Thyristor
Type | Active |
---|---|
First production | 1956 |
Pin configuration | anode, gate and cathode |
Electronic symbol | |
A thyristor (
Some sources define "
The first thyristor devices were released commercially in 1956. 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
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.
History
The silicon controlled rectifier (SCR) or thyristor proposed by William Shockley in 1950 and championed by Moll and others at Bell Labs was developed in 1956 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.
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.[1]: 12
In recent years, some manufacturers[3] 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.
Design
The thyristor is a four-layered, three-terminal semiconductor device, with each layer consisting of alternating
Thyristors have three states:
- Reverse blocking mode: Voltage is applied in the direction that would be blocked by a diode
- Forward blocking mode: Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not been triggered into conduction
- Forward conducting mode: The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"
Gate terminal
The thyristor has three
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
.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.
Switching characteristics
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.
For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors can be made by diffusing
Types
- ACS
- ACST
- AGT: Anode Gate Thyristor: A thyristor with gate on n-type layer near to the anode
- ASCR: Asymmetrical SCR
- BCT: Bidirectional Control Thyristor: A bidirectional switching device containing two thyristor structures with separate gate contacts
- BOD: Breakover Diode: A gateless thyristor triggered by avalanche current
- DIAC: Bidirectional trigger device
- Dynistor: Unidirectional switching device
- Shockley diode: Unidirectional trigger and switching device
- SIDAC: Bidirectional switching device
- Trisil, SIDACtor: Bidirectional protection devices
- BRT: Base Resistance Controlled Thyristor
- ETO: Emitter Turn-Off Thyristor[4]
- GTO: Gate Turn-Off thyristor
- DB-GTO: Distributed buffer gate turn-off thyristor
- MA-GTO: Modified anode gate turn-off thyristor
- IGCT: Integrated gate-commutated thyristor
- Ignitor: Spark generators for fire-lighter circuits
- LASCR: Light-activated SCR, or LTT: light-triggered thyristor
- LASS: light-activated semiconducting switch
- MCT: MOSFET Controlled Thyristor: It contains two additional FET structures for on/off control.
- CSMT or MCS: MOS composite static induction thyristor
- PUT or PUJT: Programmable Unijunction Transistor: A thyristor with gate on n-type layer near to the anode used as a functional replacement for unijunction transistor
- RCT: Reverse Conducting Thyristor
- SCS: Silicon Controlled Switch or Thyristor Tetrode: A thyristor with both cathode and anode gates
- SCR: Silicon Controlled Rectifier
- SITh: Static Induction Thyristor, or FCTh: Field Controlled Thyristor: containing a gate structure that can shut down anode current flow.
- TRIAC: Triode for Alternating Current: A bidirectional switching device containing two thyristor structures with common gate contact
- Quadrac: special type of thyristor which combines a DIAC and a TRIAC into a single package.
Reverse conducting thyristor
A reverse conducting thyristor (RCT) has an integrated reverse
Photothyristors
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
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.
Failure modes
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.[5]
As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including:
- Turn on di/dt: in which the rate of rise of on-state current after triggering is higher than can be supported by the spreading speed of the active conduction area (SCRs & triacs).
- Forced commutation: in which the transient peak reverse recovery current causes such a high voltage drop in the sub-cathode region that it exceeds the reverse breakdown voltage of the gate cathode diode junction (SCRs only).
- Switch on dv/dt: the thyristor can be spuriously fired without trigger from the gate if the anode-to-cathode voltage rise-rate is too great.
Applications
Thyristors are mainly used where high currents and voltages are involved, and are often used to control
Thyristors can be used as the control elements for phase angle triggered controllers, also known as
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
Thyristors have been used for decades as light dimmers in
Snubber circuits
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.[6]
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).
HVDC electricity transmission
Since modern thyristors can switch power on the scale of
Comparisons to other devices
The functional drawback of a thyristor is that, like a diode, it only conducts in one direction so it cannot be safely used with
Although thyristors are heavily used in megawatt-scale
See also
- Thyristor-controlled reactor
- Insulated-gate bipolar transistor
- Latch-up
- Quadrac
- Thyratron
- Thyristor drive
References
- ^ OCLC 232176984.
- ISBN 0-07-138421-9
- ^ Example: Silicon Carbide Inverter Demonstrates Higher Power Output Archived 2020-10-22 at the Wayback Machine in Power Electronics Technology (2006-02-01)
- ISBN 978-81-317-0246-8
- ^ "Safe Firing of Thyristors"[permanent dead link] on powerguru.org
- ^ "di/dt and dv/dt Ratings and Protection of SCR or Thyristor". Electronics Mind. 5 December 2021.
- ^ "Chapter 5.1". High Voltage Direct Current Transmission – Proven Technology for Power Exchange (PDF). Siemens. Retrieved 2013-08-04.
- ABB Asea Brown Boveri. Retrieved 2014-01-24.)
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(help - ABB Asea Brown Boveri. Archived from the original on January 22, 2009. Retrieved 2008-12-20.)
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Sources
- Wintrich, Arendt; Nicolai, Ulrich; Tursky, Werner; Reimann, Tobias (2011). Application Manual Power Semiconductors 2011 (PDF) (2nd ed.). Nuremberg: Semikron. ISBN 978-3-938843-66-6. Archived from the original(PDF) on 2013-09-16.
- Thyristor Theory and Design Considerations; ON Semiconductor; 240 pages; 2006; HBD855/D. (Free PDF download)
- Ulrich Nicolai, Tobias Reimann, Jürgen Petzoldt, Josef Lutz: Application Manual IGBT and MOSFET Power Modules, 1. Edition, ISLE Verlag, 1998,
- SCR Manual; 6th edition; General Electric Corporation; Prentice-Hall; 1979.
External links
- The Early History of the Silicon Controlled Rectifier by Frank William Gutzwiller (of G.E.)
- THYRISTORS from All About Circuits
- Universal thyristor driving circuit
- Thyristor Resources (simpler explanation)
- Thyristors of STMicroelectronics
- Thyristor basics