Et502 Power Electronic

5/25/2018 Et502 Power Electronic

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CHAPTER 1:

OVERVIEW OF POWER

ELECTRONIC DEVICES


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SCR

Structure

Terminals

Operation

Symbol


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SCR

pnpndevices

(thyristor)family

Terminals

Gate(G)

Anode(A)

Cathode(C).

Applications

relay control

time-delaycircuit

regulatedpower supplier

staticswitches

motor control

chopper inverter

batterycharges

phase control


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the anode must be positive with respect to

the cathode and no gate signal junctionj1

andj3are forward biased andj2is reversed

biased.


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cathode positive with respect to the anode,

j2is forward-biased andj1 andj3are

reverse biased. Again only leakage current

flows.


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Using 2 transistor (NPN and PNP).

From 0 ke t1 ,Vgate = 0V.

Vb2 =Vgate =0V

Base current, Ib2 =0 and Ic2 almost same to Ic0. Base current for Q1 , Ib1=Ic2=Ic0.

The current too smal to on the Q1.

So that, both transistor will OFF .

The resistance for each transistor are highbetween the collector and emitter.

So that, the circuit will open.


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When t= t1, Vg is trigger to the gate terminal.

The value is enough to onQ2(Vbe2=Vg).

Current collector Q2 will increase and will on

Q1(Ib1=Ic2).

When Q1 on,Ic1 will increase and also increase the Ib2.

Ib2 at Q2 increase and it also increase the Ic2.

This process is called Regeneration Concept where the

current collector for each transistor will increase and this

will make the process continuous for each transistor.

The resistance between anode and cathode is low.

So that, the anode current (IA) is almost same with the

cathode current (IK).

In this condition, we can assume that the SCR will on.


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Forward Breakover voltageMinimum forward voltage, gate being open, SCR

starts conducting to turn ON (50 500V)

Holding current

Minimum anode current, to maintain SCR in the

ON state


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Latching Current

Minimum anode current required

maintaining SCR in ON state immediately

after SCR has been turn on and gate currenthas been removed


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Reverse Breakover Voltage

Maximum reverse voltage can applied to

SCR without conducting in reverse direction


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Forward current rating

Maximum anode current that SCR is capable

of passing without destruction

Gate triggering voltageMinimum values of gate voltage at which

SCR is turned ON


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Gate triggering current

Resulting gate current


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Increase the forward biased voltage more or

equal to forward breakdown

Trigger positive supply when the device in

forward biased condition, Gate controlmethod are trigger with

(i)DC signal

(ii)AC signal

(iii)beat signal


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Method to off SCR

1)natural Commutation

2)Reverse Bias Turn Off3)Get turn Off.


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#To turn off SCR, the anode-cathodecurrent must be reduced to less than

the holding current, Ih

Requirement to off SCR1) Cut off the current connection of SCR.

2) Short anode and cathode.

3) Switch off the positive voltage supply at anode.4) Change the polarity of anode to negative.


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GTO

Structure

Terminals

Operation

Symbol


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4 layers of semi-conductor material

Has 3 terminals, anode, cathode and Gate.

Behave like normal thyristor, but can be

turned off using gate signal.However turning off is difficult. Need very

large reverse gate current (normally 1/5 of

anode current).


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- Can turn on by giving positive supply to the gate.

- GTO still function without positive supply.

- GTO can turn off by supply negative supply at

the gate and this will cause negative Ig flow

drastically.- Negative Ig only flow in just a few microsecond.

-When Ig reach the maximum level, current at

anode will fall down and VAK voltage will

increase


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i. Tractionii. inverters

iii. motor drives

iv. pulsed power

v. distribution lines

vi. induction heating

vii. flexible ac transmission

systems


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TRIAC

Structure

Terminals

Operation

Symbol


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http://en.wikipedia.org/wiki/File:TRIAC_(smial).jpg


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Has three terminal semiconductor for controlling

current in either direction.

The symbol looks like two SCRs in parallel(

opposite direction) with one trigger or gateterminal.


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n

n n

p

n

p

n

MT2

MT1G

The main or power terminals are designated

as MT1 and MT2 .

Bidirectional electronic switch which can

conduct current in either direction when it is

triggered (turned on). Doesn't have any

polarity.

When the voltage on the MT2 is positive

with regard to MT1 and a positive gate voltage

is applied, the left SCR conducts.

When the voltage is reversed and a negativevoltage is applied to the gate, the right SCR

conducts.

Minimum holding current, Ih, must be

maintained in order to keep a triac conducting.


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Operates in the same way as the SCR however it

operates in both a forward and reverse

direction.

It can be triggered into conduction by either (+)

or (-) gate signal.

TRIAC is capable of conducting current in both

directions and assuring that it switches "off"

during the brief zero-crossing of current can be

difficult.


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Application

1) Phase control

2) Inverter design

3) AC switching

4) Relay replacement


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Two main categories of transistors: bipolar junction transistors(BJTs)and

field effect transistors (FETs).

Transistors have 3 terminals where the

application of current (BJT)or voltage (FET)tothe input terminal increases the amount ofcharge in the active region.

