Chap 3 - Zener Diode-updated

8/9/2019 Chap 3 - Zener Diode-updated

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Y:SURINA MAT SU OH

SCHOOL OF ELECTRICAL SYSTEMENGINEERING


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Describe the characteristic of zener diode andanalyze its operation.Explain how a zener is used in voltageregulation and limitingDescribe the varactor diode and its variablecapacitance characteristicsDiscuss the operation and characteristics ofLEDs and photodiodes

Discuss the basic characteristics of the currentregulator diode, the pin diode, the step-recoverydiode, the tunnel diode, and the laser diode.


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Zener diodes Zener diode applica ions !arac or diodes Op ical diodes Tro"#les$oo in% S"&&ar'


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ener diode ! silicon pn -junction device thatis designed for operation in the reverse-biased region .

"he basic function of zener diode is to maintaina nearly constant dc voltage under properoperating condition.

"ypically it is used to provide a stable referencevoltage for use in power supplies, voltmeter andother e#uipment.

Zener diode symbol


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$rea%down voltage ! set by controllingthe doping level during manufacture.&hen diode reached reverse brea%down! voltage remains constant even though

current change drastically.'f zener diode is FB ! operates the sameas a recti(er diode.) zener diode is much li%e a normaldiode ! but it is placed in the circuit inreverse bias and operates in reversebrea%down.*ote that its forward characteristics are

+ust li%e a normal diode.


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General diode V-I characteristic


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"wo type of reverse brea%down avalanche breakdown & zenerbreakdown

ii avalanche breakdown - occurs in both recti er & zener diodes at high reverse

voltage- breakdown voltages greater than approximately 5V

ii zener breakdown- occurs at relatively low reverse voltage- a zener diode is heavily doped to reduce breakdown voltage

- this causes very thin depletion region-

as a result! intense electric eld exists within the depletionregion- "ear zener breakdown voltage #Vz ! the eld have enough

energy to pull electrons from their valence bands andcreate current$

*ote both type called zener diode brea%down voltages of /.01 ! 233

1


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"he reverse voltage 1 4 is increased ! the reverse current'4 remains extremely small up to the 5%nee6 of the curve.

4everse current ! called the zener current, ' .

)t that point, brea%down e7ect begin where zenerimpedance z begin to decrease as ' increases rapidly.

)t the bottom of the %nee- the zener brea%down voltage 1remains constant although it increase slightly as the zenercurrent, ' increase.

' 8 ! min. current re#uired to maintain voltage regulation '

9 ! max. amount of current the diode can handle without

being damage:destroyed ' " ! the current level at which the 1 rating of diode is

measured speci(ed on a data sheet

"he zener diode maintains nearly constant voltage for value

of reverse current rating from ' 8 to ' 9


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er diode e#uivalent circuit model and the characteristic curve illustratin

Zener impedance


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Z

Z Z

I

V Z

=

;ince the actual voltage is not ideally vertical,the change in zener current produces a smallchange in zener voltage

$y ohms law

*ormally - z is speci(ed at ' ".

'n most cases, assume z is constant over fullrange of zener current values and is purelyresistive.


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)s with most devices, zener diodes have given characteristicssuch astemperature coe


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ener diode can be used as1oltage regulator to provide stable reference

voltage.;imple limiter or clipper


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)s i:p voltage varies within limits ! zener diodemaintains a constant o:p voltage

)s 1 '* changes, ' will change proportionally, so i:pvoltage variations are set by the min. = max.current value ' 8 = ' 9 with which the zener canoperate.

4esistor, R !current limiting resistor


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Maintain o/p voltage

i/p vary from10.055V 32V


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V mA R I V ZM R 22)220)(100( ===

>rom data sheet, /*?@?3 /31 zener diode maintainregulation from ' 8 A3.2Bm) to ' 9 A/33m).

;o,

>or min. zener current, the voltage across the 223ohm resistor

;ince 1 4 A 1 '* - 1 ,

>or max. zener current, the voltage across resistoris

"hus

V V W P mA I Z D ZK 10,1,25.0 (max) ===

mA

V

P I

Z

D

ZM 100(max) ==

mV R I V ZK R 55)220)(25.0( ===

mV V mV V V V Z R IN 055.101055(min) =+=+

V V V V IN 321022

(max) =+=


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'n this illustration of zener regulation circuit, the zener diodewill 5ad+ust6 its impedance based on varying input voltagesand loads 4 L to be able to maintain its designated zenervoltage.

ener current will increase or decrease directly with voltageinput changes. "he zener current will increase or decreaseinversely with varying loads. )gain, the zener has a (niterange of operation.

*ote "he zener diode maintains a nearly constant voltageacross 4 L as long as the zener current is greater than ' 8 andless than ' 9


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&hen output terminal of zener diode is open4LA , the load current is zero.

)ll of the current is through the zener ACno-load condition.

&hen 4 L is connected, current is ow throughzener and 4 L.

"he total current through 4 is constant as longas zener is regulating.

)s 4 L decreases, load current ' L increases, ' decreases.

ener diode continues to regulate the voltageuntil ' reaches its minimum value, ' 8 .

)t this point, ' L is maximum and full-loadcondition exists.


