What is a thyristor? Types of thyristors and their uses
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Thyristors are an interesting class of semiconductor devices. They share similar characteristics with other solid-state components made from silicon, like diodes and transistors. Therefore, distinguishing thyristors from diodes and transistors could be difficult. To add to the difficulty, there are different types of thyristors available on the market.
In some instances, what sets thyristors apart from one another could be just a tiny detail.
Also, depending on the manufacturer, a given thyristor may be known by another name.
To apply thyristors successfully when designing circuits, it is important to know their unique characteristics, limitations, and their relationship with the circuit. That’s why we’re taking some time to sort it all out so that you can have a better understanding of what thyristor is most suitable for your application.
A thyristor is a four-layer device with alternating P-type and N-type semiconductors (P-N-P-N).
In its most basic form, a thyristor has three terminals: anode (positive terminal), cathode (negative terminal), and gate (control terminal). The gate controls the flow of current between the anode and cathode.
The primary function of a thyristor is to control electric power and current by acting as a switch. For such a small and lightweight component, it offers adequate protection to circuits with large voltages and currents (up to V, A).
It is attractive as a rectifier because it can switch rapidly from a state of conducting current to a state of non-conduction.
In addition, its cost of maintenance is low and, operating under the right conditions, remains functional in the long term without developing a fault.
Thyristors are used in a wide range of electric circuits, from simple burglar alarms to power transmission lines.
A thyristor with a P-N-P-N structure has three junctions: PN, NP, and PN. If the anode is a positive terminal with respect to the cathode, the outer junctions, PN and PN are forward-biased, while the center NP junction is reverse-biased. Therefore, the NP junction blocks the flow of a positive current from the anode to cathode. The thyristor is said to be in a forward blocking state. Similarly, the flow of a negative current is blocked by the outer PN junctions. The thyristor is in a reverse blocking state.
Another state a thyristor can exist in is the forward conducting state, whereby it receives a sufficient signal to switch on, and it starts conducting.
Let’s take a minute to highlight the unique properties thyristors bring to a circuit by going further into the nature of the signal and the thyristor’s response.
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Thyristor turn-on at-a-glance
When a sufficient positive signal current or pulse is applied to the gate terminal, it triggers the thyristor into a conducting state. Current flows from anode to cathode and will continue to do so, even when the gate signal is removed. The thyristor is said to have “latched on”.
To unlatch the thyristor, the circuit needs to be reset by reducing the anode to cathode current below a threshold value known as the holding current.
Thyristor turn-on at a semiconductor material level
The PNPN structure of a thyristor can be interpreted as two transistors coupled together. That is, the collector current from the NPN transistor feeds the base of the PNP transistor. Similarly, the collector current from the PNP transistor feeds the base of the NPN transistor.
For the thyristor to latch on and start conducting current, the sum of the common base
current gains of the two transistors must exceed unity.
When a positive current or momentary pulse is applied to the gate that sufficiently raises the loop gain to unity, regeneration occurs. This means that the pulse causes the NPN transistor to begin conducting current, which, in turn, biases the PNP transistor into conduction. If the
initial triggering current on the gate is removed, the thyristor remains in the on state, as long as the current through the thyristor is high enough to meet the unity gain criteria. This is the latching current.
A thyristor can also turn on because of avalanche breakdown of a blocking junction. For the thyristor to switch on when the gate current is zero, the applied current must reach the breakover voltage of the thyristor. This is undesirable, since breakdown damages the device. For normal operation, a thyristor is chosen so that its breakover voltage is magnitudes greater than the largest voltage that will be experienced from the power source. This way, switching a thyristor on can only take place after an intentional pulse is applied to the gate, except where the thyristor has been specifically designed to operate in breakover mode. (See types of thyristors with controlled turn off abilities below).
Thyristor turn-off
To turn off a thyristor that has latched on (switched on/turned on), the current through it must change such that the loop gain is below unity. Turn off begins when the current is reduced below the holding current.
