacs based on the rate of change of voltage over time. This triggering mechanism is crucial because it allows for control over when the device will switch on, depending on how fast the voltage is changing. It plays an important role in ensuring that these devices operate reliably, especially in applications where rapid changes in voltage can occur.
Forced Commutation: Forced commutation is a technique used in power electronics to turn off a thyristor or a triac by applying an external circuit action that disrupts the holding current. This method allows for the controlled turn-off of these devices, which are normally latched in the on-state after being triggered. By using forced commutation, designers can implement more flexible and reliable control in circuits utilizing thyristors and triacs, especially in applications requiring rapid switching.
Forward Blocking State: The forward blocking state refers to a condition in thyristors where the device remains non-conductive despite being forward-biased. In this state, the thyristor prevents current from flowing, which is essential for controlling power in various applications. Understanding this state helps in grasping the operational principles of thyristors and their role in switching and controlling electrical circuits.
Forward blocking voltage: Forward blocking voltage is the maximum voltage that can be applied in the forward direction across a thyristor or a triac without causing it to conduct. This parameter is crucial because it determines the device's ability to withstand forward voltage stress while remaining in its non-conducting state. Understanding this concept helps in the selection and application of these devices in various electronic circuits.
Forward conduction state: The forward conduction state refers to the operational condition of a thyristor or triac when it is triggered and allows current to flow from its anode to cathode, enabling the device to conduct electricity. In this state, the device remains on as long as the current stays above a certain threshold called the holding current, making it crucial for controlled power applications. Understanding this state is essential for utilizing these semiconductor devices in various electronic circuits and systems.
Full-wave rectifier: A full-wave rectifier is an electrical circuit that converts alternating current (AC) into direct current (DC) by allowing both halves of the AC waveform to be utilized. This type of rectification results in a smoother and more efficient DC output compared to half-wave rectification, as it harnesses the entire input signal, leading to better performance in powering electronic devices. In the context of semiconductor devices, full-wave rectifiers often use diodes and can be further enhanced with components like thyristors and triacs for improved control and efficiency.
Gate current: Gate current refers to the small amount of current that flows into the gate terminal of a thyristor or triac, which is essential for triggering these devices into their conducting state. This current is crucial as it allows control over the larger currents that flow through the device during operation. Understanding gate current is important for comprehending how thyristors and triacs can be turned on and off, impacting their functionality in various applications such as power control and switching circuits.
Gate trigger current: Gate trigger current is the minimum current required to be applied to the gate of a thyristor or triac in order to initiate its conduction state. This key parameter is crucial for the reliable operation of these semiconductor devices, as it determines the point at which the device transitions from its off state to its on state, allowing current to flow through it. Understanding this concept helps in analyzing the control mechanisms for power electronics applications.
Holding current: Holding current is the minimum amount of current that must flow through a thyristor or triac to keep it in the conducting state after it has been triggered into conduction. This current is essential for maintaining the device's 'on' state, as it ensures that the device remains in conduction until the current drops below this level, allowing for controlled switching in circuits.
John Bardeen: John Bardeen was an American physicist who co-invented the transistor and is the only person to have won the Nobel Prize in Physics twice. His groundbreaking work laid the foundation for modern electronics and semiconductor devices, significantly impacting technologies such as diodes, field-effect transistors, and bipolar junction transistors.
Latching: Latching refers to the process by which a device remains in a specific state after the initial triggering signal has been removed. This characteristic is crucial in devices like thyristors and triacs, allowing them to maintain conduction even after the gate current is turned off. The ability to latch enables these devices to be used in various applications, including power control and switching circuits, where maintaining a stable state is essential.
Light dimmer circuit: A light dimmer circuit is an electrical device that adjusts the brightness of light sources by controlling the power delivered to them. This is commonly achieved using components like thyristors and triacs, which allow for efficient phase control of alternating current (AC) to modify the effective voltage and current flowing to the load, ultimately resulting in varying light intensity.
Light-triggered thyristors: Light-triggered thyristors are semiconductor devices that can be turned on by the presence of light, in addition to electrical signals. They combine the characteristics of traditional thyristors with the ability to respond to optical stimuli, making them useful in applications where electrical control is impractical or undesirable. Their operation typically involves photonic triggers, which make them essential in various control and switching applications.
