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THYRISTOR 

A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse,and continue to conduct while they are forward biased (that is, while the voltage across thedevice is not reversed).

Thyristor discovery 

The idea for the thyristor was first described by Shockley in 1950. It was referred to as a

bipolar transistor with a p-n hook-collector. The mechanism for the operation of the thyristor

was analysed further in 1952 by Ebers.

Then in 1956 Moll investigated the switching mechanism of the thyristor. Development

continued and more was learned about the device such that the first silicon controlled

rectifiers became available in the early 1960s where it started to gain a significant level of 

popularity for power switching.

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.[7]

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) 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) andIGCT are two related devices 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 (only majority carriers carry the current).

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Function and PARTS 

The thyristor is a four-layer, three terminal semiconducting 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 the full four layers, and the

control terminal, called the gate, is attached to p-type material near to 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 the self-latching 

action:

Structure on the physical and electronic level, and the thyristor symbol.

Thyristors have three states:

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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).

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 V AK is increased beyond

the breakdown voltage V BO of the thyristor, avalanche breakdown of J2takes 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 suddenly.

Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until: (a) the potential V AK is removed or (b) the current through the device

(anode−cathode) is less than the holding current specified by the manufacturer.

Hence V G can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator .

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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

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.e. does not need a continuous supply of gate current to

conduct), 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 again be positively biased and retain the thyristor 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. 50 Hz or 

60 Hz), thyristors with lower values of t Q are required. Such fast thyristors are made by

diffusing into the silicon heavy metals ions such as gold or platinum which act as charge

combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the

silicon.

SAMPLE SYMBOL AND SAMPLE PIC

SCRs or Silicon Controlled Rectifiers are members of the electronic activecomponent family. They are also called Thyristors.

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The figure on the left shows the standard electronic symbol of an SCR. It showsthe three lead pin outs of the part, the upper one being the anode, the lower onethe cathode, and the central extension the gate. The symbol quite resembles anordinary rectifier diode symbol having an extra lead from the cathode side.Though SCRs are much different from diodes, they too rectify AC in response toDC electrical triggers on their gate inputs.

As you can see in the actual picture of an SCR on the right, it looks like atransistor. Externally they may look exactly like transistors, but are entirelydifferent as far as technical specifications are concerned.

Both act as switching devices, although SCRs comfortably handle high voltageACs, whereas transistors normally are dedicated for low voltage DC applications.The lead orientation specifies the first lead from the right to be the gate, theextreme left is the cathode and the center pin is the anode. The gate and theanode leads always work with respect to the ground; the cathode lead isspecified to be connected with the ground and serves as the common releaseterminal for the gate as well as the anode. The load that needs to be operated isconnected across the AC input and the anode of the SCR.

How SCRs Function

Unlike transistors, which may show an exponentially varying output current

pattern, equivalent to the applied input switching current, SCRs have specifictriggering levels below which they may not conduct properly. However, once thetrigger level crosses the optimal value, an SCR may swing into full conduction.

Another typical property associated with SCRs is their “latching” behavior withDC operated loads, where the anode to cathode conduction through the loadlatches or “holds-on” even after the gate trigger is inhibited. However, with ACoperated loads the above drawback, or rather benefit, is not available and the

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load is switched ON or OFF exactly in response to the switching of the SCR’sgate triggers.

TYPES OF THYRISTORS

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

GTO — Gate Turn-Off thyristor 

IGCT — Integrated Gate Commutated Thyristor 

DB-GTO — Distributed Buffer Gate Turn-Off thyristor 

MA-GTO — Modified Anode Gate Turn-Off thyristor 

LASCR — Light Activated SCR, or LTT — Light triggered thyristor 

LASS — Light Activated Semiconducting Switch

MCT — MOSFET Controlled Thyristor — It contains two additional FET structures for 

on/off control.

BRT — Base Resistance Controlled Thyristor* RCT — Reverse Conducting

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 

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

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Reverse conducting thyristor

A reverse conducting thyristor (RCT) 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 infrequency changers and inverters.

• A Silicon-Controlled Rectifier , or SCR, is essentially a Shockley diode with

an extra terminal added. This extra terminal is called the gate, and it is used

to trigger the device into conduction (latch it) by the application of a small

voltage.

• To trigger, or fire, an SCR, voltage must be applied between the gate and

cathode, positive to the gate and negative to the cathode. When testing an

SCR, a momentary connection between the gate and anode is sufficient inpolarity, intensity, and duration to trigger it.

• SCRs may be fired by intentional triggering of the gate terminal, excessive

voltage (breakdown) between anode and cathode, or excessive rate of 

voltage rise between anode and cathode. SCRs may be turned off by anode

current falling below the holding current value (low-current dropout), or by

"reverse-firing" the gate (applying a negative voltage to the gate). Reverse-

firing is only sometimes effective, and always involves high gate current.

