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Introduction[ edit ] The thyristor is a four-layered, three-terminal semiconductor device, with each layer consisting of alternately 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: Structure on the physical and electronic level, and the thyristor symbol. 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" Function of the gate terminal[ edit ] The thyristor has three p-n junctions serially named J1, J2, J3 from the anode.
Layer diagram of thyristor. 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. 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[ edit ] V — I characteristics. In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state i. 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. 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. After the current in a thyristor has extinguished, a finite time delay must elapse before the anode can again 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. 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 bank of six A thyristors white disks arranged in a row at top, and seen edge-on Etymology[ edit ] 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. Red trace: load output voltage Blue trace: trigger voltage.
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.
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 for the 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. 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. Snubber circuits[ edit ] Thyristors can be triggered by a high rise-rate of off-state voltage. The most common snubber circuit is a capacitor and resistor connected in series across the switch transistor.
HVDC electricity transmission[ edit ] Valve hall containing thyristor valve stacks used for long-distance transmission of power from Manitoba Hydro dams 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, both electrically triggered ETT and light-triggered LTT thyristors   are still the primary choice. Thyristors are arranged into a diode bridge circuit and to reduce harmonics are connected in series to form a 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. 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.
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. 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. Failure modes[ edit ] 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. 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[ edit ] In recent years, some manufacturers  have developed thyristors using silicon carbide SiC as the semiconductor material.
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