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Thyristor Tutorial

Thyristor Basics

In many ways the Silicon Controlled Rectifier, or the Thyristor as it is more commonly known, is similar to

the transistor. It is a multi-layer semiconductor device, hence the silicon part of its name. It reuires a !ate

si!nal to turn it "#, the controlled part of the name and once "# it $ehaves like a rectifyin! diode, the

rectifier part of the name. In fact the circuit sym$ol for the thyristorsu!!ests that this device acts like a

controlled rectifyin! diode.

Thyristor Sym$ol

However, unlike the diode which is a two layer ( P-N ) semiconductor device, or the transistor

which is a three layer ( P-N-P, or N-P-N ) device, theThyristoris a four layer ( P-N-P-N )semiconductor device that contains three PN junctions in series, and is represented y the symol

as shown!

"ike the diode, the Thyristoris a unidirectional device, that is it will only conduct current in one

direction only, ut unlike a diode, the thyristor can e made to operate as either an open-circuit

switch or as a rectifyin# diode dependin# upon how the thyristors #ate is tri##ered! $n otherwords, thyristors can operate only in the switchin# mode and cannot e used for amplification!

The silicon controlled rectifier SCR, is one of several power semiconductor devices alon# with

Triacs (Triode %&'s), iacs (iode %&'s) and *T's (nijunction Transistor) that are all capale

of actin# like very fast solid state %& switches for controllin# lar#e %& volta#es and currents! +ofor the lectronics student this makes these very handy solid state devices for controllin# %&

motors, lamps and for phase control!

The thyristor is a three-terminal device laelled .%node/, .&athode/ and .0ate/ and consistin#

of three PN junctions which can e switched .1N/ and .122/ at an e3tremely fast rate, or it cane switched .1N/ for variale len#ths of time durin# half cycles to deliver a selected amount of

power to a load! The operation of the thyristor can e est e3plained y assumin# it to e madeup of two transistors connected ack-to-ack as a pair of complementary re#enerative switchesas shown!

% Thyristors Two Transistor %nalo#y

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The two transistor e4uivalent circuit shows that the collector current of the NPNtransistor T56feeds directly into the ase of the PNP transistor T57, while the collector current

of T57feeds into the ase of T56! These two inter-connected transistors rely upon each other for

conduction as each transistor #ets its ase-emitter current from the other's collector-emittercurrent! +o until one of the transistors is #iven some ase current nothin# can happen even if an

%node-to-&athode volta#e is present!

8hen the thyristors %node terminal is ne#ative with respect to the &athode, the centre N-

Pjunction is forward iased, ut the two outerP-Njunctions are reversed iased and it ehavesvery much like an ordinary diode! Therefore a thyristor locks the flow of reverse current until at

some hi#h volta#e level the reakdown volta#e point of the two outer junctions is e3ceeded and

the thyristor conducts without the application of a 0ate si#nal!

This is an important ne#ative characteristic of the thyristor, as Thyristorscan e unintentionallytri##ered into conduction y a reverse over-volta#e as well as hi#h temperature or a rapidly

risin#dv/dtvolta#e such as a spike!

$f the %node terminal is made positive with respect to the &athode, the two outerP-Njunctions

are now forward iased ut the centreN-Pjunction is reverse iased! Therefore forward current

is also locked! $f a positive current is injected into the ase of the NPN transistor T56, theresultin# collector current flows in the ase of transistor T57! This in turn causes a collector

current to flow in the PNP transistor, T57which increases the ase current of T56and so on!

Typical Thyristor

9ery rapidly the two transistors force each other to conduct to saturation as they are connected in

a re#enerative feedack loop that can not stop! 1nce tri##ered into conduction, the current

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flowin# throu#h the device etween the %node and the &athode is limited only y the resistance

of the e3ternal circuit as the forward resistance of the device when conductin# can e very low at

less than 7:so the volta#e drop across it and power loss is also low!

Then we can see that a thyristor locks current in oth directions of an %& supply in its .122/state and can e turned .1N/ and made to act like a normal rectifyin# diode y the application of

a positive current to the ase of transistor, T56which for a silicon controlled rectifier is called the.0ate/ terminal!

The operatin# volta#e-current I-%characteristics curves for the operation of a Silicon

Controlled Rectifierare #iven as

Thyristor $-9 &haracteristics &urves

1nce the thyristor has een turned .1N/ and is conductin# in the forward direction (anodepositive), the #ate si#nal looses control due to the re#enerative latchin# action of the two internal

transistors! The application of any #ate si#nals or pulses after re#eneration is initiated will haveno effect at all ecause the thyristor is already conductin# and fully-1N!

nlike the transistor, the +&5 can not e iased to stay within some active re#ion alon# a load

line etween its lockin# and saturation states! The ma#nitude and duration of the #ate .turn-on/pulse has little effect on the operation of the device since conduction is controlled internally!

Then applyin# a momentary #ate pulse to the device is enou#h to cause it to conduct and will

remain permanently .1N/ even if the #ate si#nal is completely removed!

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%t the start of each positive half-cycle the +&5 is .122/! 1n the application of the #ate pulsetri##ers the +&5 into conduction and remains fully latched .1N/ for the duration of the positive

cycle! $f the thyristor is tri##ered at the e#innin# of the half-cycle ( > ? ?

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Thus far we have seen that a thyristor is essentially a half-wave device that conducts in only the

positive half of the cycle when the %node is positive and locks current flow like a diode when

the %node is ne#ative, irrespective of the 0ate si#nal!

But there are more semiconductor devices availale which come under the anner of .Thyristor/that can conduct in oth directions, full-wave devices, or can e turned .122/ y the 0ate

si#nal!