Different types of thyristors and their uses
Thyristors can be classified depending on the nature of their turn-on and turn-off behavior and their voltage and current characteristics: The different classes are:
SCRs are the most widely known thyristor. As explained in the general thyristor description above, an SCR remains latched on even when the gate current is removed. To unlatch, the anode to cathode current needs to be removed or the anode reset to a negative voltage relative to the cathode. This characteristic is ideal for phase control. When the anode current becomes zero, the SCR stops conducting and blocks the reverse voltage.
SCRs are used in switching circuits, DC motor drives, AC/DC static switches and inverting circuits.
Thyristors usually only allow current in the forward direction, while blocking reverse direction currents. However, an RCT consists of a SCR integrated with a reverse diode which eliminates undesired loop inductance and reduces reverse voltage transients. The RCT allows electric conduction in the reverse direction with improved commutation.
RCTs are used in inverters and DC drives for high power choppers.
These are also known as light triggered thyristors (LTT). For these devices, when light particles strike the reverse-biased junction, the number of electron-hole pairs in the thyristor increases. If the light’s intensity is greater than a critical value, the thyristor will switch on. An LASCR provides complete electrical isolation between the light source and the switching device of a power converter.
LASCRs are used in HVDC transmission equipment, reactive power compensators, and high-power pulse generators.
Traditional thyristors like SCRs turn on when sufficient gate pulse is applied. To turn them off, the main current has to be interrupted. This is inconvenient in DC to AC and DC to DC conversion circuits, where current does not naturally become zero.
A GTO differs from a standard thyristor as it can be switched off by applying a negative current (voltage) to the gate without requiring the removal of the current between the anode and cathode (forced commutation). This means the GTO can be turned off by a gate signal with a negative polarity, making it a fully controllable switch. It’s also referred to as a Gate-Controlled Switch, or GCS. The turn off time of a GTO is approximately ten times faster than an equivalent SCR.
GTOs with reverse blocking ability comparable to their forward voltage ratings are called symmetric GTOs. Asymmetric GTOs do not have considerable reverse voltage blocking capability. Reverse conducting GTOs consist of a GTO integrated with an anti-parallel diode. Asymmetric GTOs are the most popular variety on the market.
GTOs are used in DC and AC motor drives, high power inverters, and AC stabilizing power.
An MTO is a combination of a GTO and MOSFET to improve the GTO’s turn-off ability. GTO’s require a high gate turn off current to be supplied whose peak amplitude is about 20-35 % of the anode to cathode current (current to be controlled). An MTO has two control terminals, a turn-on gate and a turn-off gate also called the MOSFET gate.
To turn on an MTO, an applied gate pulse of sufficient magnitude causes the thyristor to latch on (similar to SCR and GTO).
To turn off the MTO, a voltage pulse is applied to the MOSFET gate. The MOSFET turns on, which shorts the emitter and base of the NPN transistor, thereby stopping latching. It’s a much faster process than a GTO (approximately 1-2 µs) in which case, the large negative pulse applied on the GTO’s gate aims to extract enough current from the base of the NPN transistor. In addition, the faster time (MTO) eliminates the losses associated with current transfer.
MTOs are used in high voltage applications up to 20 MVA, motor drives, flexible AC line transmissions (FACTs), and voltage source inverters for high power.
Just like an MTO, the ETO has two terminals, a normal gate, and a second gate connected in series with a MOSFET.
To turn on an ETO, positive voltages are applied to both gates which results in NMOS turning on and PMOS turning off. When a positive current is injected into the normal gate, the ETO switches on.
To turn off, when a negative voltage signal is applied to the MOSFET gate, NMOS turns off and transfers all current away from the cathode. The latching process stops and the ETO is switched off.
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ETOs are applied in voltage source inverters for high power, Flexible AC line Transmissions (FACTs), and Static Synchronous Compensator (STATCOM).
