A thyristor is a two- to four-lead solid-state semiconductor device with four layers of alternating
N and P-type material. They act exclusively as bistable switches, conducting when their
gate receives a current trigger, and continue to conduct while they are forward biased.
A three-lead thyristor is designed to control the larger current of its two leads by combining
that current with the smaller current or voltage of its other lead - known as its control lead.
In contrast, a two-lead thyristor is designed to 'switch on' if the potential difference
between its leads is sufficiently large - a value representing its breakdown voltage.
Some sources define silicon controlled rectifiers and thyristors as synonymous. Other sources
define thyristors as a larger set of devices with at least four layers of alternating N
and P-type material. The first thyristor devices were released
commercially in 1956. Because thyristors can control a relatively large amount of power
and voltage with a small device, they find wide application in control of electric power,
ranging from light dimmers and electric motor speed control to high-voltage direct current
power transmission. Thyristors may be used in power-switching circuits, relay-replacement
circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits,
light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control
circuits, etc. Originally thyristors relied only on current reversal to turn them off,
making them difficult to apply for direct current; newer device types can be turned
on and off through the control gate signal. A thyristor is not a proportional device like
a transistor. In other words, a thyristor can only be fully on or off, while a transistor
can lie in between on and off states. This makes a thyristor unsuitable as an analog
amplifier, but useful as a switch.
Introduction 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.
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:
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
The thyristor has three p-n junctions.
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. Now if VAK is increased beyond
the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and
the thyristor starts conducting. 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. Once avalanche breakdown has occurred, the
thyristor continues to conduct, irrespective of the gate voltage, until: the potential
VAK is removed or the current through the device is less than the holding current specified
by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT
relaxation oscillator. The gate pulses are characterized in terms
of gate trigger voltage and gate trigger current. 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. As long as the anode remains positively biased, it cannot be switched off until the
anode current falls below the holding current. 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 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. Attempting to positively bias the anode within this time causes the thyristor
to be self-triggered by the remaining charge carriers that have not yet recombined.
For applications with frequencies higher than the domestic AC mains supply, thyristors with
lower values of tQ are required. 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. History
The Silicon Controlled Rectifier or Thyristor proposed by William Shockley in 1950 and championed
by Moll and others at Bell Labs was developed in 1956 by power engineers at General Electric
led by Gordon Hall and commercialized by G.E.'s Frank W. "Bill" Gutzwiller.
Etymology 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. 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 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.e. the device is biased fully on. 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. This kind of protection circuit is known as a crowbar,
and has the advantage over a standard circuit breaker or fuse 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 1970s. 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. 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 lighting
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. Snubber circuits
Thyristors can be triggered by a high rise-rate of off-state voltage. This is prevented by
connecting a resistor-capacitor snubber circuit between the anode and cathode terminals in
order to limit the dV/dt. HVDC electricity transmission
Since modern thyristors can switch power on the scale of megawatts, thyristor valves have
become the heart of high-voltage direct current conversion either to or from alternating current.
In the realm of this and other very high power applications, both electrically triggered
and light triggered thyristors are still the primary choice. The valves are arranged in
stacks usually suspended from the ceiling of a transmission building called a valve
hall. Thyristors are arranged into a diode bridge circuit and to reduce harmonics are
connected in series to form a 12 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. 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 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. 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 they have virtually 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 and IGCT are two devices
related to the thyristor, which address this problem. 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.
Failure modes 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, specified for a given trigger pulse duration, is not
exceeded. 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. 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.
Switch on dv/dt — the thyristor can be spuriously fired without trigger from the gate if the
anode-to-cathode voltage rise-rate is too great.
Silicon carbide thyristors In recent years, some manufacturers have developed
thyristors using Silicon carbide as the semiconductor material. These have applications in high
temperature environments, being capable of operating at temperatures up to 350 °C.
