Chapter 16
Diodes and
Applications
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C.Y. Lee
Objectives
Understand the basic structure of semiconductors
and how they conduct current
Describe the characteristics and biasing of a pn
junction diode
Describe the basic diode characteristics
Analyze the operation of a half-wave rectifier and
a full-wave rectifier
Describe the operation of power supplies
Understand the basic operation and describe some
applications of special-purpose diodes
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Introduction to Semiconductors
Two types of semiconductive materials are silicon
and germanium (both have four valance electrons)
Ge
Si
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Introduction to Semiconductors
When silicon and
germanium atoms
combine into molecules
to form a solid material,
they arrange themselves
in a fixed pattern called
a crystal
– atoms within the crystal
structure are held
together by covalent
bonds (atoms share
valence electrons)
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Introduction to Semiconductors
Recombination occurs
when a conduction-band
electron loses energy and
falls back into a hole in
the valence band
When an electron jumps to
the conduction band, a
vacancy is left in the
valence band within the
crystal (called a hole)
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Introduction to Semiconductors
In an intrinsic semiconductor, there are relatively few
free electrons
– pure semiconductive materials are neither good
conductors nor good insulators
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Introduction to Semiconductors
Intrinsic semiconductive materials must be modified by
increasing the free electrons and holes to increase its
conductivity and make it useful for electronic devices
– by adding impurities (doping), n-type and p-type
extrinsic semiconductive material can be produced
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The PN Junction Diode
A diode consists of an n region and a p region
separated by a pn junction
– the n region has many conduction electrons
– the p region has many holes
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The PN Junction Diode
Forward bias is the condition that permits current
through a diode
I
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The PN Junction Diode
Reverse bias is the condition that prevent current
through the diode
If the external reverse-bias voltage is increased to
a large enough value, reverse breakdown occurs
I≈0
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Diode Characteristics
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Diode Characteristics
The “arrowhead” in the diode symbol points in the
direction opposite the electron flow
I
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Diode Characteristics
The simplest way to visualize diode operation is to
think of it as a switch
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Typical Diode Package
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Diode Rectifiers
A diode is connected
to an ac source that
provides the input
voltage, Vin, and to a
load resistor, RL,
forming a half-wave
rectifier
– on the positive halfcycle, the diode is
forward biased
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Diode Rectifiers
The average value of a half-wave output voltage
is the value you would measure on a dc voltmeter
V AVG =
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V p ( out )
π
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Diode Rectifiers
Peak inverse voltage (PIV) is the maximum value
of reverse voltage that a diode can withstand
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Diode Rectifiers
A full-wave rectifier allows unidirectional current
to the load during the entire input cycle
The average value for a full-wave rectifier output
voltage is twice that of the half-wave rectifier
V AVG =
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2V p ( out )
π
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Diode Rectifiers
Example: Find the average value of the full-wave rectified
output voltage
VAVG = (2)(15V)/π = 9.55 V
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Diode Rectifiers
The full-wave bridge rectifier uses four diodes
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Diode Rectifiers
For the full-wave bridge rectifier, the output voltage is
a full-wave rectified voltage with a peak value equal
to the peak secondary voltage
The PIV of the diodes must equal the peak secondary
voltage:
PIV = VP(out)
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Diode Rectifiers
Example: Determine the peak output voltage for the bridge
rectifier and what minimum PIV rating is required for the
diodes?
Vp(out) = Vp(sec) = nVp(in) = (1)(25V) = 25 V
PIV = Vp(out) = 25 V
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Power Supplies
The dc power supply converts the standard 110 V, 60 Hz
ac available at the wall outlets into a constant dc voltage
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Power Supplies
Operation of a half-wave rectifier with a capacitor-input
filter:
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直流式去顫動電擊器
低交流電壓經變壓器轉為高交流電壓，再經半波整流電路
轉為直流脈動電壓。直流脈動電壓向電容器充電後，再觸
發向心臟放電，放電脈衝為一約10ms之2kV正脈衝電壓。
－
＋
＋
－
直流式去顫動電擊器電路示意圖
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電流流經心臟的方式
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Power Supplies
For a given input frequency, ripple voltage for a full-wave
rectifier will be less than that for a half-wave rectifier
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Power Supplies
The ripple factor (r) is an indication of the effectiveness
of the filter
⎛ Vr ⎞
⎟⎟ ×100%
r = ⎜⎜
⎝ VDC ⎠
The lower the ripple factor, the better the filter factor can
be decreased by increasing the value of the filter capacitor
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Power Supplies
An integrated circuit regulator (three-terminal regulator)
is a device that is connected to the output of a filtered
rectifier and maintains a constant output voltage despite
changes in the input voltage or the load current
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Power Supplies
Typical metal and plastic package for IC regulators
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Power Supplies
A basic +5.0 V regulated power supply:
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Special Purpose Diodes
The zener diode is used to provide an output
reference voltage that is stable despite changes in
input voltage
IZ
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VZ
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Special Purpose Diodes
The light-emitting diode (LED)
– Since the electrons in the conduction band are at a
higher energy level than the holes in the valence band,
when recombination takes place, energy is released in
the form of light
I = (VS –VF )/R
VS
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血流脈動量測
將手指放置於發光二極體與光敏電阻之間，當微血管
內的血液密度隨著心臟脈動而同步改變時，受光之光
敏電阻的輸出電壓也將隨著脈動而同步改變。
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Summary
The process of adding impurities to an intrinsic
(pure) semiconductor to increase and control
conductivity is called doping
The depletion region is a region adjacent to the pn
junction containing no majority carriers
Forward bias permits majority carrier current
through the diode
Reverse bias prevents majority carrier current
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Summary
The output voltage of a bridge rectifier equals the
total secondary voltage
The PIV is the maximum voltage appearing across
the diode in reverse bias
A capacitor-input filter provides a dc output
approximately equal to the peak of the input
Ripple voltage is caused by the charging and
discharging of the filter capacitor
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Summary
Regulation of output voltage over a range of input
voltages is called input or line regulation
The zener diode operates in reverse breakdown
A zener diode maintains an essentially constant
voltage across its terminals over a specified range
of zener currents
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