Circuit symbol for a thyristor

An SCR rated about 100 amperes, 1200 volts mounted on a heat sink - the two small wires are the gate trigger leads

The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. An electronic symbol is a Pictogram used to represent various Electrical and electronic devices (such as Wires batteries, Resistors Solid-state Electronic components devices and systems are based entirely on the Semiconductor, such as Transistors Microprocessor chips and Semiconductor devices are Electronic components that exploit the electronic properties of Semiconductor materials principally Silicon, Germanium An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order They act as a switch, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed).

Some sources define silicon controlled rectifiers and thyristors as synonymous;^[1] others define SCRs as a subset of thyristors, along with gate turn-off thyristor (GTO), triode ac switch (triac), static induction transistor (SIT), static induction thyristor (SITH) and MOS-controlled thyristor (MCT). A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. Among the latter, the International Electrotechnical Commission 60747-6 standard stands out. The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes

Non-SCR thyristors include devices with more than four layers, such as triacs and DB-GTOs. A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two Silicon-controlled rectifiers ( SCRs A gate turn-off thyristor (GTO is a special type of Thyristor, a high-power Semiconductor device. ^[2]

Function

The thyristor is a four-layer semiconducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to 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 the self-latching action:

Thyristors have three states:

1. Reverse blocking mode -- Voltage is applied in the direction that would be blocked by a diode
2. Forward blocking mode -- Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
3. 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"

Function of the gate terminal

The thyristor has three p-n junctions (serially named J₁, J₂, J₃ from the anode). A bipolar (junction transistor ( BJT) is a type of Transistor. A p-n junction is a junction formed by combining P-type and N-type Semiconductors together in very close contact

Layer Diagram of Thyristor

When the anode is at a positive potential V_AK 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 V_AK is increased beyond the breakdown voltage V_BO of the thyristor, avalanche breakdown of J₂ takes place and the thyristor starts conducting (On state). Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials

If a positive potential V_G is applied at the gate terminal with respect to the cathode, the breakdown of the junction J₂ occurs at a lower value of V_AK. By selecting an appropriate value of V_G, the thyristor can be switched into the on state immediately.

It should be noted that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until either: (a) the potential V_G is removed or (b) the current through the device (anode−cathode) is less than the holding current specified by the manufacturer. Hence V_G can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator. A unijunction transistor ( UJT) is an electronic Semiconductor device that has only one junction A relaxation oscillator is an oscillator in which a Capacitor is charged gradually and then discharged rapidly

These gate pulses are characterized in terms of gate trigger voltage (V_GT) and gate trigger current (I_GT). 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. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction.

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 conduct), providing the anode current has exceeded the latching current (I_L). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (I_H).

V - I Characteristics

A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.

After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can be positively biased in the off-state. This minimum delay is called the circuit commutated turn off time (t_Q). 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. An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J In the Solid state physics of Semiconductors carrier generation and recombination are processes by which mobile Electrons and Electron holes

For applications with frequencies higher than the domestic AC mains supply (e. g. 50 Hz or 60 Hz), thyristors with lower values of t_Q are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 Platinum (ˈplætɪnəm is a Chemical element with the Atomic symbol Pt and an Atomic number of 78 Alternatively, fast thyristors may be made by neutron irradiation of the silicon. This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. Irradiation is the process by which an item is exposed to Radiation.

History

1956 The Silicon Controlled Rectifier (SCR) or Thyristor proposed by William Shockley in 1950 and championed by Moll and others at Bell Labs was developed first by power engineers at General Electric (G. Year 1956 ( MCMLVI) was a Leap year starting on Sunday (link will display full calendar of the Gregorian calendar. William Bradford Shockley ( February 13, 1910 &ndash August 12, 1989) was a British -born American Physicist Year 1950 ( MCML) was a Common year starting on Sunday (link will display the full calendar of the Gregorian calendar. Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) is the Research organization E. ) led by Gordon Hall and commercialised by G. E. 's Frank W. "Bill" Gutzwiller.

Applications

A bank of six, 2000 A Thyristors (white pucks). The clear tubes are for cooling water

Load voltage regulated by thyristor phase control.
Red trace: load voltage
Blue trace: trigger signal

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 automatically switch off; referred to as Zero Cross operation. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant A zero cross circuit is an Electrical circuit that starts operation with the AC load voltage at close to zero-phase The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical 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. Phase control ( PFC) also called phase cutting, is a method of Pulse width modulation (PWM for power limiting applied to AC voltages

Thyristors can also be found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see A circuit breaker is an automatically-operated electrical Switch designed to protect an electrical circuit from damage caused by overload or The thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor conducts, shorting the power supply output to ground (and in general blowing an upstream fuse). A Zener diode is a type of Diode that permits current in the forward direction like a normal diode but also in the reverse direction if the voltage is larger In Electronics and Electrical engineering a fuse (short for fusible link) is a type of Overcurrent protection device

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 1970s. The DIAC, or diode for alternating current, is a bidirectional trigger Diode that conducts current only after its Breakdown voltage has been Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic This article is about the Decade 1970-1979 For the Year 1970 see 1970. 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 output d. c. supply as well fluctuations on the input a. c. supply. They proved to be unpopular with the a.c. grid power supplier companies because the simultaneous switching of many television receivers, all at approximately the same time, introduced asymmetry into the supply waveform and, as a consequence injected d. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power c. back into the grid with a tendency towards saturation of transformer cores and overheating. Thyristors were largely phased out in this kind of application by the end of the decade.

