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ZenerZener DiodesDiodes
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OutlinesOutlines
Introduction to Zener Diode
Voltage regulation and limiting
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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.
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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
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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.
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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.
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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
(
(!
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