Thyristor

From Free net encyclopedia

Image:Thyristor circuit symbol.png 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 synonymous¹; others define SCRs as a subset of thyristors that includes devices with more than four layers, such as triacs and GTOs².

Contents 1 Function 2 Function of the gate terminal 3 Applications 4 Comparisons to other devices 5 Types of thyristors 6 References 7 See also

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, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a thyristor can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.

Function of the gate terminal

The thyristor has three p-n junctions (serially named J₁, J₂, J₃ from the anode). Image:Thyristor.JPG 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 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.

Applications

Image:Nelson Bipole Thyristors.JPG 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).

Comparisons to other devices

Image:SCR1369.jpg 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 the triac 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).

Types of thyristors

• Silicon controlled rectifier
• LASCRs
• Distributed Buffer - Gate Turn-off Thyristor (DB-GTO)
• Triac, a bidirectional switching device containing two thyristor structures
• Gate turn-off thyristor (GTO thyristor)
• Mosfet controlled thyristor (MCT), two additional FET structures for on and off control.
• Modified anode - gate turn-off thyristor (MA-GTO)
• Static induction thyristor (SITh) or Field controlled thyristor (FCTh), a gate structure can shut down anode current flow.

References

1. Christiansen, Donald; Alexander, Charles K. (2005); Standard Handbook of Electrical Engineering (5th ed.). McGraw-Hill, ISBN 0-07-138421-9
2. Dorf, Richard C., editor (1997), Electrical Engineering Handbook (2nd ed.). CRC Press, IEEE Press, Ron Powers Publisher, ISBN 0-8492-8574-1

See also

• Thyristor towercs:Tyristor

de:Thyristor et:Türistor es:Tiristor fr:Thyristor hr:Tiristor it:Tiristore nl:Thyristor ja:サイリスタ pl:Tyrystor pt:Tiristor ru:Тиристор fi:Tyristori tr:Tristör