• A variant of the SCR, called a Gate-Turn-Off thyristor (GTO), is specifically

designed to be turned off by means of reverse triggering. Even then, reverse

triggering requires fairly high current: typically 20% of the anode current.

• SCR terminals may be identified by a continuity meter: the only two

terminals showing any continuity between them at all should be the gate and

cathode. Gate and cathode terminals connect to a PN junction inside the SCR,

so a continuity meter should obtain a diode-like reading between these two

terminals with the red (+) lead on the gate and the black (-) lead on the

cathode. Beware, though, that some large SCRs have an internal resistor

connected between gate and cathode, which will affect any continuity

readings taken by a meter.

• SCRs are true rectifiers: they only allow current through them in one

direction. This means they cannot be used alone for full-wave AC power

control.

• If the diodes in a rectifier circuit are replaced by SCRs, you have the

makings of a controlled rectifier circuit, whereby DC power to a load may be

time-proportioned by triggering the SCRs at different points along the AC

power waveform.

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Introduction to Diac-Operation and Construction

A diac is an important member of the thyristor family and is usually employed for triggering triacs. A diac is atwo-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. This is just like a triac without gate terminal, as shown in figure. Its equivalentcircuit is a pair of inverted four layer diodes. Two schematic symbols are shown in figure. Again the terminal

designations are arbitrary since the diac, like triac, is also a bilateral device. The switching from off-state toon-state is achieved by simply exceeding the avalanche break down voltage in either direction.

Construction of a Diac.

A diac is a P-N-P-N structured four-layer, two-terminal semiconductor device, as shown in figure.A. MT2 andMTX are the two main terminals of the device. There is no control terminal in this device. From the diagram,a diac unlike a diode, resembles a bipolar junction transistor (BJT) but with the following exceptions.

• there is no terminal attached to the middle layer (base),

• the three regions are nearly identical in size,

• the doping level at the two end P-layers is the same so that the device gives symmetrical switchingcharacteristics for either polarity of the applied voltage.

DIAC Circuit Symbol

Operation of a Diac.

When the terminal MT2 is positive, the current flow path is P1-N2-P2-N3 while for positive polarity of terminalMT1 the current flow path is P2-N2-P1-N1. The operation of the diac can be explained by imagining it as twodiodes connected in series. When applied voltage in either polarity is small (less than breakover voltage) avery small amount of current, called the leakage current, flows through the device. Leakage current causeddue to the drift of electrons and holes in the depletion region, is not sufficient to cause conduction in thedevice. The device remains in non-conducting mode. However, when the magnitude of the applied voltageexeeds the avalanche breakdown voltage, breakdown takes place and the diac current rises sharply, as

shown in the characteristics shown in figure.

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Diac Characteristics

Characteristics of a Diac

Volt-ampere characteristic of a diac is shown in figure. It resembles the English letter Z because of thesymmetrical switching characteristics for either polarity of the applied voltage.

The diac acts like an open-circuit until its switching or breakover voltage is exceeded. At that point the diacconducts until its current reduces toward zero (below the level of the holding current of the device). The diac,because of its peculiar construction, does not switch sharply into a low voltage condition at a low currentlevel like the SCR or triac. Instead, once it goes into conduction, the diac maintains an almost continuousnegative resistance characteristic, that is, voltage decreases with the increase in current. This means that,unlike the SCR and the triac, the diac cannot be expected to maintain a low (on) voltage drop until its currentfalls below a holding current level.

What is a Triac?

Triacs are widely used in AC power control applications. They are able to switch high voltagesand high levels of current, and over both parts of an AC waveform. This makes triac circuits idealfor use in a variety of applications where power switching is needed. One particular use of triaccircuits is in light dimmers for domestic lighting, and they are also used in many other power control situations including motor control.

The triac is a development of the thyristor. While the thyristor can only control current over onehalf of the cycle, the triac controls it over two halves of an AC waveform. As such the triac can beconsidered as a pair of parallel but opposite thyristors with the two gates connected together and

the anode of one device connected to the cathode of the other, etc..

Triac symbol 

The basic triac symbol used on circuit diagram indicates its bi-directional properties. The triacsymbol can be seen to be a couple of thyristor symbols in opposite senses merged together.

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Triac symbol for use in circuit diagrams

Like a thyristor, a triac has three terminals. However the names of these are a little more difficultto assign, because the main current carrying terminals are connected to what is effectively acathode of one thyristor, and the anode of another within the overall device. There is a gate whichacts as a trigger to turn the device on. In addition to this the other terminals are both calledAnodes, or Main Terminals These are usually designated Anode 1 and Anode 2 or Main Terminal1 and Main Terminal 2 (MT1 and MT2). When using triacs it is both MT1 and MT2 have very

similar properties.

How does a triac work? 

