Jm610-Chapter 2 Rectfier

8/19/2019 Jm610-Chapter 2 Rectfier

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CHAPTER 2:CHAPTER 2:

DC CONVERTERSDC CONVERTERS

At the end of the lesson, students

should be able to :

1. Give defnitions o DC converters

2. Analyze controlled anduncontrolled circuit o a rectifer

3. Explain the principle operation o a

rectifer4. Analyze chopper circuit


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2.1 ntroduction and Defnitions o2.1 ntroduction and Defnitions o

DC convertersDC converters

An electronic circuit !hich converts onedirect"current volta#e into another$consistin# o an inverter ollo!ed %y astep"up or step"do!n transor&er andrectifer.

t is a class o po!er converter.

DC to DC converters are i&portant in

porta%le electronic devices such ascellular phones and laptop co&puters$!hich are supplied !ith po!er ro&%atteries pri&arily.


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Electronic devices oten containseveral su%"circuits$ each !ith its o!n

volta#e level re'uire&ent di(erentro& that supplied %y the %attery oran external supply )so&eti&es hi#heror lo!er than the supply volta#e*.

+!itched DC to DC converters o(er a&ethod to increase volta#e ro& apartially lo!ered %attery volta#e

there%y savin# space instead o usin#&ultiple %atteries to acco&plish thesa&e thin#.


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2.2 Analyze control and2.2 Analyze control and

uncontrolled circuit o a rectiferuncontrolled circuit o a rectifer

A DC,DC converter is a device that ta-es aninput DC volta#e and converts it into adi(erent output volta#e.

here are t!o cate#ories o re#ulatin#

DC,DC converters/ a linearre#ulator,controller and a s!itchin#converter, controller.

A linear re#ulator or controller can only

step do!n a hi#her input volta#e to a lo!eroutput volta#e. ts output current e'uals itsinput current plus its %iasin# or #roundcurrent.


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+!itch"%ased re#ulators &ove ener#yro& input to output in discrete

pac-ets via one %ipolar or E s!itchand diode )or another %ipolar or Es!itch*.

An inductor or capacitor is used as the

ener#y stora#e ele&ent that transersener#y ro& the input to the output othe po!er supply circuit.

hus$ the output and input currentsare not e'ual and eciencies are&uch hi#her than in a re#ulator.


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2.2.1 he principle operation o a2.2.1 he principle operation o a

rectiferrectifer

ectifcation◦ ectifcation is the process o chan#in#

alternatin# current to direct current. 5hena se&iconductor rectifer$ such as a

6unction diode$ is connected to an acvolta#e source$ it is alternately %iasedor!ard and reverse$ in step !ith the acvolta#e$ as sho!n in i#ure 2.1

Figure 2.1: Rectification Process


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2.2.2 Analyze volta#e and current2.2.2 Analyze volta#e and current

!aveor& in the circuit !ith load!aveor& in the circuit !ith loada. esistive loada. esistive load

1. 8al"!ave ectifer1. 8al"!ave ectifer

n i#ure 2.2 a diode is placed in series !itha source o ac po!er and a load resistor.

his is called a hal"!ave rectifer circuit. he transor&er provides the ac input to

the circuit.

he diode provides the rectifcation o the

ac.

Figure 2.2: Half-wave rectifier circuit and signal


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he load resistor serves t!o purposes/)1* t li&its the a&ount o current :o! in

the circuit to a sae level$ and )2* itdevelops an output si#nal due to thecurrent :o! throu#h it.

Assu&e$ in i#ure 2.3$ that the top othe transor&er secondary is positiveand the %otto& ne#ative.

Figure 2.3: Output of half-wave rectifier circuit


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5ith this polarity$ the diode is or!ard%iased$ resistance o the diode is verylo!$ and current :o!s throu#h thecircuit in the direction o the arro!s.

he output )volta#e drop* across theload resistor ollo!s the !ave shape o

the positive hal o the ac input.5hen the ac input #oes in a ne#ative

direction$ the top o the transor&ersecondary %eco&es ne#ative and the

diode %eco&es reverse %iased.


