PowerPedia:Thyristor

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Circuit symbol for a thyristor:Electronic symbols are symbols used to represent various electrical and electronic devices in a drawing of an electrical or electronic circuit. Although some convention exists, these symbols can vary from country to country.

The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. 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. Some sources define silicon controlled rectifiers and thyristors as synonymous ^{[1] (http://www.peswiki.com/index.php/PowerPedia:Thyristor#endnote_Christiansen)}; others define SCRs as a subset of thyristors. Non-SCR thyristors include devices with more than four layers, such as triacs and GTOs ^{[2] (http://www.peswiki.com/index.php/PowerPedia:Thyristor#endnote_Dorf)}

Function

The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N-type or P-type material, for example P-N-P-N. The main terminals, labelled 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. 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). 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). 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 must be noted that V_G need not be applied after the avalanche breakdown has occurred. Hence V_G can be a voltage pulse, such as the voltage output from an UJT relaxation oscillator. 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.

Switching characteristics

In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state, 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). 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. For applications with frequencies higher than the domestic AC mains supply (e.g. 50Hz or 60Hz), thyristors with lower values of t_Q are required. Such fast thyristors are made by diffusing into the silicon heavy metal ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.

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.E.) led by Gordon Hall and commercialised by G.E.'s Frank W. "Bill" Gutzwiller.

Applications

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. 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.

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. 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 opens, shorting the power supply output to ground (and in general blowing an upstream fuse).

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.

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. 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.

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. 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. 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. One major problem associated with SCRs is that they are not fully controllable switches. The GTO (Gate Turn-off Thyristor) is another related device which addresses this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising out of bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).

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] (http://www.peswiki.com/index.php/PowerPedia:Thyristor#endnote_Example)} 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.

Types of thyristors

• Silicon controlled rectifier (SCR)
• 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 — a bidirectional switching device containing two thyristor structures
• GTO (thyristor) — gate turn-off thyristor
  • MA-GTO — Modified anode gate turn-off thyristor
  • DB-GTO — Distributed buffer gate turn-off thyristor
• MCT — MOSFET controlled thyristor containing two additional FET structures for on/off control.
  • 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.

Related concepts

Thyristor tower

A thyristor tower (or valve hall) marks the tower-like arrangement in series of switched thyristors in a static inverter valve of a HVDC- station. Thyristor towers in static inverter-resounds on insulators set up or at the cover hung up. As a rule a thyristor tower four valve functions beeinhaltet. For the setting up of a complete static inverter in a HVDC station three thyristor towers are necessary in this case. A valve hall is a building which contains the valves of the static inverters of a HVDC plant. The valves consist of thyristors, or at older plants, mercury arc rectifiers. Mercury arc rectifiers are usually supported by insulators mounted on the floor, while thyristor valves may be either supported by insulators or hung from the roof of the valve hall. The latter required a stronger ceiling structure, however the hall and the static inverter can better survive earthquakes compared to valve structures standing on the floor.

A valve hall is equipped with heating and cooling equipment to control the temperature of the mercury arc rectifiers (which operate best over a narrow temperature range) or thyristors. The valve hall also protects the valves from weather and dust. Several valve assemblies, connected in series for the required terminal voltage, may be installed in the valve hall building. High voltage bushings are supported through the walls of the valve hall, to allow connections between the converter transformers on the one side and the DC switchyard on the other. Beside the valve hall there is often an additional building, in which are control electronics, equipment for valve cooling and valve monitoring, station service power distribution, and amenities for the plant workers. Because very high voltages are present while the inverters are in operation, access to the valve halls is limited while the static inverter is running. The auxiliary control building may have windows to observe the valve hall, but usually the converter is remotely controlled. To protect communication systems from electromagnetic interference, valve hall buildings must have shielding installed to control emission of radio-frequency energy. At some HVDC converters such as at Cabora-Bassa, outdoor valves are installed in oil-immersed containers. At such exceptional plants no valve halls are required.

Latchups

Latchups are inadvertent creations of a low-impedance path between the power supply rails of an electronic component, triggering a parasitic device, which then acts as a short circuit, leading to ceasement of proper function of the part and perhaps even its destruction with the overcurrent. A power cycle is required to correct the situation.

