Zener diodes (1)

8/7/2019 Zener diodes (1)

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ZenerZener DiodesDiodes



OutlinesOutlines

Introduction to Zener Diode

Voltage regulation and limiting



IntroductionIntroductionThe zener diode is a silicon pn junction devices that diff er s f r om rectifier

diodes because it is designed for operation in the reverse-breakdown

region. The breakdown voltage of a zener diode is set by caref ully

contr olling the level dur ing manuf acture. The basic f unction of zener

diode is to maintain a specific voltage acr oss it¶s ter minals within given

limits of line or load change. Typically it is used for pr oviding a stable

ref erence voltage for use in power supplies and other equipment.

This par ticular zener cir cuit will work to maintain 10 V acr oss the load.



Zener DiodesZener Diodes

A zener diode is much like a nor mal diode. The exception being is that it is placed in the cir cuit in rever se bias and operates in rever se breakdown.

This typical character istic curve illustrates the operating range for a zener .

Note that it¶s for ward character istics are just like a nor mal diode.

Volt-ampere character istic is shown in this Figure with nor mal operating regions for rectifier diodes and for zener diodes shown as shaded areas. 4



Zener BreakdownZener Breakdown

Zener diodes are designed to operate in rever se breakdown. Two types

of rever se breakdown in a zener diode are avalanche and zener . Theavalanche break down occur s in both rectifier and zener diodes at a

sufficiently high rever se voltage. Zener breakdown occur s in a zener

diode at low rever se voltages.

A zener diode is heavily doped to reduced the breakdown voltage. This causes a very thin depletion region. As a result, an intense

electr ic field exists within the depletion region. Near the zener

breakdown voltage (Vz), the field is intense enough to pull electr ons

f r om their valence bands and create current. The zener diodes

breakdown character istics are deter mined by the doping pr ocess

Low voltage zener s less than 5V operate in the zener breakdown

range. Those designed to operate more than 5 V operate mostly in

avalanche breakdown range. Zener s are commer cially available with

voltage breakdowns of 1.8 V to 200 V.



Breakdown CharacteristicsBreakdown Characteristics

Figure

shows

the rever

se p

or tion

of

a zen

er dio

de¶s

chara

cter istic

curve. As the rever se voltage (VR ) is increased, the rever se current (IR )

remains extremely small up to the ³k nee´ of the curve. The rever se

current is also called the zener current, IZ. At this point, the breakdown

eff ect begins; the inter nal zener resistance, also called zener impedance

(ZZ),

begins

to

decrea

se a

srever

securre

nt inc

reases

rapidly.



Zener Equivalent CircuitZener Equivalent Circuit

Zener diode equivalent circuit models and the characteristic curve illustrating ZZener diode equivalent circuit models and the characteristic curve illustrating ZZZ..

Figure (b) represents the practical model of a zener diode, where the

zener impedance (ZZ) is included. Since the actual voltage curve is not

ideally ver tical, a change in zener current (IZ) pr oduces a small change

in zener voltage (VZ), as illustrated in Figure (c).

Z

Z

Z

I

V Z

(

(!

Zener diodes (1)

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