A semiconductor device that acts as a switch; also often called a silicon-controlled rectifier (SCR). It is made of a sandwich of p-n-p-n semiconductor material. A flow of current is initiated by a signal, and the current then becomes independent of the signal. This flow will stop only if the voltage across the thyristor is reversed. A triac is a thyristor which, once triggered, remains on until the voltage across it falls to zero. Thyristors are replacing gas-filled electronic valves.
Some sources define silicon controlled rectifiers and thyristors as synonymous; Non-SCR thyristors include devices with more than four layers, such as triacs and GTOs.
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 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:
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 yet 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
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).
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. 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). By selecting an appropriate value of VG, the thyristor can be switched into the on state immediately. 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 (IL).
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.
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. 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.
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.
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;
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).
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 1970's. The stabilized high voltage d.c 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 a.c supply input.
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.
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.
Modern thyristors can switch large amounts of power (up to megawatts). In high-frequency applications, thyristors are poor candidates due to large switching times arising out of bipolar conduction.
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 &Silicon carbide thyristors
In recent years, some manufacturers have developed thyristors using Silicon carbide (SiC) as the semiconductor material.
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