The parasitic structure is usually an equivalent of a thyristor (or SCR), a PNPN structure which acts as a PNP and an NPN transistor stacked on each other. During a latchup when one of the transistors is conducting, the other one begins conducting too. They both keep each other in saturation for as long as the structure is forward-biased and some current flows through it - which usually means until a power-down. The SCR parasitic structure is formed as a part of the totem-pole PMOS and NMOS transistor pair on the output drivers of the gates.

The latchup does not have to happen between the power rails; it can happen any place where the required parasitic structure exists. A spike of positive or negative voltage on an input or output pin of a digital chip. Exceeding the rail voltage by more than a diode drop, is a common cause of a latchup. Another cause is supply voltage exceeding the absolute maximum rating, leading to a breakdown of some internal junction - a common source are transient spikes in power supply. A common problem with circuits with multiple supply voltages that do not come up in the proper order after a power-up, leading to voltages on data lines exceeding the input rating of parts that do not get their supply voltage yet.

Yet another common cause of latchups is ionizing radiation. It is possible to design chips that are latchup-resistant, where layer of insulating oxide (called a trench) surrounds both the NMOS and the PMOS transistors. This breaks the parasitic SCR structure between these transistors. Such parts are important in the cases where the proper sequencing of power and signals can not be guaranteed, eg. in hot swap devices. Most SOI devices are inherently latchup-resistant. Another possibility for a latchup prevention is the Latchup Protection Technology circuit. When a latchup is detected, the LPC circuit shuts down the chip and holds it powered-down for a preset time.

Thyristor drives

A thyristor drive is a motor and controller combination including the drive shaft,where AC supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor. Thyristor drives are very simple and were first introduced in the 1960s. They remained the predominant type of industrial motor controller until the end of the 1980s when the availabilty of low cost electronics led to their replacement by chopper drives for high performance systems and inverters for high reliability with AC motors.

They are still employed in very high power applications, such as locomotives, where the high power capability of the thyristors and the simplicity of the design can make them a more attractive proposition than transistor based controllers. A derivative of the thyristor drive is the simple AC phase controller. This uses a single phase controlled Triac to provide a variable voltage AC output for regulating a universal motor. This is the type of motor speed control most commonly used in domestic appliances such as food mixers and small AC powered tools, such as electric drills.

References and external articles

Citations

• ^  Christiansen, Donald; Alexander, Charles K. (2005); Standard Handbook of Electrical Engineering (5th ed.). McGraw-Hill, ISBN 0-07-138421-9
• ^  Dorf, Richard C., editor (1997), Electrical Engineering Handbook (2nd ed.). CRC Press, IEEE Press, Ron Powers Publisher, ISBN 0-8492-8574-1.
• ^  Example: Silicon Carbide Inverter Demonstrates Higher Power Output (http://powerelectronics.com/news/silicon-carbide-inverter/) in Power Electronics Tecnology (2006-02-01)

General

• General Electric Corporation, SCR Manual, 6th edition, Prentice-Hall, 1979.
• Wikipedia contributors (http://en.wikipedia.org/wiki/Special:Recentchanges), Wikipedia: The Free Encyclopedia. Wikimedia Foundation. <http://en.wikipedia.org>.
• The Early History of the Silicon Controlled Rectifier (http://www.semiconductormuseum.com/Transistors/GE/OralHistories/Gutzwiller/Gutzwiller_Page12.htm) — by Frank William Gutzwiller (of G.E.)
• Latch-up in CMOS designs (http://www.ece.drexel.edu/courses/ECE-E431/latch-up/latch-up.html)
• Analog Devices: Winning the battle against latchup in CMOS analog devices (http://www.analog.com/library/analogDialogue/archives/35-05/latchup/)
• Single-event latchup protection of integrated circuits (http://www.nasatech.com/Briefs/July98/0798ETB2.html)

See also

- PowerPedia main index
- PESWiki home page
- PES Network, Inc. (http:pureenergysystems.com)