Special Purpose Diode NOTES

8/16/2019 Special Purpose Diode NOTES

1/36

EKT 102

CHAPTER 3

SPECIAL-PURPOSE DIODE

1


2/36

3.1 Zener Diode

Zener diode is a p-n junction diode

that is designed to operate in the

reverse breakdown region.

Two things happen when the

reverse breakdown voltage (VBR) isreached:

The diode current increases

drasticall!

The reverse voltage (VR

) across

the diode re"ains relativel

constant!

#n other words$ the voltage across

a %ener diode operated in this

region is relatively constant over arange o& reverse current and nearly

equal to its %ener voltage (VZ)

rating!

3. SPECIAL-PURPOSE DIODE

'

#ZVZ

node ()

*athode (+) +

Fig.3-1: Zener diode

s"bol!

Fig.3-2: Zener diode voltage-curent (V-#) characteristic!,


3/36

Inventor of ener Dio!e


C"#ren$e %e"vin ener was a pro&essor at *arnegie ellon

.niversit in the depart"ent o& /hsics! 0e developed the

Zener iode in 1234 and e"ploed it in "odern co"puter

circuits!

5


4/36

3.1.1 ener &re#'!o(n

There are two tpes o& reverse breakdown:

1! valanche breakdown!

,! Zener breakdown!


Avalanche breakdown is a high-&ielde&&ect that occurs when the

electrostatic field strength associated

with the p-n junction is strong enough

to pull electrons out o& the valence

band within the depletion region!Zener breakdown is a tpe o& reverse breakdown that occurs at relativel

low reverse voltages! The n-tpe and p-tpe "aterials o& a %ener diode

are heavil doped$ resulting in a very narrow depletion region. There&ore$

the electric &ield e6isting within this region is intense enough to pull

electrons &ro" their valence bands and create current at a low reversevoltage (VR)!

)ote*Zener diodes with low VZ ratings e6perience %ener breakdown$ while

those with high VZ ratings usuall e6perience avalanche breakdown!7


5/36

Fig.3-3: Reverse characteristic o&

a %ener diode!

VR

VZ

#Z

8VR

8#R

VBR

#Z+

#ZT

#Z

Zener knee current

Zener test current

"a6i"u" Zener current

3.1.2 &re#'!o(n C+#r#$teri,ti$,

The characteristic that indicates the abilit o&the %ener diode to keep the reverse voltage

across its ter"inals nearl constant "akes

the diode is use&ul as a voltage regulator !

9our "ain characteristics o& the %ener diode

are:Zener voltage (V Z ) is the no"inal %ener

voltage at the breakdown voltage!

Zener knee current (I ZK ) is the "ini"u"

current reuired to "aintain the diode in

breakdown &or the voltage regulation!

Zener test current (I ZT ) is the current level at

which the VZ rating o& the diode is

"easured!

Zener maximum current (I ZM ) is the"a6i"u" reverse current$ which "a not

be e6ceeded! t this current level$ the

diode can work without being da"aged or

destroed!


3


6/36

3.1.3 I!e#"-#n!-Pr#$ti$#" ener Eiv#"ent Cir$it,


V9VR

#9

#R

VZ

Fig.3-/* #deal "odel and

characteristic curve o& a %ener

diode in reverse breakdown!

The constant voltage drop ;

the no"inal %ener voltage!

Fig.3-* /ractical "odel and characteristic curve o& a %ener

diode$ where the %ener i"pedance (resistance)$ ZZ is

included!

change in %ener current (8#Z) produces a s"all

changes in %ener voltage (8VZ)!<


7/36


3.1./ Teer#tre Coeffi$ient

The %ener voltage o& a %ener diode is ver sensitive to the te"perature o& operation!

The &or"ula &or calculating the change in %ener voltage due to a change in

te"perature is

)( 01 T T x xT V V

C Z Z −=∆

where$ VZ ; no"inal %ener voltage at the re&erence te"perature o& ,3o*!

T* ; te"perature coe&&icient!

T1 ; new te"perature level!

T4 ; re&erence te"perature o& ,3o*!