Thyristors have been used for decades as lighting dimmers in television, motion pictures, and theater, where they replaced inferior technologies such as autotransformers and rheostats. Dimmers are devices used to vary the brightness of a light. By decreasing or increasing the RMS voltage and hence the mean power to the lamp it is possible Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic Theatre (or theater, see spelling differences) is the branch of the Performing arts defined by Bernard Beckerman as what "occurs when one An autotransformer (sometimes called autoformer) is an electrical Transformer with only one winding. A potentiometer is a three-terminal Resistor with a sliding contact that forms an adjustable Voltage divider. They have also been used in photography as a critical part of flashes (strobes).

Snubber circuits

Because thyristors can be triggered on by a high rate of rise of off-state voltage, in many applications this is prevented by connecting a resistor-capacitor (RC) snubber circuit between the anode and cathode terminals in order to limit the dV/dt (i. |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors A snubber is a device used to suppress ("snub" voltage transients in electrical systems pressure transients in fluid systems or excess force or rapid movement in mechanical e. , rate of change of voltage versus time).

Comparisons to other devices

The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a TRIAC, is able to work in both directions. A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two Silicon-controlled rectifiers ( SCRs 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. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant 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. In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting A snubber is a device used to suppress ("snub" voltage transients in electrical systems pressure transients in fluid systems or excess force or rapid movement in mechanical 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.

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. A gas-filled tube, also known as a discharge tube, is an arrangement of Electrodes in a Gas within an insulating, temperature-resistant Envelope A thyratron is a type of Gas filled tube used as a high Energy electrical Switch and controlled Rectifier. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals

Modern thyristors can switch large amounts of power (up to megawatts). In the realm of very high power applications, they are still the primary choice. HVDC or high-voltage Direct current Electric power transmission systems contrast with the more common Alternating current systems as a means However, in low and medium power (from few tens of watts to few tens of kilowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBTs. A Power MOSFET is a specific type of Metal Oxide Semiconductor Field-Effect Transistor ( MOSFET) designed to handle large power The insulated-gate bipolar transistor or IGBT is a three-terminal Power semiconductor device, noted for high efficiency and fast switching One major problem associated with SCRs is that they are not fully controllable switches. The GTO (Gate Turn-off Thyristor) and IGCT are two related devices which address this problem. A gate turn-off thyristor (GTO is a special type of Thyristor, a high-power Semiconductor device. The Integrated Gate Commutated Thyristor (IGCT is a new high-power Semiconductor device. In high-frequency applications, thyristors are poor candidates due to large 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). Majority carrier denotes the Charge carrier which is primarily responsible for current transport in a piece of Semiconductor.

Failure modes

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).

Silicon carbide thyristors

In recent years, some manufacturers^[3] have developed thyristors using Silicon carbide (SiC) as the semiconductor material. Silicon carbide ( is a compound of Silicon and Carbon bonded together to form Ceramics but it also occurs in nature as the extremely rare mineral These have applications in high temperature environments, being capable of operating at temperatures up to 350 °C.

Types of thyristor

• SCR — Silicon Controlled Rectifier
• ASCR — Asymmetrical SCR
• RCT — Reverse Conducting Thyristor
• LASCR — Light Activated SCR, or LTT — Light Triggered Thyristor
• DIAC & SIDAC — Both forms of trigger devices
• BOD — Breakover Diode — A gateless thyristor triggered by avalanche current, used in protection applications
• TRIAC — Triode for Alternating Current — A bidirectional switching device containing two thyristor structures
• GTO — Gate Turn-Off thyristor
• IGCT — Integrated Gate Commutated Thyristor
  • MA-GTO — Modified Anode Gate Turn-Off thyristor
  • DB-GTO — Distributed Buffer Gate Turn-Off thyristor
• MCT — MOSFET Controlled Thyristor — It contains two additional FET structures for on/off control. A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. Reverse conducting thyristor (RCT is high-power switching Semiconductor device similar to Thyristor with integrated reverse Diode. The DIAC, or diode for alternating current, is a bidirectional trigger Diode that conducts current only after its Breakdown voltage has been The DIAC, or diode for alternating current, is a bidirectional trigger Diode that conducts current only after its Breakdown voltage has been An avalanche diode is a Diode (usually made from Silicon, but can be made from another Semiconductor) that is designed to go through Avalanche breakdown A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two Silicon-controlled rectifiers ( SCRs A gate turn-off thyristor (GTO is a special type of Thyristor, a high-power Semiconductor device. The Integrated Gate Commutated Thyristor (IGCT is a new high-power Semiconductor device. A gate turn-off thyristor (GTO is a special type of Thyristor, a high-power Semiconductor device. MOS Controlled Thyristor (or MCT) is voltage controlled fully controllable Thyristor. The field-effect transistor (FET is a type of Transistor that relies on an Electric field to control the shape and hence the conductivity of a 'channel'
  • BRT — Base Resistance Controlled Thyristor
• SITh — Static Induction Thyristor, or FCTh — Field Controlled Thyristor containing a gate structure that can shut down anode current flow. The static induction thyristor ( SI-thyristor, SITh) is a Thyristor with a buried gate structure in which the gate electrodes are placed in n-base region

The GTO is a tri state device. with an 8-function setup. it also has an equation: v=j-o x n/n o

• LASS — Light Activated Semiconducting Switch

References

Footnotes

Bibliography

• General Electric Corporation, SCR Manual, 6th edition, 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

See also

• Thyristor tower
• Latchup