+uch devices include .0ate Turn-122 Thyristors/ (0T1), .+tatic $nduction Thyristors/ (+$TH),

.A1+ &ontrolled Thyristors/ (A&T), .+ilicon &ontrolled +witch/ (+&+), .Triode Thyristors/(T5$%&) and ."i#ht %ctivated Thyristors/ ("%+&5) to name a few, with all these devices

availale in a variety of volta#e and current ratin#s makin# them attractive for use in applications

at very hi#h power levels!

Thyristor +ummary

Silicon Controlled Rectifiersknown commonly as Thyristorsare three-junction PNPN

semiconductor devices which can e re#arded as two inter-connected transistors that can e usedin the switchin# of heavy electrical loads! They can e latched-.1N/ y a sin#le pulse of positivecurrent applied to their 0ate terminal and will remain .1N/ indefinitely until the %node to

&athode current falls elow their minimum latchin# level!

+tatic &haracteristics of a Thyristor

Thyristors are semiconductor devices that can operate only in the switchin# mode!

Thyristor are current operated devices, a small 0ate current controls a lar#er %node

current!

&onducts current only when forward iased and tri##erin# current applied to the 0ate!

The thyristor acts like a rectifyin# diode once it is tri##ered .1N/!

Blocks current flow when reverse iased, no matter if 0ate current is applied!

1nce tri##ered .1N/, will e latched .1N/ conductin# even when a #ate current is no

lon#er applied providin# %node current is aove latchin# current!

Thyristors are hi#h speed switches that can e used to replace electromechanical relays in many

circuits as they have no movin# parts, no contact arcin# or suffer from corrosion or dirt! But in

addition to simply switchin# lar#e currents .1N/ and .122/, thyristors can e made to control

the mean value of an %& load current without dissipatin# lar#e amounts of power! % #oode3ample of thyristor power control is in the control of electric li#htin#, heaters and motor speed!

$n the ne3t tutorial we will look at some asic Thyristor Circuitsand applications usin# oth

%& and & supplies

2.

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Thyristor &ircuit

The Thyristor Switch

In the previous tutorial we looked at the basic construction and operation of the Silicon ControlledRectifier more commonly known as a Thyristor. This time we will look at how we can use the

thyristor switching circuits to control larger loads such as lamps, motors, or heaters etc.

We said previously that in order to get the Thyristorto turn-ON we need to inject a small trigger pulse of

current (not a continuous current into the !ate" ( ! terminal when the thyristor is in its forward direction"

that is the #node" (# is positive with respect to the $athode" (%" for regenerative latching to occur&

Typical Thyristor

!enerally" this trigger pulse need only 'e of a few micro-seconds in duration 'ut the longer the !ate pulse

is applied the faster the internal avalanche 'readown occurs and the faster the turn-ON time of the

thyristor" 'ut the ma)imum !ate current must not 'e e)ceeded& Once triggered and fully conducting" the

voltage drop across the thyristor" #node to $athode" is reasona'ly constant at a'out *&+, for all values of

#node current up to its rated value&

ut remem'er though that once a Thyristorstarts to conduct it continues to conduct even with no !ate

signal" until the #node current decreases 'elow the devices holding current" (./ and 'elow this value it

automatically turns-O00& Then unlie 'ipolar transistors and 01T2s" thyristors cannot 'e used for

amplification&

Thyristors are semiconductor devices that are specifically designed for use in high-power switching

applications& Thyristors can operate only in the switching mode" where they act lie either an open or

closed switch and once triggered it will remain conducting& Therefore in 3$ circuits and some highly

inductive #$ circuits the current has to 'e artificially reduced 'y a separate switch or turn off circuit&

3$ Thyristor $ircuit

When connected to a direct current 3$ supply" the thyristor can 'e used as a 3$ switch to control larger

3$ currents and loads& When using the Thyristor as a switch it 'ehaves lie an electronic latch 'ecause

once activated it remains in the ON state until manually reset& $onsider the 3$ thyristor circuit 'elow&

3$ Thyristor Switching $ircuit

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This simple on-off thyristor firing circuit uses the thyristor as a switch to control a lamp" 'ut it could also

'e used as an on-off control circuit for a motor" heater or some other such 3$ load& The thyristor is

forward 'iased and is triggered into conduction 'y 'riefly closing the normally-open ON push

'utton" S*which connects the !ate terminal to the 3$ supply via the !ate resistor" 4!thus allowing

current to flow into the !ate& .f the value of 4!is set too high with respect to the supply voltage" the

thyristor may not trigger&

Once the circuit has 'een turned-ON" it self latches and stays ON even when the push 'utton is

released providing the load current is more than the thyristors latching current& #dditional operations of

push 'utton" S*will have no effect on the circuits state as once latched the !ate looses all control& The

thyristor is now turned fully ON (conducting allowing full load circuit current to flow through the device in

the forward direction and 'ac to the 'attery supply&

One of the main advantages of using a thyristor as a switch in a 3$ circuit is that it has a very high

current gain& The thyristor is a current operated device'ecause a small !ate current can control a much

larger #node current&

The !ate-cathode resistor 4!%is generally included to reduce the !ate2s sensitivity and increase its dv5dt

capa'ility thus preventing false triggering of the device&

#s the thyristor has self latched into the ON state" the circuit can only 'e reset 'y interrupting the power

supply and reducing the #node current to 'elow the thyristors minimum holding current (./ value&

Opening the normally-closed O00 push 'utton" S6'reas the circuit" reducing the circuit current flowing

through the Thyristorto 7ero" thus forcing it to turn O00 until the application again of another !ate

signal&

/owever" one of the disadvantages of this 3$ thyristor circuit design is that the mechanical normally-

closed O00 switch S6needs to 'e 'ig enough to handle the circuit power flowing through 'oth the

thyristor and the lamp when the contacts are opened& .f this is the case we could just replace the thyristor

with a large mechanical switch& One way to overcome this pro'lem and reduce the need for a larger more

ro'ust O00 switch is to connect the switch in parallel with the thyristor as shown&

