Transcript OTHER MEMBERS OF THYRISTOR FAMILY

Power Electronics-I
OTHER MEMBERS OF THYRISTOR
FAMILY
Savdas AmbaliyaAmrit OjhaBhadresa PayalBhalodiya ParthBhatt Harsh-
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There are many other members of thyristor other
than SCR which are listed as follows:
 PUT (Programmable Unijunction Transistor)
 SUS (Silicon Unilateral Switch)
 SCS (Silicon Controlled Switch)
 Light Activated Thyristor
 Diac (Bidirectional Thyristor Diode)
 Triac
 ASCR (Asymmetrical Thyristor)
 RCT (Reverse Conducting Thyristor)
PUT (Programmable Unijunction Transistor)
 It is a pnpn device like SCR, but major difference is that gate
is connected to n-type material near the anode as shown in
figure.PUT is used mainly in time delay, logic and SCR
trigger circuit
 In a PUT, G is always biased positive with respect to cathode.
When anode voltage exceeds the gate voltage by about 0.7V
junction J1 gets forward biased and PUT turns ON. When
anode voltage becomes less than gate voltage, PUT is turned
OFF.
SUS (Silicon Unilateral Switch)
 A SUS is similar to a PUT but with an inbuilt low voltage
avalanche diode between gate and cathode as shown in fig.
because of presence of diode ,SUS turn on far a fixed anode
to cathode voltage unlike an SCR whose trigger voltage vary
widely with change in ambient temperature
 It sis used mainly in timing ,logic and trigger circuits.
SCS (Silicon Controlled Switch)
 SCS is a tetrode,ie. Four electrode thyristor. It has two gates
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one anode gate (AG) like a PUT and another cathode gate
(KG) like an SCR.
In other words SCS is a four layer, four terminal pnpn device
; anode A, cathode K, anode gate AG and cathode gate KG.
SUS can be turned on by either gate.
When a negative pulse is applied to gate AG, junction J1 is
forward biased and SCS is turned on.
A positive pulse at AG will reverse bias junction J1 and turn
off the SCS.
A positive pulse at gate KG turns on the device and a
negative pulse at KG turn it off.
 This can be operated like an OR gate. Its application
included:
 Timing,logic and triggering circuits, pulse generator, voltage
sensors, oscillators, et al.
Light Activated Thyristor
 Light Activated Thyristor is also called LASCR.
 LASCRs are turned on by throwing a pulse of light on the
silicon wafer of thyristor.
 The pulse of appropriate wavelength is guided by optical
fibres to the special sensitive area of the wafer.
 IF the intensity of light exceeds a certain value, excess
electron-hole pair are generated due to radiation and forward
biased thyristor gets turned on.
 A light fired thyristor has complete electrical isolation
between the light triggering source and high voltage anode
cathode circuit.
 The primaryb use of light fired thyristor is in high voltage
high current application, static relative p[ower
compensation,etc
Diac (Bidirectional Thyristor Diode)
 If voltage V12,with terminal 1 positive with respect to
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terminal 2,, then the structure pnpn conducts exceeds break
over voltage VBO1
In case 2 terminal is positive wit respect to terminal 1 and
when V21 exceeds breakover voltage VBO2 ,structure pn pn’
conducts.
the term diac is obtained from capital letter Diode that can
work on AC.
It is seen that diac has symmetrical breakdown characteristic.
Its lead are interchangable.
Its turn off voltage is about 30 V.
 When conducting it act like a low resistance with about 3 V
drop acrossit.
 When not conducting, it acts like a low resistance with about
3 V drop across it.
 When not conducting, it acts like an open switch.
 A diac is sometimes called a gateless triac.
Triac
 An SCR is a unidirectional device as it can conduct from
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anode to cathode only and not from cathode to anode,
A triac can however conduct in both the directions.
A triac is thus a bidirecrtional thyristor with three terminals.
It is used extensively for the control of power in ac circuit.
Triac is the word derived by combing the capital letters from
the word TRIode and AC.
When in operation , a triac is equivalent to two scrs
connnected in antiparallel.
As triac can conduct in both direction , the term anode and
cathode are not applicable to triac.
 Its three terminals are usully designated as MT1 (main
terminal 1),MT2 and the gate by Gas in a thyristor.
ASCR (Asymmetrical Thyristor)
 A conventional thyristor is able to block a large reverse
voltage but this blocking capability is not required in several
industrial application
 A freewheeling diode is usually connected in anti parallel in
across each thyristor.
 An asymmetrical thyristor or ASCR is specially fabricated to
have limited reverse voltage capability; this permits a turn
ON turn OFF time and on state voltage drop in ASCR.
 A typical ASCR may have reverse blocking capability of 20 to
30 volt and forward blocking voltage to 400 to 2000 volt.
RCT (Reverse Conducting Thyristor)
 A reverse conducting thyristor is a special asymmetrical
thyristor with a monolithically integrated antiparallel diode
on the same silicon chip.
 This construction reduces to zero the reverse blocking
capacity of RCT.
 A current pulse through the diode in a single device reduces
the heat sink size and leads to compactness of the converter.
 The undesirable stray loop inductance between ASCR and
diode is also eliminated and unwanted reverse voltage
transients across ASCR are avoided; this leads to better turn
off behavior of RCT.
 RCTs with 2000V and 500A rating are avilable.
 For high performance inverter and chopper circuits, RCTs
can now be tailor made.
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