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Chapter 16Diodes and

Applications

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ObjectivesUnderstand the basic structure of semiconductorsand how they conduct currentDescribe the characteristics and biasing of a pn junction diodeDescribe the basic diode characteristicsAnalyze the operation of a half-wave rectifier and a full-wave rectifierDescribe the operation of power suppliesUnderstand the basic operation and describe some applications of special-purpose diodes

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Introduction to Semiconductors

Two types of semiconductive materials are siliconand germanium (both have four valance electrons)

Si Ge

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Introduction to SemiconductorsWhen 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 SemiconductorsWhen an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal (called a hole)

Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band

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Introduction to SemiconductorsIn 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 SemiconductorsIntrinsic 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 DiodeA 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 diodeIf 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

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Diode CharacteristicsThe 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 half-

cycle, 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

)out(p

AVG

VV =

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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

)out(p

AVG

VV

2=

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Diode Rectifiers

Example: Find the average value of the full-wave rectified output voltage

VAVG = (2)(15V)/ = 9.55 V

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Diode RectifiersThe full-wave bridge rectifier uses four diodes

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Diode RectifiersFor 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 RectifiersExample: 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 SuppliesThe 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 SuppliesOperation of a half-wave rectifier with a capacitor-input filter:

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10ms2kV

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Power SuppliesFor 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 SuppliesThe ripple factor (r) is an indication of the effectiveness of the filter

The lower the ripple factor, the better the filter factor can be decreased by increasing the value of the filter capacitor

%VVr

DC

r 100

=

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Power SuppliesAn 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 SuppliesTypical 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 DiodesThe zener diode is used to provide an output reference voltage that is stable despite changes in input voltage

IZ VZ

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Special Purpose DiodesThe 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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Summary

The process of adding impurities to an intrinsic (pure) semiconductor to increase and control conductivity is called dopingThe depletion region is a region adjacent to the pn junction containing no majority carriersForward bias permits majority carrier current through the diodeReverse bias prevents majority carrier current

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Summary

The output voltage of a bridge rectifier equals the total secondary voltageThe PIV is the maximum voltage appearing across the diode in reverse biasA capacitor-input filter provides a dc output approximately equal to the peak of the inputRipple 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 regulationThe zener diode operates in reverse breakdownA zener diode maintains an essentially constant voltage across its terminals over a specified range of zener currents

Chapter 16ObjectivesIntroduction to SemiconductorsIntroduction to SemiconductorsIntroduction to SemiconductorsIntroduction to SemiconductorsIntroduction to SemiconductorsThe PN Junction DiodeThe PN Junction DiodeThe PN Junction DiodeDiode CharacteristicsDiode CharacteristicsDiode CharacteristicsTypical Diode PackageDiode RectifiersDiode RectifiersDiode RectifiersDiode RectifiersDiode RectifiersDiode RectifiersDiode RectifiersDiode RectifiersPower SuppliesPower SuppliesPower SuppliesPower SuppliesPower SuppliesPower SuppliesPower SuppliesSpecial Purpose DiodesSpecial Purpose DiodesSummarySummarySummary