Scr Triac, Bjt)

Chapter One Basic Electronic Devices andTheir Applications

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CHAPTER 1BASIC ELECTRONIC DEVICES AND THEIR APPLICATION

POWER ELECTRONIC SWITCHES• Thyristors-SCR (Silicon Controlled Rectifier), TRIAC,DIAC, PUT

(Programmable Unijunction Transistor), GTO-SCR (Gate Turnoff SCR)

• Transistors- BJT (Bipolar Junction Transistor), FET (Field EffectTransistor).

• Other Components – Opto-coupler, Zener diode, Phototransistor, Photo thyristor, Photo diode, Operational Amplifier


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Thyristors• Also call Silicon Controlled Rectifier (SCR).• Thyristor is used for requiring high speed & high power

switching.• Handle V & I up to 1000A & 1 kV.• Anode – high +ve voltage with relative to cathode & gate

at small +ve potential w.r.t cathode.

Regenerative Action When a positive current is applied to terminal Gate.

Transistor Q2 is biased into conduction, causing itscollector current to rise and saturated.

The collector current of Q2 cause Q1 is biased intoconduction, causing the collector current to rise andsaturated.

Both transistors are driven into saturation, and theimpedance between A and C is very low.

The positive current applied to terminal G, which servedto trigger the self-regenerative action, is no longerrequired since the collector of PNP transistor Q1 nowsupplies more than enough current to drive Q2

The circuit will remain on until it is turned off by areduction in the collector current to a value below thatnecessary to maintain conduction.


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Circuit Symbol

Biasing

No current can flow through the SCR in both circuit (a) & (b). WHY?(a) (b)

P

N

N

PJ2

J1

J3

P

N

N

PJ2

J1

J3


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Working of SCR

J 1J 3

J 2

When Gate is Open

• J2 R.biased & J1, J3 F. biased.• No current thro RL• V increased till breaks down

& SCR ON-stroke.

Gate is +ve w.r.t cathode• J2 R.biased & J3 F. biased.• Switch ON- V small Vb/over decreases. IG flowing• & Ia increases more at J2 – breaks up – SCR ON.• Gate loses control. Remove VG SCR still ON

Working of SCRSwitching SCR OFF

• Reduce apply voltage to almost zero which the anode currentis reduced below certain value called holding current.

Advantage of SCR• No moving parts – noiseless – high. operating freq.• Very high switching speed ( 109/s ).• Higher control over IL (20-150A) – small IG (mA)• Small size – reliable – longer life.


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V-I Characteristic

In a conventional thyristor, once it hasbeen switched on by the gateterminal, the device remains latchedin the on-state (i.e. do not need acontinuous supply of gate current toconduct), providing the anode currenthas exceeded the latching current (IL).

As long as the anode remainspositively biased, it cannot beswitched off until the anode currentfalls below the holding current (IH)

Important Terms Break over voltage

Minimum forward voltage, gate being open, SCR starts conducting to turn ON (50 – 500V). Holding current

Min anode current, to maintain SCR in the ON state. Latching Current

Minimum anode current required to maintain SCR in ON state immediately after SCR hasbeen turn on and gate current has been removed

Peak Reverse Voltage (PRV)Maximum reverse voltage can applied to SCR without conducting in reverse direction.

Forward current ratingMaximum anode current that SCR is capable of passing without destruction.

Gate triggering voltage, VGT

Minimum values of gate voltage at which SCR is turned ON. Gate triggering current, IGT

Resulting gate current to turn on SCR.


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Firing and TriggeringLamp load

Pushbutton

V

DC firing signal

Load

V

Pulse signal

Turning OFFConsidered the SCR circuit with pulse signal at gate:

Once fired, SCR remains ON even when triggering pulse is removed

This is referred as LATCHING – ability to remain ON even when gatecurrent is removed

Methods used to turn SCR OFF:

1. anode current interruption

2. reducing current through SCR below holdingcurrent


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Phase Controlled Half Wave Rectification

• OFF in -ve half cycle & +ve halfcycle,

• only ON in +ve half cycle & ifproper IGT is provided.

Phase Controlled Half Wave Rectification

L

in

L

SCRinL

R

v

R

vvi

cos12

mav

vv

cos12

L

mav

R

vI

Average o/p current

Average o/p voltage


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CENTER-TAP TRANSFORMER SCR FULL WAVE RECTIFIER

cos1 L

mav

R

vI

TRIAC


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TRIAC, from Triode for Alternating Current, is an electroniccomponent which can conduct current in either direction when it istriggered (turned on), and is formally called a bidirectional triodethyristor or bilateral triode thyristor.