Before looking at how a triac works, it helps to have an understanding of haow a thyristor works.In this way the basic concepts can be grasped for the simpler device and then applied to a triacwhich is more complicated. The operation of the thyristor is covered in the article in this sectionand accessible through the "Related Articles" box on the left of the page and below the mainmenu.

For the operation of the triac, it can be imagined from the circuit symbol that the triac consists of two thyristors in parallel but around different ways. The operation of the triac can be looked on inthis fashion, although the actual operation at the semiconductor level is rather more complicated.

Equivalent circuit of a triac

When the voltage on the MT1 is positive with regard to MT2 and a positive gate voltage isapplied, one of the thyristors conducts. When the voltage is reversed and a negative voltage isapplied to the gate, the other thyristor conducts. This is provided that there is sufficient voltageacross the device to enable a minimum holding current to flow.

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Using triacs

there are a number of points to note when using triacs. Although these devices operate very well,to get the best performance out of them it is necessary to understand a few hints on tips on usingtriacs.

It is found that because of their internal construction and the slight differences between the twohalves, triacs do not fire symmetrically. This results in harmonics being generated: the lesssymmetrical the triac fires, the greater the level of harmonics that are produced. It is not normallydesirable to have high levels of harmonics in a power system and as a result triacs are notfavoured for high power systems. Instead for these systems two thyristors may be used as it iseasier to control their firing.

To help in overcoming the problem non-symmetrical firing ad the resulting harmonics, a deviceknown as a diac (diode AC switch) is often placed in series with the gate of the triac. Theinclusion of this device helps make the switching more even for both halves of the cycle. Thisresults from the fact that the diac switching characteristic is far more even than that of the triac.Since the diac prevents any gate current flowing until the trigger voltage has reached a certain

voltage in either direction, this makes the firing point of the triac more even in both directions.

Overview of using triacs

Triacs are ideal devices for use in many AC small power applications. Triac circuits for use asdimmers are widespread and they are simple and easy to implement. When using triacs, diacsare often included in the circuit as mentioned above to help reduce the level of harmonicsproduced.

• A TRIAC acts much like two SCRs connected back-to-back for bidirectional

(AC) operation.

• TRIAC controls are more often seen in simple, low-power circuits than

complex, high-power circuits. In large power control circuits, multiple SCRs

tend to be favored.

• When used to control AC power to a load, TRIACs are often accompanied

by DIACs connected in series with their gate terminals. The DIAC helps the

TRIAC fire more symmetrically (more consistently from one polarity to

another).

• Main terminals 1 and 2 on a TRIAC are not interchangeable.

• To successfully trigger a TRIAC, gate current must come from the main

terminal 2 (MT2) side of the circuit!

Other types of thyristor 

There is a number of different types thyristor - these are variants of the basic thyristor

component, but they offer different capabilities that can be used in various instances and may

be useful for certain circuits.

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• Reverse conducting thyristor, RCT: Although thyristors normally block current in

the reverse direction, there is a form of thyristor called a reverse conducting thyristor.

This has an integrated reverse diode to provide conduction in the reverse direction,

although there is no control in this direction.

Within a reverse conducting thyristor, the thyristor itself and the diode do not conductat the same time. This means that they do not produce heat simultaneously. As a

result they can be integrated and cooled together.

The reverse conducting thyristor can be used where a reverse or freewheel diode

would otherwise be needed. Reverse conducting thyristors are often used in frequency

changers and inverters.

• Gate Assisted Turn-Off Thyristor, GATT: The GATT is used in circumstances

where a fast turn-off is needed. To assist in this process a negative gate voltage can

sometimes be applied. In addition to reducing the anode cathode voltage. This reverse

gate voltage helps in draining the minority carriers stored on the n-type base region

and it ensures that the gate-cathode junction is not forward biased.

The structure of the GATT is similar to that of the standard thyristor, except that the

narrow cathode strips are often used to enable the gate to have more control because

it is closer to the centre of the cathode.

• Gate Turn-Off Thyristor, GTO: The gate turn-off thyristor is sometimes also

referred to as the gate turn off switch. This device is unusual in the thyristor family

because it can be turned off by simply applying a negative voltage to the gate - there

is no requirement to remove the anode cathode voltage. See further page in this

series more fully describing the GTO.

•  Asymmetric Thyristor: The asymmetric thyristor is used in circuits where the

thyristor does not see a reverse voltage and therefore the rectifier capability is not

needed. As a result it is possible to make the second junction, often referred to as J2(see page on Thyristor structure) can be made much thinner. The resulting n-base

region provides a reduced Von as well as improved turn on time and turn off time.

DIFFERENCE OF THYRISTORS AND RELAYS

Triac versus Relay 

Triacs are solid-state devices, whereas relays are electromechanical devices. Triacs

can switch both AC and DC, but as XTL said, they will not stop the current flow 

unless the current between MT1 and MT2 falls below a threshold level, or you

forcibly commutate the device off.