#lternative 3$ Thyristor $ircuit

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/ere the thyristor switch receives the re8uired terminal voltage and !ate pulse signal as 'efore 'ut the

larger normally-closed switch of the previous circuit has 'e replaced 'y a smaller normally-open switch in

parallel with the thyristor& #ctivation of switch S6momentarily applies a short circuit 'etween the thyristors

#node and $athode stopping the device from conducting 'y reducing the holding current to 'elow its

minimum value&

#$ Thyristor $ircuit

When connected to an alternating current #$ supply" the thyristor 'ehaves differently from the previous

3$ connected circuit& This is 'ecause #$ power reverses polarity periodically and therefore any thyristor

used in an #$ circuit will automatically 'e reverse-'iased causing it to turn-O00 during one-half of each

cycle& $onsider the #$ thyristor circuit 'elow&

#$ Thyristor $ircuit

The a'ove thyristor firing circuit is similar in design to the 3$ S$4 circuit e)cept for the omission of an

additional O00 switch and the inclusion of diode 3*which prevents reverse 'ias 'eing applied to the

!ate& 3uring the positive half-cycle of the sinusoidal waveform" the device is forward 'iased 'ut with

switch S*open" 7ero gate current is applied to the thyristor and it remains O00& On the negative half-

cycle" the device is reverse 'iased and will remain O00 regardless of the condition of switch S*&

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9OW14 1:1$T4ON.$S /#N3OO%" Third 1dition

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Current rice; $lic to see&&&

9rice 3isclaimer

.f switch S*is closed" at the 'eginning of each positive half-cycle the thyristor is fully O00 'ut shortly

after there will 'e sufficient positive trigger voltage and therefore current present at the !ate to turn the

thyristor and the lamp ON&

The thyristor is now latched-ON for the duration of the positive half-cycle and will automatically turn

O00 again when the positive half-cycle ends and the #node current falls 'elow the holding current

value&

3uring the ne)t negative half-cycle the device is fully O00 anyway until the following positive half-cycle

when the process repeats itself and the thyristor conducts again as long as the switch is closed&

Then in this condition the lamp will receive only half of the availa'le power from the #$ source as the

thyristor acts lie a rectifying diode" and conducts current only during the positive half-cycles when it is

forward 'iased& The thyristor continues to supply half power to the lamp until the switch is opened&

.f it were possi'le to rapidly turn switch S*ON and O00" so that the thyristor received its !ate signal at the

pea (

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9hase control is the most common form of thyristor #$ power control and a 'asic #$ phase-control circuit

can 'e constructed as shown a'ove& /ere the thyristors !ate voltage is derived from the 4$ charging

circuit via the trigger diode" 3*&

3uring the positive half-cycle when the thyristor is forward 'iased" capacitor" $charges up via

resistor 4*following the #$ supply voltage& The !ate is activated only when the voltage at point #has

risen enough to cause the trigger diode 3*" to conduct and the capacitor discharges into the !ate of the

thyristor turning it ON& The time duration in the positive half of the cycle at which conduction starts is

controlled 'y 4$ time constant set 'y the varia'le resistor" 4*&

.ncreasing the value of 4*has the effect of delaying the triggering voltage and current supplied to the

thyristors !ate which in turn causes a lag in the devices conduction time& #s a result" the fraction of the

half-cycle over which the device conducts can 'e controlled 'etween + and *?+ o" which means that the

average power dissipated 'y the lamp can 'e adjusted& /owever" the thyristor is a unidirectional deviceso only a ma)imum of >+= power can 'e supplied during each positive half-cycle&

There are a variety of ways to achieve *++= full-wave #$ control using thyristors& One way is to include

a single thyristor within a diode 'ridge rectifier circuit which converts #$ to a unidirectional current

through the thyristor while the more common method is to use two thyristors connected in inverse parallel&

# more practical approach is to use a single Triacas this device can 'e triggered in 'oth directions"

therefore maing them suita'le for #$ switching applications&

3.

Triac Tutorial

Triac Tutorial and Basic Principles

In the previous tutorial we looked at the construction and operation of the Silicon Controlled Rectifier more

commonly known as a Thyristor, which can $e used as a solid state switch to control lamps, motors, or

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heaters etc. owever, one of the pro$lems of usin! a thyristor for controllin! such circuits is that like a diode,

the thyristor is a unidirectional device, meanin! that it passes current in one direction only,

fromAnodeto Cathode.

2or & switchin# circuits this .one-way/ switchin# characteristic may e acceptale as once

tri##ered all the & power is delivered strai#ht to the load! But in +inusoidal %& +witchin#&ircuitsthis unidirectional switchin# may e a prolem as it only conducts durin# one half of thecycle (like a half-wave rectifier) when the %node is positive irrespective of whatever the 0ate

si#nal is doin#! Then for %& operation only half the power is delivered to the load y a thyristor!

$n order to otain full-wave power control we could connect a sin#le thyristor inside a full-wave

rid#e rectifier which tri##ers on each positive half-wave, or to connect two thyristors to#etherin inverse parallel (ack-to-ack) as shown elow ut this increases oth the comple3ity and

numer of components used in the switchin# circuit!

Thyristor &onfi#urations

There is however, another type of semiconductor device called a .Triode %& +witch/

or Triacfor short which is also a memer of the thyristor family that e used as a solid statepower switchin# device ut more importantly it is a .idirectional/ device! $n other words,

a Triaccan e tri##ered into conduction y oth positive and ne#ative volta#es applied to its%node and with oth positive and ne#ative tri##er pulses applied to its 0ate terminal makin# it a

two-4uadrant switchin# 0ate controlled device!