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5ith reverse %ias applied to the diode$the resistance o the diode %eco&es

very #reat$ and current :o! throu#hthe diode and load resistor %eco&eszero.

)e&e&%er that a very s&all current

!ill :o! throu#h the diode.* heoutput$ ta-en across the load resistor$!ill %e zero. the position o the diode!ere reversed$ the output !ould %ene#ative pulses.


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8al 5ave ectifer8al 5ave ectifer

5aveor&s5aveor&s

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he dc or avera#e =utput volta#e

he dc output current orresistive load

he r&s output current andvolta#e

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he avera#e po!er a%sor%ed %y

the resistor

Eciency

!here$

>dc ? @dc.dc and >ac ? @r&s . r&s

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Exa&ple 1Exa&ple 1

or the hal"!ave rectifer !ith load$ the po!er supply is asinusoidal volta#e o 12


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Exa&ple 2

Given a hal"!ave rectifer !ith

load$ !ith pea- supply volta#e o12


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2. ull"!ave rectifer

• n a hal !ave rectifer$ a hal cycle o

po!er is produced across the loadresistor or each ull cycle o inputpo!er.

• o increase the output po!er$ a ull

!ave rectifer can %e used.A ull"!ave rectifer converts the !hole othe input !aveor& to one o constantpolarity )positive or ne#ative* at its output.

ull"!ave rectifcation converts %othpolarities o the input !aveor& to DC)direct current*$ and yields a hi#her &eanoutput volta#e.


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i.i. ull !ave rectifer !aveor&ull !ave rectifer !aveor&)Center)Center

ap* ap*

1;


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Figure 2.5: Full-wave rectifier using a center tap transforer

and 2 diodes.

i#ure a%ove sho!s a ull"!ave rectifer$!hich is$ in e(ect$ t!o hal"!ave rectifers

co&%ined into one circuit. n this circuit a load resistor is used to li&it

current :o! and develop an output volta#e$t!o diodes to provide rectifcation$ and a

transor&er to provide an ac input to thecircuit.


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he transor&er$ used in ull"!ave rectifercircuits$ &ust %e center"tapped to co&plete thepath or current :o! throu#h the load resistor.

Assu&in# the polarities on the transor&er isthe positive hal cycle $ diode D1 !ill %e or!ard%iased and current !ill :o! ro& #roundthrou#h the load resistor$ throu#h diode D1$ tothe top o the transor&er.

5hen the ac input chan#es direction$ the

transor&er secondary !ill assu&e an oppositepolarity. Diode D2 is no! or!ard %iased andcurrent !ill :o! in the opposite direction$ ro&#round throu#h the load resistor$ throu#h diodeD2$ to the %otto& hal o the transor&er.


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5hen one diode is or!ard %iased$ the other isreverse %iased. o &atter !hich diode isor!ard %iased$ current !ill :o! throu#h the

load resistor in the sa&e directionB so theoutput !ill %e a series o pulses o the sa&epolarity. y reversin# %oth diodes$ the outputpolarity !ill %e reversed.

his !ill #ive ull !ave rectifcation. Even ateryouve rectifed the !ave$ its still !avvy. +othen you have to flter it to #et it nice ands&ooth li-e the DC current you &i#ht #et outo a %attery.


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@olta#e and@olta#e and

currentcurrent 23


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ii. ull"!ave rid#e ectiferii. ull"!ave rid#e ectifer

Another$ &ore popular ull"!ave rectifer

desi#n exists$ and it is %uilt around a our"diode %rid#e conf#uration. his desi#n iscalled a full-wave bridge.

he %rid#e rectifer di(ers ro& the ull"!ave

rectifer in that a %rid#e rectifer does notre'uire a center tapped transor&er$ %utdoes re'uire t!o additional diodes.

Figure 2.!: Full-wave "ridge rectifier.