3.1./ ener Po(er Di,,i#tion #n! Der#ting

The "a6i"u" current that "a be carried b a given %ener diode depends on both the

zener voltage and the maximum dc power dissipation capabilit o& the diode! The dcpower dissipation o& the %ener diode is given b the &or"ula$

Z Z D V I P =

(5-1)

(5-,)=


8/36


The "a6i"u" power dissipation o& a %ener diode is speci&ied &or te"perature at or below

a certain value (34o*$ &or e6a"ple)!

bove the speci&ied te"perature$ the "a6i"u" power dissipation is reduced according

to a derating &actor!

The derating &actor is e6pressed in ">?o*!

The "a6i"u" derated power can be deter"ined with the &ollowing &or"ula:

T C mW P P o

Dderated D

∆−= )/((max))(

(5-5)

@


9/36


10/36


9or an ideal "odel o& a certain %ener diode$ the minimum zener current (I ZK

is speci&ied

on datasheet! 0owever$ the maximum zener current is not given on datasheet but can

calculated &ro" the maximum diode power specification$ which is given on datasheet b

using the euation:

Z

D

ZM

V

P

I (max)=

9or the "ini"u" %ener current$ the voltage across the resistor is deter"ined b:

Z R IN V V V +=(min)

Thus$ the "ini"u" input voltage that can be regulated b the %ener diode$

R I V ZK R =

(5-7)

(5-3)

(5-


11/36


9or the "a6i"u" %ener current$ the voltage across the resistor is deter"ined b:

Z R IN V V V +=

'

(max)

Thus$ the "a6i"u" input voltage that can be regulated b the %ener

diode$

R I V ZM R

='

(5-=)

(5-@)

11


12/36


3.2.2 ener Reg"#tion (it+ # #ri#6"e Lo#!

The %ener diode "aintains a nearl constant voltage across RA as long as the %ener

current is greater than I ZK and less than I Z! !

>hen the output ter"inals o& the %ener regulator are open (RA ; ) or a no-load

condition$ the load current (#A) ; 4 and all o& the current is through the %ener!

>hen a load resistor (RA) is connected$ a part o& the total current is through the %enerand an other part through RA!

s RA is decreased$ the load current #A increases and #Z decreases! The %ener diode

continues to regulate the voltage until #Z reaches its "ini"u" value$ #Z+! t this point #A is

"a6i"u"$ and a &ull-load condition e6ists!

Fig.3-7: Zener voltage regulation

with a variable load

1,


13/36


B using "athe"aticall &or"ula$ when #Z is "a6i"u"$ we obtain:

)(0(min) ∞== L L R A I

R

V V I I Z IN

T Z

−==(max)

thus$

>hen #Z is "ini"u" (#Z ; #Z+)$ so

ZK T L I I I −=(max)

(max)

(min)

L

Z

L

I

V R =

(5-2)

(5-14)

(5-11)

(5-1,)

15


14/36


#n addition to voltage regulation applications$

%ener diode can be used in ac applications to

li"it voltage swings to desired levels!

Fig.3-8.

/art (a) shows a %ener used to li"it the

positive peak o& a signal voltage to the

selected %ener voltage!

uring the negative alternation$ the %ener acts

as a &orward-biased diode and li"its the

negative voltage to -4!= V!

>hen the %ener is turned around$ as in part

(b)$ the negative peak is li"ited b %ener action

and the positive voltage is li"ited to '4!= V!

Two back-to-back %eners li"it both peaks to

the %ener voltage C4!= V$ as shown in part (c)!

uring the positive alternation$ , is

&unctioning as the %ener li"iter and 1 is

&unctioning as a &orward-biased diode! uring

the negative alternation$ the roles are

reversed!

3.2.3 ener Reg"#tion (it+ # #ri#6"e Lo#!

17


15/36


3.3 Varactor Diode

Varactor is a tpe o& p-n junction diode thatoperates in reverse bias! The capacitance o& the

junction is controlled b the a"ount o& reverse

bias!

Varactor diodes are also re&erred to as varicaps

or tuning diodes and the are co""onl used in

co""unication sste"s!

3.3.1 ,i$ Oer#tion

The capacitance o& a reverse-biased varactor

junction is &ound as:

Fig.3-10: Reverse-biased varactor

diode acts as a variable capacitor!

Fig.3-9: Varactor diode s"bol

d

AC

ε =

where$ * ; the total junction capacitance! ; the plate area!