% Triac ehaves just like two conventional thyristors connected to#ether in inverse parallel

(ack-to-ack) with respect to each other and ecause of this arran#ement the two thyristorsshare a common 0ate terminal all within a sin#le three-terminal packa#e!

+ince a triac conducts in oth directions of a sinusoidal waveform, the concept of an %node

terminal and a &athode terminal used to identify the main power terminals of a thyristor are

replaced with identifications of AT7, forMain Terminal 1and AT6forMain Terminal 2with the0ate terminal 0referenced the same!

$n most %& switchin# applications, the triac #ate terminal is associated with the AT7terminal,similar to the #ate-cathode relationship of the thyristor or the ase-emitter relationship of the

transistor! The construction, P-N dopin# and schematic symol used to represent a Triacis #iven

elow!

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Triac +ymol and &onstruction

8e now know that a .triac/ is a -layer, PNPN in the positive direction and a NPNP in thene#ative direction, three-terminal idirectional device that locks current in its .122/ stateactin# like an open-circuit switch, ut unlike a conventional thyristor, the triac can conduct

current in either direction when tri##ered y a sin#le #ate pulse! Then a triac has four possile

tri##erin# modes of operation as follows!

C D Aode ? AT6current positive (Dve), 0ate current positive (Dve)

C E Aode ? AT6current positive (Dve), 0ate current ne#ative (-ve)

CCC D Aode ? AT6current ne#ative (-ve), 0ate current positive (Dve)

CCC E Aode ? AT6current ne#ative (-ve), 0ate current ne#ative (-ve)

%nd these four modes in which a triac can e operated are shown usin# the triacs $-9

characteristics curves!

Triac $-9 &haracteristics &urves

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$n Fuadrant , the triac is usually tri##ered into conduction y a positive #ate current, laelled

aove as mode +! But it can also e tri##ered y a ne#ative #ate current, mode ! +imilarly, in

Fuadrant , tri##erin# with a ne#ative #ate current,EC0is also common, mode alon# with

mode +! Aodes and +are, however, less sensitive confi#urations re4uirin# a #reater

#ate current to cause tri##erin# than the more common triac tri##erin# modes of +and !

%lso, just like silicon controlled rectifiers (+&5's), triac's also re4uire a minimum holdin#

current $Hto maintain conduction at the waveforms cross over point! Then even thou#h the twothyristors are comined into one sin#le triac device, they still e3hiit individual electricalcharacteristics such as different reakdown volta#es, holdin# currents and tri##er volta#e levels

e3actly the same as we would e3pect from a sin#le +&5 device!

Triac %pplications

The Triacis most commonly used semiconductor device for switchin# and power control of %&

systems as the triac can e switched .1N/ y either a positive or ne#ative 0ate pulse, re#ardless

of the polarity of the %& supply at that time! This makes the triac ideal to control a lamp or %&

motor load with a very asic triac switchin# circuit #iven elow!

Triac +witchin# &ircuit

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The circuit aove shows a simple & tri##ered triac power switchin# circuit! 8ithswitch +87open, no current flows into the 0ate of the triac and the lamp is therefore .122/!

8hen +87is closed, 0ate current is applied to the triac from the attery supply 90viaresistor 5and the triac is driven into full conduction actin# like a closed switch and full power isdrawn y the lamp from the sinusoidal supply!

%s the attery supplies a positive 0ate current to the triac whenever switch +87is closed, the

triac is therefore continually #ated in modes +and +re#ardless of the polarity ofterminal AT6!

1f course, the prolem with this simple triac switchin# circuit is that we would re4uire an

additional positive or ne#ative 0ate supply to tri##er the triac into conduction! But we can also

tri##er the triac usin# the actual %& supply volta#e itself as the #ate tri##erin# volta#e! &onsiderthe circuit elow!

Triac +witchin# &ircuit

The circuit shows a triac used as a simple static %& power switch providin# an .1N/-.122/

function similar in operation to the previous & circuit! 8hen switch +87is open, the triac actsas an open switch and the lamp passes =ero current! 8hen +87is closed the triac is #ated .1N/

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via current limitin# resistor 5 and self-latches shortly after the start of each half-cycle, thus

switchin# full power to the lamp load!

%s the supply is sinusoidal %&, the triac automatically unlatches at the end of each %& half-cycle

as the instantaneous supply volta#e and thus the load current riefly falls to =ero ut re-latchesa#ain usin# the opposite thyristor half on the ne3t half cycle as lon# as the switch remains closed!

This type of switchin# control is #enerally called full-wave control due to the fact that othhalves of the sine wave are ein# controlled!

%s the triac is effectively two ack-to-ack connected +&5's, we can take this triac switchin#circuit further y modifyin# how the #ate is tri##ered as shown elow!

Aodified Triac +witchin# &ircuit

%s aove, if switch +87is open at position %, there is no #ate current and the lamp is .122/! $f

the switch is moved to position B#ate current flows at every half cycle the same as efore and

full power is drawn y the lamp as the triac operates in modes +and !

However this time when the switch is connected to position &, the diode will prevent the

tri##erin# of the #ate when AT6is ne#ative as the diode is reverse iased! Thus the triac only

conducts on the positive half-cycles operatin# in mode $D only and the lamp will li#ht at halfpower! Then dependin# upon the position of the switch the load is Off, atHalf PowerorFll!

ON!