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Current directions or the ull"!ave %rid#erectifer circuit are as sho!n in i#ure 2.7 orpositive hal"cycle and i#ure 2.9 or ne#ativehal"cycles o the AC source !aveor&.

Figure 2.#: $lectron flow for positive half-c%cles

Figure 2.&: $lectron flow for negative half-c%cles.


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ote that re#ardless o the polarity o the input$the current :o!s in the sa&e direction throu#hthe load.

hat is$ the ne#ative hal"cycle o source is apositive hal"cycle at the load.

he current :o! is throu#h t!o diodes in seriesor %oth polarities.

hus$ t!o diode drops o the source volta#e arelost )


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ull !ave rectifer !aveor&sull !ave rectifer !aveor&s

)rid#e*)rid#e*

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rid#e 5aveor&srid#e 5aveor&s

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or %oth the %rid#e and center"tapped transor&er rectifers$ the dc

or avera#eoutput current is/

he dc output current or the resistiveload is

he r&s output volta#e and currentare

2;


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Exa&pleExa&ple

Given a centre"tapped rectifer ini#ure 2.4$ !ith pea- supplyvolta#e o 12


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Exa&pleExa&ple

Dra! the e'uivalent circuitdia#ra& or a sin#le"phase ull"!ave %rid#e rectifer$ the source

volta#e$ the output volta#e andthe volta#es across all the diodes.

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%. nductive load%. nductive load

i. 5ithout ree!heelin#i. 5ithout ree!heelin#diodediode

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The circuit arrangement of single phase half

wave rectifier is shown below with highlyinductive load.

During positive half cycle, diode is forward

biased and the load is connected to the input

supply. Due to the inductive load , diode will continue

conduct beyond ωt = , even though the input

voltage is already negative.


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@olta#e and Current 5aveor&s@olta#e and Current 5aveor&s

or ectifer !ith "F loador ectifer !ith "F load

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ii. 5ith ree!heelin# diodeii. 5ith ree!heelin# diode

A ree!heelin# diode so&eti&es called a snu%%erdiode$ :y%ac- diode $ suppressor diode or catchdiode.

his diode used to eli&inate :y%ac-$ !hich is the

sudden volta#e spi-e seen across an inductive load!hen its supply volta#e is suddenly reduced orre&oved.

hese diodes are connected in reverse direction inparallel !ith inductive loads.

hese diodes helps in providin# a s&ooth current tothe load and also eli&inates the ne#ative volta#eacross the load.


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8al !ave rectifer !ith ree"!heelin#Diode


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Rectifier circuit with inductive load


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Current is :o!in# do!n ro& the positiveter&inal o the volta#e source to its ne#ativeter&inal$ throu#h the inductor.

5hen the s!itch is opened )i#ure 2*$ theinductor !ill atte&pt to resist the sudden dropo current %y usin# its stored &a#netic feldener#y to create its o!n volta#e.

An extre&ely lar#e ne#ative potential iscreated !here there once !as positivepotential$ and a positive potential is created!here there !as once ne#ative potential.

he s!itch$ ho!ever$ re&ains at the volta#e othe po!er supply$ %ut it is still in contact !iththe inductor pullin# do!n a ne#ative volta#e.

+ince no connection is physically &ade to


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+ince no connection is physically &ade toallo! current to continue to :o! )due to thes!itch %ein# open*$ the lar#e potentialdi(erence can cause electrons to arc across

the air"#ap o the open s!itch )or 6unction o atransistor*. his is undesira%le or the reasons&entioned a%ove and &ust %e prevented.

A :y%ac- diode solves this starvation"arc

pro%le& %y allo!in# the inductor to dra!current ro& itsel )thus$ :y%ac-* in acontinuous loop until the ener#y is dissipatedthrou#h losses in the !ire and across the diode)i#ure 3*.

5hen the s!itch is closed the diode is reverse"%iased a#ainst the po!er supply and doesntexist in the circuit or practical purposes.