The thyristors discussed so far have been unidirectional and are used as rectifiers, DC-DC converters, and inverters. To use these thyristors for AC voltage control, two thyristors will need to be connected in anti-parallel, resulting in two separate control circuits that would involve more wire connections. Bidirectional thyristors that are able to conduct current in both directions when triggered were developed specifically to overcome this problem.
TRIACS are the second-most widely used thyristors after SCRs. They can provide control on both halves of the alternating waveform thereby using available power more efficiently. However, TRIACs are generally only used for low power applications because of their inherent non-symmetrical construction. In high power applications, TRIACs present some disadvantages when switching at different gate voltages during each half cycle. This creates additional harmonics that cause an imbalance in the system and affect EMC performance.
Low-power TRIACs are used as light dimmers, speed controls for electric fans and other electric motors, and in computerized control circuits of household appliances.
DIACS are low power devices and are mainly used in conjunction with TRIACS (placed in series with the gate terminal of a TRIAC).
Because TRIACS are unsymmetrical by nature, a DIAC prevents any current flowing through the TRIAC’s gate until the DIAC reaches its trigger voltage in either direction. This ensures TRIACS used in AC switches trigger evenly in either direction.
DIACs are found in light bulb dimmers.
A SIDAC behaves the same way electrically as a DIAC. The main difference between the two is that SIDACs have a higher breakover voltage and greater power handling capability than DIACs. A SIDAC is a five-layer device that can be used directly as a switch instead of as a trigger for another switching device (like DIACs are for TRIACS).
If the applied voltage matches or exceeds its breakover voltage, a SIDAC begins to conduct current. It remains in this conducting state even if the applied voltage changes, until the current can be reduced below its rated holding current. The SIDAC returns to its nonconductive state to repeat the cycle.
SIDACs are used in relaxation oscillators and other special purpose devices.
Click here to shop thyristors or other circuit protection devices from MDE Semiconductor.
Our two-terminal thyristor P Series is designed for the Telecommunication Industry. These products Provide Protection in Accordance with FCC Part 68, UL , Bellcore . ITU-TK, 20& K. 21
A thyristor ( , from a combination of Greek language θύρα, meaning "door" or "valve", and transistor[1] ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage of current in one direction but not the other, often under control of a gate electrode, that is used in high power applications like inverters and radar generators. It usually consists of four layers of alternating P- and N-type materials.[2]: 12 It acts as a bistable switch (or a latch).[2]: 12 There are two designs, differing in what triggers the conducting state. In a three-lead thyristor, a small current on its gate lead controls the larger current of the anode-to-cathode path. In a two-lead thyristor, conduction begins when the potential difference between the anode and cathode themselves is sufficiently large (breakdown voltage). The thyristor continues conducting until the voltage across the device is reverse-biased or the voltage is removed (by some other means),[2]: 12 or through the control gate signal on newer types.
Some sources define "silicon-controlled rectifier" (SCR) and "thyristor" as synonymous.[3] Other sources define thyristors as more complex devices that incorporate at least four layers of alternating N-type and P-type substrate.
The first thyristor devices were released commercially in . Because thyristors can control a relatively large amount of power and voltage with a small device, they find wide application in control of electric power, ranging from light dimmers and electric motor speed control to high-voltage direct-current power transmission. Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. Originally, thyristors relied only on current reversal to turn them off, making them difficult to apply for direct current; newer device types can be turned on and off through the control gate signal. The latter is known as a gate turn-off thyristor, or GTO thyristor.
Unlike transistors, thyristors have a two-valued switching characteristic, meaning that a thyristor can only be fully on or off, while a transistor can lie in between on and off states. This makes a thyristor unsuitable as an analog amplifier, but useful as a switch.
The silicon controlled rectifier (SCR) or thyristor proposed by William Shockley in and championed by Moll and others at Bell Labs was developed in by power engineers at General Electric (GE), led by Gordon Hall and commercialized by GE's Frank W. "Bill" Gutzwiller. The Institute of Electrical and Electronics Engineers recognized the invention by placing a plaque at the invention site in Clyde, New York, and declaring it an IEEE Historic Milestone.