Triac Phase &ontrol

%nother common type of triac switchin# circuit uses phase control to vary the amount of volta#e,and therefore power applied to a load, in this case a motor, for oth the positive and ne#ative

halves of the input waveform! This type of %& motor speed control #ives a fully variale and

linear control ecause the volta#e can e adjusted from =ero to the full applied volta#e as shown!

Triac Phase &ontrol

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This asic phase tri##erin# circuit uses the triac in series with the motor across an %& sinusoidal

supply! The variale resistor, 957is used to control the amount of phase shift on the #ate of thetriac which in turn controls the amount of volta#e applied to the motor y turnin# it 1N at

different times durin# the %& cycle!

77< Thyristor Projects sin# +&5s and T5$%&s

Price isclaimer

The triac's tri##erin# volta#e is derived from the 957 E &7comination via the /iac(The diac is a

idirectional semiconductor device that helps provide a sharp tri##er current pulse to fully turn-

1N the triac)!

%t the start of each cycle, &7char#es up via the variale resistor, 957! This continues until thevolta#e across &7is sufficient to tri##er the diac into conduction which in turn allows

capacitor, &7to dischar#e into the #ate of the triac turnin# it .1N/!

1nce the triac is tri##ered into conduction and saturates, it effectively shorts out the #ate

tri##erin# phase control circuit connected in parallel across it and the triac takes control for the

remainder of the half-cycle!

%s we have seen aove, the triac turns-122 automatically at the end of the half-cycle and

the 957 E &7tri##erin# process starts a#ain on the ne3t half cycle!

However, ecause the triac re4uires differin# amounts of #ate current in each switchin# mode of

operation, for e3ample +and , a triac is therefore asymmetrical meanin# that it may nottri##er at the e3act same point for each positive and ne#ative half cycle!

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This simple triac speed control circuit is suitale for not only %& motor speed control ut for

lamp dimmers and electrical heater control and in fact is very similar to a triac li#ht dimmer used

in many homes! However, a commercial triac dimmer should not e used as a motor speedcontroller as #enerally triac li#ht dimmers are intended to e used with resistive loads only such

as incandescent lamps!

Then we can end this Triac Tutorialy summarisin# its main points as follows

% .Triac/ is another -layer, G-terminal thyristor device similar to the +&5!

The Triac can e tri##ered into conduction in either direction!

There are four possile tri##erin# modes for a Triac, of which 6 are preferred!

lectrical %& power control usin# a Triacis e3tremely effective when used properly to controlresistive type loads such as incandescent lamps, heaters or small universal motors commonly

found in portale power tools and small appliances!

But please rememer that these devices can e used and attached directly to the mains %& power

source so circuit testin# should e done when the power control device is disconnected from themains power supply! Please rememer safety first!

4.

$nsulated 0ate Bipolar Transistor

$nsulated 0ate Bipolar Transistor

The Insulated 0ate 1ipolar Transistor also called an I01T for short, is somethin! of a cross $etween a

conventionalBipolar Junction Transistor, 213T4 and a Field Effect Transistor, 25"S&6T4 makin! it ideal as a

semiconductor switchin! device.

The"#BT transistortakes the est parts of these two types of transistors, the hi#h input

impedance and hi#h switchin# speeds of a A1+2T with the low saturation volta#e of a ipolar

transistor, and comines them to#ether to produce another type of transistor switchin# devicethat is capale of handlin# lar#e collector-emitter currents with virtually =ero #ate current drive!

Typical I01T

The $nsulated 0ate Bipolar Transistor, ($0BT) uses the insulated #ate (hence the first part of itsname) technolo#y of the A1+2T with the output performance characteristics of a conventional

ipolar transistor, (hence the second part of its name)! The result of this hyrid comination is

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that the .$0BT Transistor/ has the output switchin# and conduction characteristics of a ipolar

transistor ut is volta#e-controlled like a A1+2T!

$0BTs are mainly used in power electronics applications, such as inverters, converters and power

supplies, were the demands of the solid state switchin# device are not fully met y poweripolars and power A1+2Ts! Hi#h-current and hi#h-volta#e ipolars are availale, ut their

switchin# speeds are slow, while power A1+2Ts may have hi#h switchin# speeds, ut hi#h-volta#e and hi#h-current devices are e3pensive and hard to achieve!

The advanta#e #ained y the insulated #ate ipolar transistor device over a B*T or A1+2T isthat it offers #reater power #ain than the ipolar type to#ether with the hi#her volta#e operation

and lower input losses of the A1+2T! $n effect it is an 2T inte#rated with a ipolar transistor

in a form of arlin#ton confi#uration as shown!

$nsulated 0ate Bipolar Transistor

8e can see that the insulated #ate ipolar transistor is a three terminal, transconductance devicethat comines an insulated #ate N-channel A1+2T input with a PNP ipolar transistor output

connected in a type of arlin#ton confi#uration! %s a result the terminals are laelled

as Collector,6mitterand 0ate! Two of its terminals (&-) are associated with a conductancepath and the third terminal (0) associated with its control!

The amount of amplification achieved y the inslated $ate bi%olar transistoris a ratio etween

its output si#nal and its input si#nal! 2or a conventional ipolar junction transistor, (B*T) the

amount of #ain is appro3imately e4ual to the ratio of the output current to the input current,

called Beta!

2or a metal o3ide semiconductor field effect transistor or A1+2T, there is no input current as

the #ate is isolated from the main current carryin# channel! Therefore, an 2T's #ain is e4ual to

the ratio of output current chan#e to input volta#e chan#e, makin# it a transconductance device

and this is also true of the $0BT! Then we can treat the $0BT as a power B*T whose ase currentis provided y a A1+2T!