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8o!ever$ !hen the s!itch is opened$ thediode %eco&es or!ard"%iased relative to theinductor )instead o the po!er supply as

%eore*$ allo!in# it to conduct current in acircular loop ro& the positive potential atthe %otto& o the inductor to the ne#ativepotential at the top )assu&in# the po!ersupply !as supplyin# positive volta#e at thetop o the inductor prior to the s!itch %ein#opened*.

he volta#e across the inductor !ill &erely%e a unction o the or!ard volta#e drop o

the :y%ac- diode. otal ti&e or dissipationcan vary$ %ut it !ill usually last or a e!&illiseconds.


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Controlled ectiferControlled ectifer

o o%tain controlled outputvolta#e $ phase controlledthyristor are used instead o

diodes. he output volta#e o thyristor

rectifer is varied %y controllin#

the delay or frin# an#le othyristor.

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+in#le >hase ully Control+in#le >hase ully Control

8al 5ave ectifer H8al 5ave ectifer H

esistive Foadesistive Foad

i#. a%ove$ sho!s the circuit dia#ra& o

a sin#le phase ully controlled hal !averectifer supplyin# a purely resistiveload.

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At It ? < !hen the input supply volta#e%eco&es positive the thyristor %eco&esor!ard %iased. 8o!ever$ unli-e a diode$ it

does not turn = till a #ate pulse isapplied at It ? J.

Durin# the period < K It L J$ the thyristor%loc-s the supply volta#e and the load

volta#e re&ains zero. Conse'uently$ noload current :o!s durin# this interval.

As soon as a #ate pulse is applied to thethyristor at It ? J it turns =. he

volta#e across the thyristor collapses toal&ost zero and the ull supply volta#eappears across the load.

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ro& this point on!ards the load volta#eollo!s the supply volta#e. he load %ein#purely resistive$ so the load current io is

proportional to the load volta#e.At It ? M as the supply volta#e passes

throu#h the ne#ative #oin# zero crossin#the load volta#e and hence the loadcurrent %eco&es zero and tries to reversedirection. n the process the thyristorunder#oes reverse recovery and starts%loc-in# the ne#ative supply volta#e.

hereore$ the load volta#e and the loadcurrent re&ains cla&ped at zero till thethyristor is ired a#ain at It ? 2M N J.

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8al 5ave ectifer )esistive8al 5ave ectifer )esistive

Foad* 5aveor&sFoad* 5aveor&s

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tif ti i


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ectifcation eciency /

..

or& actor/

ipple actor/

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8al 5ave ectifer H F Foad8al 5ave ectifer H F Foadi# %elo! sho!s the circuit dia#ra& o

a sin#le phase ully controlled hal!ave rectifer supplyin# a resistiveinductive load.

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As in the case o a resistive load$ thethyristor %eco&es or!ard %iased!hen the supply volta#e %eco&es

positive at It ?


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inally$ at It ? O )O P M* the loadcurrent %eco&es zero and thethyristor under#oes reverse recovery.ro& this point on!ards the thyristor

starts %loc-in# the supply volta#e andthe load volta#e re&ains zero until thethyristor is turned = a#ain in the

next cycle.

0<


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+in#le >hase ully Control 8al+in#le >hase ully Control 8al

5ave ectifer )F Foad*5ave ectifer )F Foad*

"5aveor&s"5aveor&s

01


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Exa&ple 3Exa&ple 3

Analyze a sin#le phase hal !avecontrolled rectifer !ith load toproduce an avera#e volta#e o 4<

volt across 1


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+olution+olution

03

i l h ll l


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ull 5ave ectifer H Foadull 5ave ectifer H Foad


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Exa&ple 4 )Q@*Exa&ple 4 )Q@*

A hal !ave controlled rectiferconnected to 10< @$ < 8z source issupplyin# a resistive load o 1< oh&. the delay an#le J ? 3< de#$ solve orthe B

i.Raxi&u& load current

ii.R+ load current

iii.>o!er supplied to the loadiv.ipple re'uency

v.>o!er actor

Jm610-Chapter 2 Rectfier

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