Types of thyristor ACS
ACST AGT — Anode Gate Thyristor — A thyristor
with gate on n-type layer near to the anode ASCR — Asymmetrical SCR
BCT — Bidirectional Control Thyristor — A bidirectional switching device containing
two thyristor structures with separate gate contacts
BOD — Breakover Diode — A gateless thyristor triggered by avalanche current
DIAC — Bidirectional trigger device Dynistor — Unidirectional switching device
Shockley diode — Unidirectional trigger and switching device
SIDAC — Bidirectional switching device Trisil, SIDACtor — Bidirectional protection
devices
BRT — Base Resistance Controlled Thyristor ETO — Emitter Turn-Off Thyristor
GTO — Gate Turn-Off thyristor DB-GTO — Distributed buffer gate turn-off
thyristor MA-GTO — Modified anode gate turn-off thyristor
IGCT — Integrated gate-commutated thyristor Ignitor — Spark generators for fire-lighter
ckts LASCR — Light-activated SCR, or LTT — light-triggered
thyristor LASS — light-activated semiconducting switch
Latchup MCT — MOSFET Controlled Thyristor — It
contains two additional FET structures for on/off control.
CSMT or MCS — MOS composite static induction thyristor
PUT or PUJT — Programmable Unijunction Transistor — A thyristor with gate on n-type layer
near to the anode used as a functional replacement for unijunction transistor
RCT — Reverse Conducting Thyristor SCS — Silicon Controlled Switch or Thyristor
Tetrode — A thyristor with both cathode and anode gates
SCR — Silicon Controlled Rectifier SITh — Static Induction Thyristor, or FCTh
— Field Controlled Thyristor — containing a gate structure that can shut down anode
current flow. TRIAC — Triode for Alternating Current — A
bidirectional switching device containing two thyristor structures with common gate
contact Quadrac — special type of thyristor which
combines a DIAC and a TRIAC into a single package.
Reverse conducting thyristor A reverse conducting thyristor has an integrated
reverse diode, so is not capable of reverse blocking. These devices are advantageous where
a reverse or freewheel diode must be used. Because the SCR and diode never conduct at
the same time they do not produce heat simultaneously and can easily be integrated and cooled together.
Reverse conducting thyristors are often used in frequency changers and inverters.
Photothyristors
Photothyristors are activated by light. The advantage of photothyristors is their ability
to be insensitive to electrical signals, which can cause faulty operation in electrically
noisy environments. A light triggered thyristor has an optically sensitive region in its gate,
into which electromagnetic radiation is coupled via an optical fiber. Since no electronic
boards need to be provided at the potential of the thyristor in order to trigger it, light
triggered thyristors can be an advantage in high voltage applications such as HVDC. Light
triggered thyristors are available with in-built over-voltage protection which triggers the
thyristor when the forward voltage across it becomes too high; they have also been made
with in-built forward recovery protection, but not commercially. Despite the simplification
they can bring to the electronics of an HVDC valve, light triggered thyristors may still
require some simple monitoring electronics and have the disadvantage of being available
from only very few manufacturers. Two common photothyristors include the light-activated
SCR and the light-activated triac. A LASCR acts as a switch that turns on when exposed
to light. Following light exposure, when light is absent, if the power is not removed and
the polarities of the cathode and anode have not yet reversed, the LASCR is still in the
'on' state. A light-activated triac resembles a LASCR, except that it is designed for ac
currents. See also
Latchup Quadrac
Thyristor drive Thyristor tower
References
Further reading Wintrich, Arendt; Nicolai, Ulrich; Tursky,
Werner; Reimann, Tobias. [PDF-Version Application Manual 2011]. Nuremberg: Semikron. ISBN 978-3-938843-66-6.
Thyristor Theory and Design Considerations; ON Semiconductor; 240 pages; 2006; HBD855/D.
Dr. Ulrich Nicolai, Dr. Tobias Reimann, Prof. Jürgen Petzoldt, Josef Lutz: Application
Manual IGBT and MOSFET Power Modules, 1. Edition, ISLE Verlag, 1998, ISBN 3-932633-24-5.
SCR Manual; 6th edition; General Electric Corporation; Prentice-Hall; 1979.
External links The Early History of the Silicon Controlled
Rectifier — by Frank William Gutzwiller THYRISTORS — from All About Circuits
Universal thyristor driving circuit Thyristor Resources
Thyristors of STMicroelectronics Thyristor basics