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The Insulated 0ate 1ipolar Transistorcan e used in small si#nal amplifier circuits in much

the same way as the B*T or A1+2T type transistors! But as the $0BT comines the low

conduction loss of a B*T with the hi#h switchin# speed of a power A1+2T an optimal solidstate switch e3ists which is ideal for use in power electronics applications!

%lso, the $0BT has a much lower .on-state/ resistance, 51Nthan an e4uivalent A1+2T! This

means that the $65drop across the ipolar output structure for a #iven switchin# current is muchlower! The forward lockin# operation of the $0BT transistor is identical to a power A1+2T!

8hen used as static controlled switch, the insulated #ate ipolar transistor has volta#e andcurrent ratin#s similar to that of the ipolar transistor! However, the presence of an isolated #ate

in an $0BT makes it a lot simpler to drive than the B*T as much less drive power is needed!

%n insulated #ate ipolar transistor is simply turned .1N/ or .122/ y activatin# and

deactivatin# its 0ate terminal! % constant positive volta#e input si#nal across the 0ate and themitter will keep the device in its .1N/ state, while removal of the input si#nal will cause it to

turn .122/ in much the same way as a ipolar transistor or A1+2T!

$0BT &haracteristics

Because the $0BT is a volta#e-controlled device, it only re4uires a small volta#e on the 0ate tomaintain conduction throu#h the device unlike B*T's which re4uire that the Base current is

continuously supplied in a sufficient enou#h 4uantity to maintain saturation!

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%lso the $0BT is a unidirectional device, meanin# it can only switch current in the .forward

direction/, that is from &ollector to mitter unlike A1+2T's which have i-directional current

switchin# capailities (controlled in the forward direction and uncontrolled in the reversedirection)!

The principal of operation and 0ate drive circuits for the insulated #ate ipolar transistor are

very similar to that of the N-channel power A1+2T! The asic difference is that the resistanceoffered y the main conductin# channel when current flows throu#h the device in its .1N/ stateis very much smaller in the $0BT! Because of this, the current ratin#s are much hi#her when

compared with an e4uivalent power A1+2T!

The main advanta#es of usin# the Insulated 0ate 1ipolar Transistorover other types of

transistor devices are its hi#h volta#e capaility, low 1N-resistance, ease of drive, relatively fastswitchin# speeds and comined with =ero #ate drive current makes it a #ood choice for moderate

speed, hi#h volta#e applications such as in pulse-width modulated (P8A), variale speed

control, switch-mode power supplies or solar powered &-%& inverter and fre4uency converterapplications operatin# in the hundreds of kilohert= ran#e!

% #eneral comparison etween B*T's, A1+2T's and $0BT's is #iven in the followin# tale!

$0BT &omparison Tale

/evice

Characteristic

ower

1ipolar

ower

5"S&6TI01T

9olta#e 5atin# Hi#h J7k9 Hi#h J7k9 9ery Hi#h K7k9

&urrent 5atin# Hi#h JL

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losses and its aility to switch hi#h volta#es without dama#e makes this transistor ideal for

drivin# inductive loads such as coil windin#s, electroma#nets and & motors!

5.

iac Tutorial

The iac

The /Iode 8C switch, or /iac for short, is another solid state, three-layer, two-9unction semiconductor device

$ut unlike the transistor theDiachas no $ase connection makin! it a two terminal device, la$elled 8*and 8:.

/iacs have no control or amplification $ut act much like a $idirectional switchin! diode as they can conduct

current from either polarity of a suita$le 8C volta!e supply.

$n our tutorial aoutSCR;sand Triacs, we saw that in 1N-122 switchin# applications, these

devices could e tri##ered y simple circuits producin# steady state #ate currents as shown!

8hen switch, +7is open no #ate current flows and the lamp is .122/! 8hen switch +7is closed,

#ate current $0flows and the +&5 conducts on the positive half cycles only as it is operatin# in

4uadrant !

8e rememer also that once #ated .1N/, the +&5 will only switch .122/ a#ain when its supply

volta#e falls to a values such that its %node current, $% is less than the value of its holdin#current, $H!

$f we wish to control the mean value of the lamp current, rather than just switch it .1N/ or

.122/, we could apply a short pulse of #ate current at a pre-set tri##er point to allow conduction

of the +&5 to occur over part of the half-cycle only! Then the mean value of the lamp current

would e varied y chan#in# the delay time, Tetween the start of the cycle and the tri##erpoint! This method is known commonly as .phase control/!

But to achieve phase control, two thin#s are needed! 1ne is a variale phase shift circuit (usually

an 5& passive circuit), and two, some form of tri##er circuit or device that can produce the

re4uired #ate pulse when the delayed waveform reaches a certain level! 1ne such solid statesemiconductor device that is desi#ned to produce these #ate pulses is the /iac!

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The diac is constructed like a transistor ut has no ase connection allowin# it to e connected

into a circuit in either polarity! iacsare primarily used as tri##er devices in phase-tri##erin# and

variale power control applications ecause a diac helps provide a sharper and more instanttri##er pulse (as opposed to a steadily risin# ramp volta#e) which is used to turn .1N/ the main

switchin# device!

The diac symol and the volta#e-current characteristics curves of the diac are #iven elow!

iac +ymol and $-9 &haracteristics

8e can see from the aove diac $-9 characteristics curves that the diac locks the flow of current

in oth directions until the applied volta#e is #reater than 9B5, at which point reakdown of thedevice occurs and the diac conducts heavily in a similar way to the =ener diode passin# a suddenpulse of volta#e! This 9B5point is called the iacs reakdown volta#e or reakover volta#e!

$n an ordinary =ener diode the volta#e across it would remain constant as the current increased!

However, in the diac the transistor action causes the volta#e to reduce as the current increases!