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(note: I spent 13 years in industrial motor control, we designed equipment which

switched up to many thousands of Amps and many thousands of Volts through

thyristors.)

Relays are pretty simple devices to use; you energize the coil and the contacts engage.

 You de-energize the coil and the contacts open. A simple transistor can drive it, but you'll want some snubbering (a reverse-biased diode across the relay coil at a

minimum) to prevent your transistor from dying due to inductive kickback. Your

control signal and your controlled signal are completely isolated from one another.

Relay contacts aren't invincible; if you open up the contacts under load you can cause

them to "ice up" (meaning they won't open up). Also, if you use a relay rated for

power and try to switch small signals, the contacts can eventually get dirty and you

 won't get a good connection between the contacts.

Triacs (and their unidirectional equivalent, SCRs), being solid-state, are mostly 

silent. Unless you use a pulse transformer or optoisolator, your control circuit will beat the potential of your controlled circuit (generally the Neutral for your 120/220V 

circuits). Thyristors can be used to phase-control a load, meaning you can dim lights

or (roughly) control the speed of an AC motor. This is pretty much impossible with

relays. You can also do neat tricks like allowing only 'x' entire cycles through to do

less "noisy" phase control. SCRs are also good for dumping all the energy in a

capacitor into a load (flash or railgun type applications). Some power supplies use

SCRs as crowbar devices as well; they turn on and short out the supply (blowing the

fuse in the process), protecting the load from an overvoltage.

Thyristors don't really enjoy sharp voltage or current spikes when they're turned off;

these can cause them to turn on by accident or can destroy the devices. Simplesnubbering helps control these failure modes.

Thyristors also don't completely isolate the load from the source; if you measure the

 voltage on a load with the thyristor off, you'll measure full voltage. They thyristor is

off, but off doesn't mean "open" -- it means "high resistance". This can cause trouble

 with some applications.

If you are switching an AC signal, thyristors are pretty painless; they will shut

themselves off around the next zero crossing. If you're controlling DC... again... you

have more to think about. DC is also problematic for relays because you will almost

always be opening up the relay contacts under load, so you must size your relay forthis.

Long story short: Yes, triacs can replace SCRs in almost every application. If you

don't want to bother with the snubbering and isolation you can always buy solid state

relays; they're triacs with the appropriate control circuitry to make them work almost

the same as relays.

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APPLICATIONS OF THYRISTORS

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 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.

They can also be found in power supplies for digital circuits, where they are used as a sort of 

"circuit breaker " or "crowbar" to prevent a failure in the power supply from damaging downstreamcomponents. A 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 will conduct, then

short-circuit 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 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 (if the rising slope was used the output voltage would always rise towards the peak inputvoltage when the device was triggered and thus defeat the aim of regulation). The precise

switching point was determined by the load on the output DC supply as well fluctuations on the

input AC supply.

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 (strobes).

Snubber circuitsThyristors can be triggered by a high rate of rise of off-state voltage. This is prevented by

connecting a resistor -capacitor  (RC) snubber circuit between the anode and cathode terminals in

order to limit the dV/dt (i.e., rate of change of voltage versus time).

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Powered Shockley diode equivalent circuit.

Both the series inductor and parallel resistor-capacitor “snubber” circuit help

minimize the Shockley diode's exposure to excessively rising voltage.

The voltage rise limit of a Shockley diode is referred to as the critical rate of voltage

rise. Manufacturers usually provide this specification for the devices they sell.

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HVDC electricity transmission

Two of three thyristor valve stacks used for long distance transmission of power from Manitoba Hydrodams

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 electronically switched

(ETT) and light switched (LTT) 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 Graetz bridge circuit and to avoid harmonics are connected in series to form a

12 pulse converter. Each thyristor is cooled with deionized water , and the entire arrangementbecomes one of multiple identical modules forming a layer in a multilayer valve stack called

a quadruple valve. Three such stacks are typically hung from the ceiling of the valve building of a

long distance transmission facility

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a Graetz bridge 

Thyristor applications 

Thyristors, SCRs are used in many areas of electronics where they find uses in a variety of 

different applications. Some of the more common applications for thyristors are outlined

below:

• AC power control (including lights, motors,etc).

• Overvoltage protection crowbar for power supplies.

• AC power switching.

• Control elements in phase angle triggered controllers.

• Within photographic flash lights where they act as the switch to discharge a stored

voltage through the flash lamp, and then cut it off at the required time.

Thyristors are able to switch high voltages and withstand reverse voltages making them ideal

for switching applications, especially within AC scenarios.

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).

Switch on dv/dt — the thyristor can be spuriously fired without trigger 

from the gate if the rate of rise of voltage anode to cathode is too great.

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