The physics of "transistor action" is quite

different for the BJT and FET.


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In analog circuits, transistors are used inamplifiers and linear regulated powersupplies.

In digital circuits they function aselectrical switches, including logic gates,random access memory (RAM), andmicroprocessors


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BJT

Structure

Terminals

Operation

Symbol


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http://upload.wikimedia.org/wikipedia/commons/5/5a/Transistors.agr.jpghttp://upload.wikimedia.org/wikipedia/commons/5/5a/Transistors.agr.jpg


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A bipolar transistoressentially consists ofa pair of PN Junctiondiodesthat are joinedback-to-back.

There are thereforetwo kinds of BJT, theNPNand PNPvarieties.


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The three layers of the sandwich areconventionally called the Collector, Base,andEmitter.

The term bipolar was tagged onto thename to distinguish the fact that both

carrier types play important roles in the

operation


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-Energy levels in an NPN

transistor under no externallyapplying voltages.-In each of the N-type layersconduction can take place bythe free movement of electronsin the conduction band.In the P-type (filling) layer

conduction can take place bythe movement of the free holesin the valence band.-However, in the absence of anyexternally applied electric field,we find that depletion zones

form at both PN-Junctions, sono charge wants to move fromone layer to another.


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What happens when weapply a moderate voltagebetween the collectorand base parts.

The polarity of theapplied voltage is chosen

to increase the forcepulling the N-typeelectrons and P-typeholes apart.

This widens the depletionzone between thecollector and base and sono current will flow.

In effect we havereverse-biassedthe Base-Collector diode junction.


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Apply a relatively smallEmitter-Base voltage whosepolarity is designed toforward-bias the Emitter-Basejunction.

This 'pushes' electrons from theEmitter into the Base regionand sets up a current flowacross the Emitter-Baseboundary.

As a result the electrons whichget into the Base move swiftlytowards the Collector andcross into the Collector region.

Hence a Emitter-Collectorcurrent magnitude is set by thechosen Emitter-Base voltage


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Some electron fall into a hole

-Some of free electronscrossing the Base encounter a

hole and 'drop into it'.

-As a result, the Base region

loses one of its positive

charges (holes).

-The Base potential would

become more negative

(because of the removal of the

holes) until it was negative

enough to repel any more

electrons from crossing theEmitter-Base junction.

The current flow would then

stop.


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Normal operation B-E junction forward biased; B-C junction reverse

biased.


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fet

FET

Structure

Terminals

Operation

Symbol


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FET Field Effect Transistor-are unipolar transistors since their operationdepends primarily on a single carrier type.

There are two main types of FETs:

a) Junction Field-Effect Transistor (JFET)

b) Metal Oxide Semiconductor Field Effect Transistor

(MOSFET)

The JFET and MOSFET are voltage controlled devices:

that is a small change in input voltage causes a large

change in output current.


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FET operation involves an electric field which

controls the flow of a charge (current ) through

the device.

Bipolar transistor employs a small input current

to control a large output current.

The source, drain, and gate terminal of the FET

are analogous to the emitter, collector, and base

of a bipolar transistor .

N-channel and P-channel refer to the materialwhich the drain and source are connected.


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The JFET channel is controlled

by the size of the depletionregion of the reverse-biased PN

junction.


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Identify the characteristics of FET

Identify the characteristics of FET


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MOSFET have two types:

a) Depletion type MOSFET ( DE MOSFET)

b) Enhancement type MOSFET (EN MOSFET)

The channel is controlled by the action of the electric

field.

DE MOSFET is similar to JFET.


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DE MOSFET- Depletion & Enhancement mode MOSFET

(MOSFET Ragam Susutan & Peningkatan)

Gate voltage can be varied through both negative and positive values. Voltage is positive valueDE MOSFET will operate in depletion mode.

Voltage is negative valueDE MOSFET will operate in enhancement

mode.

The channel has connection from drain and source. The current drain can

flow even the voltage gate ,VGS =0.


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-Has no PN junction at

the gate

- SiO2 is used to separate the

Gate terminal with the channel.

- Cause the gate current is

smaller than JFET.


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EN MOSFET- Enhancement mode MOSFET (MOSFETRagam Peningkatan)

Operate if the voltage gate,VGS is large.

The channel has no connection from drain and source. The current drain

cant flow if the voltage gate ,VGS =0.


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Until one level, the depletion area will cover

all the channel and the current ID cannot

flow.

# VGS , depletion area , and the ID

current will


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Types FET BJT

Input Resistance -Big

109-1011

-Small

103- 105

Current flow - unipolar - bipolar Operate device - Voltage-operated

device

- Current-operated

device

Power gain - Higher than BJT - Lower than FET

Power requirement - FET digital circuitneed much less

power

- BJT circuit needmore power


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Insulated Gate Bipolar Transistor have three terminals. The terminals are

collector(C), gate (G) and emitter (E).


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combines features from both the MSOFET

and BJT.

useful in high voltage and high current

switching applications


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APPLICATION

electric cars

Trains

variable speed refrigerators air-conditioners

stereo systems with switching amplifiers

switched-mode power supplies

traction motor control

induction heating


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