1nce in the conductin# state, the resistance of the diac falls to a very low value allowin# arelatively lar#e value of current to flow! 2or most commonly availale diacs their reakdown

volta#e typically ran#es from aout M6L to GL volts!

This action #ives the diac the characteristic of a ne#ative resistance as shown aove! %s the diac

is a symmetrical device, it therefore has the same characteristic for oth positive and ne#ative

volta#es and it is this ne#ative resistance action that makes the /iacsuitale as a tri##erin#device for +&5's or triacs!

iac %pplications

%s stated aove, the diac is commonly used as a tri##erin# device for other semiconductorswitchin# devices, mainly +&5's and triacs! Triacs are widely used in applications such as lamp

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dimmers and motor speed controllers and as such the diac is used in conjunction with the triac to

provide full-wave control of the %& supply as shown!

iac %& Phase &ontrol

%s the %& supply volta#e increases at the e#innin# of the cycle, capacitor, &is char#ed throu#h

the series comination of the fi3ed resistor, 57and the potentiometer, 957and the volta#e across

its plates increases! 8hen the char#in# volta#e reaches the reakover volta#e of the diac (aoutG< 9), the diac reaks down and the capacitor dischar#es throu#h the diac, producin# a sudden

pulse of current, which fires the triac into conduction! The phase an#le at which the triac is

tri##ered can e varied usin# 957, which controls the char#in# rate of the capacitor!

1nce the triac has een fired into conduction, it is maintained in its .1N/ state y the loadcurrent flowin# throu#h it, while the volta#e across the resistorEcapacitor comination is limited

y the .1N/ volta#e of the triac and is maintained until the end of the present half-cycle of the

%& supply!

%t the end of the half cycle the supply volta#e falls to =ero, reducin# the current throu#h the triacelow its holdin# current, $Hturnin# it .122/ and the diac stops conduction! The supply volta#e

then enters its ne3t half-cycle, the capacitor volta#e a#ain e#ins to rise (this time in the opposite

direction) and the cycle of firin# the triac repeats over a#ain!

Triac &onduction 8aveform

Then we have seen that the /iacis a very useful device which can e used to tri##er triacs and

ecause of its ne#ative resistance characteristics this allows it to switch .1N/ rapidly once a

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The =ni9unction Transistor or =3T for short, is another solid state three terminal device that can $e used in

!ate pulse, timin! circuits and tri!!er !enerator applications to switch and control either thyristors and triacs

for 8C power control type applications. 7ike diodes, uni9unction transistors are constructed from separate -

type and #-type semiconductor materials formin! a sin!le 2hence its name =ni-3unction4 #-9unction within

the main conductin! #-type channel of the device.

%lthou#h the 'ni(nction Transistorhas the name of a transistor, its switchin# characteristics are

very different from those of a conventional ipolar or field effect transistor as it can not e used

to amplify a si#nal ut instead is used as a 1N-122 switchin# transistor! *T's haveunidirectional conductivity and ne#ative impedance characteristics actin# more like a variale

volta#e divider durin# reakdown!

"ike N-channel 2T's, the *T consists of a sin#le solid piece of N-type semiconductor material

formin# the main current carryin# channel with its two outer connections marked as Base 2( B6)

andBase 1( B7)! The third connection, confusin#ly marked as the)mitter( ) is located alon#the channel! The emitter terminal is represented y an arrow pointin# from the P-type emitter to

the N-type ase!

The mitter rectifyin# p-n junction of the nijunction Transistoris formed y fusin# the P-type

material into the N-type silicon channel! However, P-channel *T's with an N-type mitterterminal are also availale ut these are little used!

The mitter junction is positioned alon# the channel so that it is closer to terminal B6than B7! %narrow is used in the *T symol which points towards the ase indicatin# that the mitter

terminal is positive and the silicon ar is ne#ative material! Below shows the symol,construction, and e4uivalent circuit of the *T!

nijunction Transistor +ymol and &onstruction

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Notice that the symol for the unijunction transistor looks very similar to that of the junction

field effect transistor or *2T, e3cept that it has a ent arrow representin# the mitter( ) input!8hile similar in respect of their ohmic channels, *2T's and *T's operate very differently and

should not e confused!

+o how does it work 8e can see from the e4uivalent circuit aove, that the N-type channelasically consists of two resistors 5B6and 5B7in series with an e4uivalent (ideal)diode, representin# the p-n junction connected to their center point! This mitter p-n junction is

fi3ed in position alon# the ohmic channel durin# manufacture and can therefore not e chan#ed!

5esistance 5B7is #iven etween the mitter, and terminal B7, while resistance 5B6is #iven

etween the mitter, and terminal B6!

%s the physical position of the p-n junction is closer to terminal B6than B7the resistive valueof 5B6will e less than 5B7!

The total resistance of the silicon ar (its 1hmic resistance) will e dependent upon the

semiconductors actual dopin# level as well as the physical dimensions of the N-type siliconchannel ut can e represented y 5BB! $f measured with an ohmmeter, this static resistance wouldtypically measure somewhere etween aout k: and 7(eta)! Typical standard values of Qran#e from

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However, when the mitter input volta#e is increased and ecomes #reater

than 95B7(orQ9BBD

ecomes forward iased and the unijunction transistor e#ins to conduct! The result is thatmitter current, Q$now flows from the mitter into the Base re#ion!

The effect of the additional mitter current flowin# into the Base reduces the resistive portion of

the channel etween the mitter junction and the B7terminal! This reduction in the valueof 5B7resistance to a very low value means that the mitter junction ecomes even more forwardiased resultin# in a lar#er current flow! The effect of this results in a ne#ative resistance at the

mitter terminal!