D ; the dielectric constant (per"ittivit)!

d ; the width o& the depletion region

(plate separation)!

(5-15)

13


16/36


The abilit o& a varactor to act as a voltage-controlled capacitor is de"onstrated in 9ig!

5-14!

Fig.3-10: Varactor diode capacitance varies with reverse voltage!

s the reverse-bias voltage increases$ the depletion region widens$ increasing the plateseparation$ thus decreasing the capacitance!

>hen the reverse-bias voltage decreases$ the depletion region narrows$ thus increasing

the capacitance! 1<


17/36


3.3.2 #r#$tor A"i$#tion

"ajor application o& varactor is in tuning circuits$ &or e6a"ple$ V09$ .09$ and satelite

receivers utili%e varactors! Varactors are also used in cellular co""unications!

>hen used in a parallel resonant circuit$ as shown in 9ig! 5-11$ the varactor acts as a

variable capacitor " thus allowing the resonant &reuenc to be adjusted b a variable

voltage level!

Fig.3-11: resonant

band-pass &ilter!

1=


18/36


*1 prevents a dc path &ro" potentio"eter wiper back to the ac source through the

inductor and R1!

*, prevents a dc path &ro" cathode to the anode o& the varactor through the inductor!

*5 prevents a dc path &ro" the wiper to a load on the output through the inductor!

*7 prevents a dc path &ro" the wiper to ground!

R,$ R5$ R7 and R3 &unction as a variable dc voltage divider &or biasing the varactor!

The parallel resonant &reuenc o& the A* circuit is

LC f r

π 21≅

where$ A ; the inductance o& an inductor (0)

* ; the capacitance o& a capacitor (9)!

(5-17)

1@


19/36


3.4 Optical Diodes

There are two popular tpes o& optoelectronic devices: light#emitting diode ($%& and

photodiode.

3./.1 T+e Lig+t-Eitting Dio!e :LED;

AE is diode that e"its light when biased in the &orward direction o& p-n junction!

node *athode

Fig.3-12: The sche"atic s"bol and construction &eatures!

(b) (c)

12

2 DIODES


20/36

2. DIODES

Fig.3-13: AE that are produced in an arra o& shapes and si%es!

AE characteristics:

characteristic curves are ver si"ilar

to those &or p-n junction diodes

higher &orward voltage (V9)

lower reverse breakdown voltage (VBR)!,4


21/36


The basic operation o& AE is as illustrated in 9ig!

5-17:

“When the device is forward-biased, electronscross the p-n junction from the n-type materialand recombine with holes in the p-type material.These free electrons are in the conduction bandand at a higher energy than the holes in thevalence band.

When recombination takes place, therecombining electrons release energy in theform photons .

A large eposed surface area on one layer of

the semiconductive material permits thephotons to be emitted as visible light.!

This process is called electroluminescence.

Various i"purities are added during the doping

process to establish the wavelength o& the e"itted

light! The wavelength deter"ines the color o&

visible light! Fig.3


22/36

TBAE 5-1: *o""on AEs


L! "emiconductor Materials

The color e"itted b a given AE depends on the co"bination o& ele"ents used to

produce the co"ponent! Fo"e co""on ele"ent co"binations are identi&ied in Table

5-1!

*o"pound 9orward Voltage (V9) *olor E"itted

Gas 1!3 V #n&rared (invisible)

lGas 1!@ V Red

Ga/ ,!7 V Green

Gas/ ,!4 V Hrange

GaI 7!1 V >hite

lGa#n/ ,!4 V "ber (ellow)

lGa#nI 5!< V Blue

V9 is "easured at #9 ; ,4 " in each case!

,,

3 SPECIAL PURPOSE DIODE


23/36


#urrent$Limiting %esistor

>hen used in "ost practical applications$ AE reuire the use o& a series current-

li"iting resistor$ as shown in 9ig! 5-1< (a)! The resistor ensures that the "a6i"u"

current rating o& the AE can not be e6ceeded b the circuit current!

The a"ount o& power output translated into light is directly proportional to the &orward

current$ as indicated in 9ig! 5-1< (b)

Fig.3-15: Basic operation o& a AE! ,5



24/36


The li"iting resistor (RA##T) is deter"ined using the &ollowing uestion:

F

F Bias

LIMIT

I

V V R

−

=

,7



25/36


A&&lication

The seven segment display is an example of !Ds "se for display ofdecimal digits.