MT2

MT1MT1

MT2

A TRIAC is approximately equivalent to twocomplementary SCRs (joined in inverse parallel(paralleled but with the polarity reversed) and withtheir gates connected together.

It can be triggered by either a positive or a negativevoltage being applied to its gate electrode (withrespect to MT1.

Once triggered, the device continues to conductuntil the current through it drops below a certainthreshold value, the holding current, such as at theend of a half-cycle of alternating current (AC)mains power.

Equivalent circuit


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APPLICATIONS TRIAC is a very convenient switch for AC circuits, allowing

the control of very large power flows with milliampere-scalecontrol currents.

In addition, applying a trigger pulse at a controllable point inan AC cycle allows one to control the percentage of currentthat flows through the TRIAC to the load (phase control).

STATC CHARACTERISTIC CURVE FOR TRIAC


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GTO (gate-turn-off) SCRGate turn-off thyristor (GTO) is a special type of thyristor, a high-power semiconductor device. GTOs, as opposed to normal thyristors,are fully controllable switches which can be turned on and off by theirthird lead, the GATE lead.

structuresymbol

OPERATION OF GTO-SCR Normal SCR are not fully controllable switches (a "fully

controllable switch" can be turned on and off at will).

SCR can only be turned ON and cannot be turned OFF.

SCR are switched ON by a gate signal, but even after the gatesignal is removed, the SCR remains in the ON-state until anyturn-off condition occurs (which can be the application of areverse voltage to the terminals, or when the current flowingthrough (forward current) falls below a certain threshold valueknown as the "holding current").

Thus, a SCR behaves like a normal semiconductor diode after itis turned on .


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OPERATION OF GTO-SCR

The GTO can be turned-on by a gate signal, and can also beturned-off by a gate signal of negative polarity.

Turn on is accomplished by a "positive current" pulse between thegate and cathode terminals. The turn on phenomenon in GTO ishowever, not as reliable as an SCR and small positive gate currentmust be maintained even after turn on to improve reliability.

Turn off is accomplished by a "negative voltage" pulse betweenthe gate and cathode terminals.

OPERATION OF GTO-SCR GTO thyristors suffer from long switch off times, whereby after

the forward current falls, there is a long tail time where residualcurrent continues to flow until all remaining charge from thedevice is taken away. This restricts the maximum switchingfrequency to approx. 1 kHz.

It may however be noted that the turn off time of a comparableSCR is ten times that of a GTO.

Thus switching frequency of GTO is faster than an SCR.


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FIELD EFFECT TRANSISTOR

The Field-Effect Transistor (FET) relies on an electricfield to control the shape and hence the conductivity of achannel of one type of charge carrier in a semiconductormaterial.

FETs are sometimes called unipolar transistors to contrasttheir single-carrier-type operation with the dual-carrier-typeoperation of bipolar (junction transistors (BJT).

FIELD EFFECT TRANSISTOR


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Types of field-effect transistors The channel of a FET is doped to produce either an N-type semiconductor or a P-

type semiconductor. The drain and source may be doped of opposite type to thechannel, in the case of depletion mode FETs, or doped of similar type to thechannel as in enhancement mode FETs. Field-effect transistors are alsodistinguished by the method of insulation between channel and gate. Types ofFETs are:

The JFET (Junction Field-Effect Transistor) uses a reverse biased p-njunction to separate the gate from the body.

The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor)utilizes an insulator (typically SiO2) between the gate and the body.

JFET Construction and symbol


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N-Channel JFET Conduction


The depletion layer is thickerwhen Vgs high

Less current can flowfrom Drain to source

The current flow from Drain tosource can be controlled bycontrolling gate voltage.


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The depletion layer isthinner when Vgs = 0N

More current can flowfrom Drain to source

The current flow from Drain tosource can be controlled bycontrolling gate voltage.


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Assignment 1

State the advantages and disadvantages of SCR and TRIAC compared to magnetic contactors.

Differentiate characteristics between FET and Bipolar Junction Transistor (BJT).

Explain the structure, symbol, static characteristics and application of the followingdevices:-

• Bidirectional diode (DIAC)

• Programmable Unijunction Transistor (UJT)

• Opto-coupler

• Zener diode

• Photo transistor

• Photo thyristor

• Photo diode

• Operational Amplifier

Submission date : Before 30 Jan 2010Prepared by Lab GroupAssessments :-Assignment report : 5 MarkInterview : 5 Mark

Scr Triac, Bjt)

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