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"ikewise, if the input volta#e applied etween the mitter and B7terminal decreases to a value

elow reakdown, the resistive value of5B7 increases to a hi#h value! Then the =ni9unction

Transistorcan e thou#ht of as a volta#e reakdown device!

+o we can see that the resistance presented y 5B7is variale and is dependant on the value of

mitter current, $! Then forward iasin# the mitter junction with respect to B7causes more

current to flow which reduces the resistance etween the mitter, and B7!

$n other words, the flow of current into the *T's mitter causes the resistive value of 5B7to

decrease and the volta#e drop across it, 95B7must also decrease, allowin# more current to flowproducin# a ne#ative resistance condition!

nijunction Transistor %pplications

Now that we know how a ni(nction transistorworks, what can they e used for! The mostcommon application of a unijunction transistor is as a tri##erin# device forSCR;sand Triacsut

other *T applications include sawtoothed #enerators, simple oscillators, phase control, andtimin# circuits! The simplest of all *T circuits is the 5ela3ation 1scillator producin# non-

sinusoidal waveforms!

$n a asic and typical *T rela3ation oscillator circuit, the mitter terminal of the unijunction

transistor is connected to the junction of a series connected resistor and capacitor, 5& circuit as

shown elow!

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nijunction Transistor 5ela3ation 1scillator

8hen a volta#e (9s) is firstly applied, the unijunction transistor is .122/ and the capacitor &7isfully dischar#ed ut e#ins to char#e up e3ponentially throu#h resistor 5G! %s the mitter of the*T is connected to the capacitor, when the char#in# volta#e 9cacross the capacitor ecomes

#reater than the diode volt drop value, the p-n junction ehaves as a normal diode and ecomes

forward iased tri##erin# the *T into conduction! The unijunction transistor is .1N/! %t this

point the mitter to B7 impedance collapses as the mitter #oes into a low impedance saturatedstate with the flow of mitter current throu#h 57takin# place!

%s the ohmic value of resistor 57is very low, the capacitor dischar#es rapidly throu#h the *T

and a fast risin# volta#e pulse appears across 57! %lso, ecause the capacitor dischar#es more

4uickly throu#h the *T than it does char#in# up throu#h resistor 5G, the dischar#in# time is a lotless than the char#in# time as the capacitor dischar#es throu#h the low resistance *T!

8hen the volta#e across the capacitor decreases elow the holdin# point of the p-n junction

( 9122), the *T turns .122/ and no current flows into the mitter junction so once a#ain the

capacitor char#es up throu#h resistor 5Gand this char#in# and dischar#in# processetween 91Nand 9122is constantly repeated while there is a supply volta#e, 9sapplied!

*T 1scillator 8aveforms

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Then we can see that the unijunction oscillator continually switches .1N/ and .122/ without

any feedack! The fre4uency of operation of the oscillator is directly affected y the value of thechar#in# resistance 5G, in series with the capacitor &7and the value of Q! The output pulse shape

#enerated from the Base7 (B7) terminal is that of a sawtooth waveform and to re#ulate the timeperiod, you only have to chan#e the ohmic value of resistance, 5Gsince it sets the 5&time

constant for char#in# the capacitor!

The time period, Tof the sawtoothed waveform will e #iven as the char#in# time plus the

dischar#in# time of the capacitor! %s the dischar#e time, R7is #enerally very short in comparison

to the lar#er 5&char#in# time, R6the time period of oscillation is more or less e4uivalent to T R6!The fre4uency of oscillation is therefore #iven y S ? 7T!

*T 1scillator 3ample No7

The data sheet for a 6N6OO nijunction Transistor #ives the intrinsic stand-off ratio >as

a 7

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Then the value of char#in# resistor re4uired in this simple e3ample is calculated as @L!Gk:'sto thenearest preferred value! However, there are certain conditions re4uired for the *T rela3ation

oscillator to operate correctly as the resistive value of 5Gcan e too lar#e or too small!

2or e3ample, if the value of 5Gwas too lar#e, (Ae#ohms) the capacitor may not char#e up

sufficiently to tri##er the nijunction's mitter into conduction ut must also e lar#e enou#h to

ensure that the *T switches .122/ once the capacitor has dischar#ed to elow the lower tri##ervolta#e!

"ikewise if the value of 5Gwas too small, (a few hundred 1hms) once tri##ered the current

flowin# into the mitter terminal may e sufficiently lar#e to drive the device into its saturation

re#ion preventin# it from turnin# .122/ completely! ither way the unijunction oscillator circuitwould fail to oscillate!

*T +peed &ontrol &ircuit

1ne typical application of the unijunction transistor circuit aove is to #enerate a series of pulsesto fire and control a thyristor! By usin# the *T as a phase control tri##erin# circuit in

conjunction with an +&5 or Triac, we can adjust the speed of a universal %& or & motor as

shown!

nijunction Transistor +peed &ontrol

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sin# the circuit aove, we can control the speed of a universal series motor (or whichever type

of load we want, heaters, lamps, etc) y re#ulatin# the current flowin# throu#h the +&5! Tocontrol the motors speed, simply chan#e the fre4uency of the sawtooth pulse, which is achieved

y varyin# the value of the potentiometer!

nijunction Transistor +ummary

8e have seen that a =ni9unction Transistoror =3Tfor short, is an electronic semiconductordevice that has only one p-n junction within a N-type (or P-type) li#htly doped ohmic channel!

The *T has three terminals one laelled mitter () and two Bases (B7and B6)!

Two ohmic contacts B7and B6are attached at each ends of the semiconductor channel with the

resistance etween B7and B6, when the emitter is open circuited ein# called the interaseresistance, 5BB! $f measured with an ohmmeter, this static resistance would typically measure

somewhere etween aout k: and 7(eta)!

Typical standard values of Qran#e from