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Determine the minimum and maximum load current for

which the zener diode will maintain regulation. &hat isthe minimum 4 L that can be used 1 A/21, ' 8 A/m) and' 9 AB3m). )ssume A3F and 1 remains a constant/21 over range of current values.


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;tep / &hen ' LA3) 4 LA , ' is maximum ande#ual to the total current, ' " .

;tep 2 ' max- is less than ' 9 , so 3) is min. valuefor ' L because zener can handle all of 2B.Bm).4 L can be removed from the circuit. ' L min- A 3)

;tep G "he max value of ' L occurs when ' is

min ' A' 8

;tep ? "he minimum value of 4 L is

mA R

V V I I Z inT z 5.25

470

1224(max)

=

=

==

mAmAmA I I I ZK T L 5.2415.25(max) ===

=== 4905.24

12

(max)(min)

L

Z

L I

V R


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>igure below shown a /*?@GG zener regulatedcircuit. Hiven 1 A /3 1 at ' " A /3m),' 8 A Bm), A 23 and ' 9 A 23m). Determine the

maximum permissible load currents.


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"he change in zener voltage, "he change in zener current, ener voltage for min and max values are

9aximum and minimum zener current

9aximum and minimum load current

Z Z Z Z I V =

Z Z Z

Z Z Z

V V V

V V V

+=

=

(max)

(min)

R

V

R

V V I I

R

V

R

V V I I

R Z inT Z

R Z in

T Z

=

==

=

==

(min)(min)

(max)(max)

ZM T L

ZK T L

I I I

I I I

=

=

(min)

(max)

ZT ZM ZK ZT Z I I @ I I I =


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Z Z Z V V V =

(min)

"o (nd ' L max- , let analyze the circuitIzener

diode connected in parallel with load resistor,4 L. >or parallel circuit, 1 same but ' di7erent. ) certain amount of ' from source will ow

through zener diode and 4 L. (f ( throughzener diode is max! ( )ow through * + is

min$ (f ( through zener diode is min! (through * + is max$ 1 for both zener and 4 L isthe ;)9E.

;o, when current through zener diode, ' ismin, 1 also min. Jn the other hand, ' L will bemax as well as 1 L max.

*ow, (nd V ,#min from the formula. Z Z Z

Z I V = ZK ZT Z

I I I =


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"he change in zener current is >ind 1 min-

;o, 1 min- AK.021. "hen, we need to (nd totalcurrent through the circuit, ( ,#min $ sing theformula,

>inally (nd ' L max- using the formula below.Hiven in the #uestion, ' 8 ABm). ;o, ' L max- is

mA I I I ZK ZT Z 9110 ===V A Z I V

Z Z Z 18.0)20(009.0 ===

V V V V Z Z Z 82.918.010(min) ===

mA A R

V

R

V V I I R

Z in

T Z 8.1411418.0100

82.924(min)(min)

==

==

==

mA I I I ZK T L 8.13658.141(max) ===


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ener diode also can be used in ac applications to limitvoltage swings to desired levela "o limit the Mve pea% of a signal voltage to the selectedzener voltage

During !ve alternation, zener acts as FB diode =limits the !ve voltage to -3.@1

b ener diode is turn around "he !ve pea% is by >$ zener action = Mve voltage islimited to M3.@1

c "wo bac%-to-bac% zeners limit both pea%s to the zenervoltage N3.@1

During the Mve alternation, D2 is functioning as thezener limiter ! D/ is functioning as a FB diode.During the !ve alternation-the roles are reversed


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Determine the output voltage for eachlimiting circuit shown below.


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&hen one zener is operating in brea%down, the

other one is >$ with approximately 3.@1 acrossit. >or (gure a, during Mve cycle, D/ G.G1 >$while D2 B./1 is zener limiter. >or (gure b,D/ O.21 is zener limiter and D2 /B1 is >$during Mve cycle.


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Jptical DiodePurrent 4egulator Diode

;chott%y DiodeQ'* Diode;tep-recovery Diode

"unnel DiodeLaser Diode


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)lthough precise power supplies typically use 'P typeregulators, zener diodes can be used alone as a voltageregulator. )s with all troubleshooting techni#ues wemust %now what is normal.

) properly functioning zener will wor% to maintain the outputvoltage within certain limits despite changes in load.


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&ith an open zener diode, the full unregulatedvoltage will be present at the output without aload. 'n some cases with full or partial loadingan open zener could remain undetected.

no voltage dropped betweenthe (ltered o:p of the powersupply = the o:p terminal

undetected


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&ith excessive zener impedance the voltagewould be higher than normal but less than thefull unregulated output.

"he zener has failed such that its internalimpedance is more than it should be.


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"he zener diode operates in reversebrea%down. ) zener diode maintains a nearly constantvoltage across its terminals over a speci(edrange of currents. Line regulation is the maintenance of a speci(cvoltage with changing input voltages.

Load regulation is the maintenance of aspeci(c voltage for di7erent loads.

Light emitting diodes LED emit eitherinfrared or visible light when forward-biased.

Qhotodiodes exhibit an increase in reversecurrent with light intensity.

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Chap 3 - Zener Diode-updated

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