Motor control: Motor control refers to the process of regulating and directing the movements of muscles and limbs to achieve a specific action or goal. This involves the brain's ability to send signals to various muscles, ensuring coordination, precision, and timing in movements. The effectiveness of motor control is crucial for the operation of devices like thyristors and triacs, as they can influence the performance of electric motors in various applications.
Natural Commutation: Natural commutation refers to the process by which a thyristor or triac turns off when the current flowing through it drops below a certain level, usually when the alternating current (AC) waveform crosses zero. This behavior is essential in controlling the switching of these semiconductor devices, allowing them to operate effectively in AC circuits. The natural commutation mechanism is what enables these devices to be used in various applications like phase control and power regulation.
On-state voltage drop: The on-state voltage drop refers to the voltage across a semiconductor device, such as a thyristor or triac, when it is in the conducting state and allowing current to flow. This voltage drop is an important parameter because it directly affects the efficiency and power dissipation in circuits utilizing these devices. The on-state voltage drop can vary based on factors such as temperature, device construction, and the amount of current passing through the device.
Phase Control: Phase control is a technique used to regulate the power delivered to a load by adjusting the phase angle of the input voltage waveform. This method is commonly applied in semiconductor devices, particularly thyristors and triacs, to enable effective control of AC power. By altering the point in the AC cycle at which a thyristor or triac is triggered, phase control can precisely manage the power flow, leading to efficient energy use and improved device performance.
Pnpn structure: The pnpn structure refers to a semiconductor device configuration consisting of alternating layers of p-type and n-type materials, creating two p-n junctions. This arrangement is crucial in devices like thyristors and triacs, enabling them to control electrical power efficiently by switching between conducting and non-conducting states.
Reverse blocking state: The reverse blocking state is a condition in thyristors and triacs where the device prevents current from flowing in the reverse direction when a reverse voltage is applied. This state is critical for controlling power in circuits, as it allows the devices to block reverse current effectively, ensuring that they only conduct when triggered appropriately. Understanding this state is essential for analyzing how these devices operate in AC applications and their role in managing load current.
Reverse recovery time: Reverse recovery time is the duration it takes for a diode to switch from the conducting state to the non-conducting state when the voltage polarity is reversed. This time is crucial because it affects the efficiency and performance of power diodes and semiconductor devices, especially in switching applications. A shorter reverse recovery time allows for faster operation and reduced power losses in circuits using diodes and thyristors.
Snubber circuits: Snubber circuits are electrical devices used to protect sensitive components from voltage spikes and transients, particularly in switching applications involving thyristors and triacs. They work by absorbing excess energy during turn-off events, thereby preventing damage to semiconductor devices and ensuring reliable operation. Understanding snubber circuits is crucial for designing robust systems that utilize these components effectively.
Thyristor: A thyristor is a four-layer, three-junction semiconductor device that acts as a switch, controlling power in electrical circuits. When triggered by a small control current, it allows a larger current to flow, making it ideal for applications like light dimmers and motor speed controls. Thyristors are known for their ability to handle high voltages and currents while maintaining efficiency.
Triac: A triac is a type of semiconductor device that can control current flow in both directions, making it an essential component in AC power control applications. It is similar to a thyristor but differs by allowing conduction for both positive and negative halves of an AC signal, enabling it to function as a switch or amplifier. Triacs are widely used in dimmers, motor speed controls, and other applications requiring precise control of AC loads.
Triggering: Triggering refers to the process of initiating the conduction state of a thyristor or triac, allowing current to flow through the device. This process is essential for controlling power in various electronic applications, as it enables these semiconductor devices to switch from an off state to an on state in response to an external control signal.
Turn-off Time: Turn-off time is the duration required for a thyristor or triac to stop conducting current after the control gate signal has been removed. This time is crucial because it affects how quickly these devices can switch off in various applications, impacting their performance and efficiency in circuits. Understanding turn-off time is essential for designing reliable circuits, as it determines the maximum frequency at which the device can operate and influences overall system stability.
William Shockley: William Shockley was an American physicist and co-inventor of the transistor, a groundbreaking semiconductor device that revolutionized electronics. His work laid the foundation for modern semiconductor technology, influencing various electronic devices and components, including transistors and diodes, as well as impacting the fields of recombination and injection processes in semiconductor physics.