Fig.3-17: The =-seg"ent AE displa!,3



26/36


3./.2 T+e P+oto!io!e

/hotodiode is a p-n junction that can convert

light energ into electrical energ!

#t operates in reverse bias voltage (VR)$ as

shown in 9ig! 5-1@$ where I= is the reverse light

current!

#t has a s"all transparent window that allows

light to strike the p-n junction!

The resistance o& a photodiode is calculated b

the &or"ula as &ollows:

λ I

V R

R R =

Fig.3-18: /hotodiode!

,<



27/36


>hen its p-n junction is e6posed to light$ the reverse current increases with the light

intensit as shown b the graph in 9ig! 5-12 e6pressed as irradiance (">?c",)!

>hen there is no incident light$ the reverse current is al"ost negligible and is called

the dark current !

Fig.3-19: Tpical photodiode characteristics!,=



28/36


9ig! 5-,4 illustrates that the photodiode is placed in the circuit in reverse bias! s with

"ost diodes when in reverse bias$ no current &lows when in reverse bias$ but when lightstrikes the e6posed junction through a tin window$ reverse current increases

proportional to light intensit!

Fig.3-20: Hperation o& photodiode! ,@

3 SPECIAL-PURPOSE DIODE


29/36


3.# Other Types of Diodes

3..1 T+e S$+ott'4 Dio!e

Fchottk diode s"bol is shown in 9ig! 5-,1(a)! The Fchottk diodeJs signi&icant

characteristic is its fast switching speed ! This is use&ul &or high &reuencies and digital

applications! #t is not a tpical diode in that it does not have a p-n junction! #nstead$ itconsists o& a doped se"iconductor (usuall n-tpe) and "etal bound together$ as

shown in 9ig! 5-,1(b)!

Fig.3-21: (a) Fchottk diode s"bol and (b) basic internal construction

o& a Fchottk diode!,2



30/36


3..2 T+e L#,er Dio!e

The laser diode ("ight #"pli&ication b ,ti"ulated e"ission o& r adiation) produces a

"onochro"atic (single color) light! Aaser diodes in conjunction with photodiodes are

used to retrieve data &ro" co"pact discs!

Fig.3-22: Basic laser diode construction and operation!54



31/36


3..3 T+e PI) Dio!e

The pin diode is also used in "ostl "icrowave &reuenc applications! #ts variable

&orward series resistance characteristic is used &or attenuation$ "odulation$ and

switching! #n reverse bias it e6hibits a nearl constant capacitance!

Fig.3-23: /#I diode

51



32/36

3. SPECIAL PURPOSE DIODE

3../ Crrent Reg"#tor Dio!e

*urrent regulator diodes keeps a constant current value over a speci&ied range o&

&orward voltages ranging &ro" about 1!3 V to < V!

Fig.3-2/: F"bol &or a current regulator diode!

5,



33/36


3.. T+e Ste-Re$over4 Dio!e

The step-recover diode is also used &or &ast switching applications! This is achievedb reduced doping at the junction!

3..5 T+e Tnne" Dio!e

The tunnel diode has negative resistance! #t will actuall conduct well with low &orward

bias! >ith &urther increases in bias it reaches the negative resistance range where

current will actuall go down! This is achieved b heavil-doped p and n "aterials that

creates a ver thin depletion region!

Fig.3-2: Tunnel diode s"bol and characteristic curve!55



34/36


3.6 Troubleshooting

lthough power suppl outputs generall use #* regulators$ %ener diodes can be usedas a voltage regulator when less precise regulation and low current is acceptable!

Fig.3-2* Zener-regulated power suppl test!

The "eter readings

o& 13!3 V &or no-load

check and 17!@ V &or

&ull-load test indicateappro6i"atel the

e6pected output

voltage o& 13 V!

properl &unctioning

%ener will work to

"aintain the outputvoltage within certain

li"its despite

changes in load!

57



35/36

3 S C U OS O

C#,e-1* ener Dio!e Oen

#n no-load check$ output voltage is ,7 V as shown in 9ig! 5-,


36/36

Documents

Special Purpose Diode NOTES