In , Transitron Electronic Corporation marketed an PNPN tetrode with a high current gain at the time referred to as Binistor.[4][5] Device was predicted to be applied both in switching and data storage circuits.[4][5] One research paper using Transitron's tetrode demonstrated an "unusual" (at the time) switched-mode power supply circuit.[6][6]: 41
An earlier gas-filled tube device called a thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived.[2]: 12
In recent years, some manufacturers[7] have developed thyristors using silicon carbide (SiC) as the semiconductor material. These have applications in high temperature environments, being capable of operating at temperatures up to 350°C.
The thyristor is a four-layered, three-terminal semiconductor device, with each layer consisting of alternating N-type or P-type material, for example P-N-P-N. The main terminals, labelled anode and cathode, are across all four layers. The control terminal, called the gate, is attached to p-type material near the cathode. (A variant called an SCS—silicon controlled switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause a self-latching action.
Thyristors have three states:
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state quickly.
Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until: (a) the potential VAK is removed or (b) the current through the device (anode−cathode) becomes less than the holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.
The gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.
In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to remain in the on state), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off unless the current drops below the holding current (IH). In normal working conditions the latching current is always greater than holding current. In the above figure IL has to come above the IH on y-axis since IL>IH.
A thyristor can be switched off if the external circuit causes the anode to become negatively biased (a method known as natural, or line, commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the anode of the first thyristor. This method is called forced commutation.
Once the current through the thyristor drops below the holding current, there must be a delay before the anode can be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.
Thyristors is designed for low frequency applications i.e. those below 1kHz.[8] Frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), require thyristors with lower values of tQ. Such fast thyristors can be made by diffusing heavy metal ions such as gold or platinum which act as charge combination centers into the silicon. Today, fast thyristors are more usually made by electron or proton irradiation of the silicon, or by ion implantation. Irradiation is more versatile than heavy metal doping because it permits the dosage to be adjusted in fine steps, even at quite a late stage in the processing of the silicon.
A reverse conducting thyristor (RCT) has an integrated reverse diode, so is not capable of reverse blocking. These devices are advantageous where a reverse or freewheel diode must be used. Because the SCR and diode never conduct at the same time they do not produce heat simultaneously and can easily be integrated and cooled together. Reverse conducting thyristors are often used in frequency changers and inverters.
Photothyristors are activated by light. The advantage of photothyristors is their insensitivity to electrical signals, which can cause faulty operation in electrically noisy environments. A light-triggered thyristor (LTT) has an optically sensitive region in its gate, into which electromagnetic radiation (usually infrared) is coupled by an optical fiber. Since no electronic boards need to be provided at the potential of the thyristor in order to trigger it, light-triggered thyristors can be an advantage in high-voltage applications such as HVDC. Light-triggered thyristors are available with in-built over-voltage (VBO) protection, which triggers the thyristor when the forward voltage across it becomes too high; they have also been made with in-built forward recovery protection, but not commercially. Despite the simplification they can bring to the electronics of an HVDC valve, light-triggered thyristors may still require some simple monitoring electronics and are only available from a few manufacturers.
Two common photothyristors include the light-activated SCR (LASCR) and the light-activated TRIAC. A LASCR acts as a switch that turns on when exposed to light. Following light exposure, when light is absent, if the power is not removed and the polarities of the cathode and anode have not yet reversed, the LASCR is still in the "on" state. A light-activated TRIAC resembles a LASCR, except that it is designed for alternating currents.
Thyristor manufacturers generally specify a region of safe firing defining acceptable levels of voltage and current for a given operating temperature. The boundary of this region is partly determined by the requirement that the maximum permissible gate power (PG), specified for a given trigger pulse duration, is not exceeded.[10]
As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including:
Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automatically, referred to as "zero cross" operation. The device can be said to operate synchronously; being that, once the device is triggered, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required, i.e., the device is biased fully on. This is not to be confused with asymmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.
Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.
They can also be found in power supplies for digital circuits, where they are used as a sort of "enhanced circuit breaker" to prevent a failure in the power supply from damaging downstream components. A thyristor is used in conjunction with a Zener diode attached to its gate, and if the output voltage of the supply rises above the Zener voltage, the thyristor will conduct and short-circuit the power supply output to ground (in general also tripping an upstream breaker or fuse). This kind of protection circuit is known as a crowbar, and has the advantage over a standard circuit breaker or fuse in that it creates a high-conductance path to ground from damaging supply voltage and potentially for stored energy (in the system being powered).
The first large-scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early s.[clarification needed] The stabilized high voltage DC supply for the receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the AC supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation). The precise switching point was determined by the load on the DC output supply, as well as AC input fluctuations.
Thyristors have been used for decades as light dimmers in television, motion pictures, and theater, where they replaced inferior technologies such as autotransformers and rheostats. They have also been used in photography as a critical part of flashes (strobes).
Thyristors can be triggered by a high rise-rate of off-state voltage. Upon increasing the off-state voltage across the anode and cathode of the thyristor, there will be a flow of charges similar to the charging current of a capacitor. The maximum rate of rise of off-state voltage or dV/dt rating of a thyristor is an important parameter since it indicates the maximum rate of rise of anode voltage that does not bring thyristor into conduction when no gate signal is applied. When the flow of charges due to rate of rise of off-state voltage across the anode and cathode of the thyristor becomes equal to the flow of charges as injected when the gate is energized then it leads to random and false triggering of thyristor which is undesired.[11]
This is prevented by connecting a resistor-capacitor (RC) snubber circuit between the anode and cathode in order to limit the dV/dt (i.e., rate of voltage change over time). Snubbers are energy-absorbing circuits used to suppress the voltage spikes caused by the circuit's inductance when a switch, electrical or mechanical, opens. The most common snubber circuit is a capacitor and resistor connected in series across the switch (transistor).
Since modern thyristors can switch power on the scale of megawatts, thyristor valves have become the heart of high-voltage direct current (HVDC) conversion either to or from alternating current. In the realm of this and other very high-power applications,[2]: 12 both electrically triggered (ETT) and light-triggered (LTT) thyristors[12][13] are still the primary choice. Thyristors are arranged into a diode bridge circuit and to reduce harmonics are connected in series to form a 12-pulse converter. Each thyristor is cooled with deionized water, and the entire arrangement becomes one of multiple identical modules forming a layer in a multilayer valve stack called a quadruple valve. Three such stacks are typically mounted on the floor or hung from the ceiling of the valve hall of a long-distance transmission facility.[14][15]
The functional drawback of a thyristor is that, like a diode, it only conducts in one direction so it cannot be safely used with AC current. A similar self-latching 5-layer device, called a TRIAC, is able to work in both directions. This added capability, though, also can become a shortfall. Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn off during the zero-voltage instants of the AC power cycle. Because of this, use of TRIACs with (for example) heavily inductive motor loads usually requires the use of a "snubber" circuit around the TRIAC to assure that it will turn off with each half-cycle of mains power. Inverse parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike TRIACs, are sure to turn off. The "price" to be paid for this arrangement, however, is the added complexity of two separate, but essentially identical gating circuits.
Although thyristors are heavily used in megawatt-scale rectification of AC to DC, in low- and medium-power (from few tens of watts to few tens of kilowatts) applications they have virtually been replaced by other devices with superior switching characteristics like power MOSFETs or IGBTs. One major problem associated with SCRs is that they are not fully controllable switches. The GTO thyristor and IGCT are two devices related to the thyristor that address this problem. In high-frequency applications, thyristors are poor candidates due to long switching times arising from bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).
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