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ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsEighth EditionEighth Edition

Chapter 3Diodes

Charles A. Schuler

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• The PN Junction• Characteristic Curves of Diodes• Diode Lead Identification• Diode Types and Applications• Photovoltaic Energy Sources

INTRODUCTION

Diodes are two-terminalworkhorses.

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P

The P-side of a junction diode is doped with acceptor atoms.

N

The N-side of a junction diode is doped with donor atoms.

Schematic Symbol

CathodeAnode

junction

A silicon crystal

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Zero Bias

Depletion region

The electrons near the junction cross over and fill the holes near the junction.

Having no carriers, the depletion region is an insulator.

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Forward Bias

The carriers move toward the junctionand collapse the depletion region.

The diode is on (conducting).

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Reverse Bias

The carriers move away from the junction.

The depletion region is reestablishedand the diode is off.

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Diode quiz

At zero bias, diodes show a depletion regionwhich acts as an________. insulator

The depletion region is collapsed by applying_________ bias. forward

The depletion region is made wider by applying________ bias. reverse

A forward-biased diode has its anode________ with respect to its cathode. positive

Diode forward voltage drop decreases astemperature ________. increases

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mA

0 0.5 1.0 1.50

25

50

75

100

125

150

175

200

Volts

10 ohms

5 ohms

20 ohms

Resistor volt-ampere characteristic curves

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For

war

d c

urr

ent

in m

A

0 0.5 1.0 1.50

25

50

75

100

125

150

175

200

Forward bias in volts

Silicon diode volt-ampere characteristic curve

knee

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Linearity

• The volt-ampere characteristic curve for a resistor is a straight line (linear).

• A diode has a non-linear characteristic curve.

• The barrier potential produces a knee in the diode curve.

• The knee voltage is approximately 0.6 to 0.7 volts for silicon diodes.

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For

war

d c

urr

ent

in m

A

0 0.5 1.0 1.50

25

50

75

100

125

150

175

200

Forward bias in volts

The effect of temperature

-50 oC

25 oC

100 oC

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0200400600

20

40

60

80

100

120

140

Reverse bias in Volts

Reversecurrentin mA

Silicon diode reverse bias characteristic curve

breakdown

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Cathode lead

Anode lead

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Anode lead

0

V

mA

Cathode lead

The diode is forward biased by the ohmmeter.

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Cathode lead

Anode lead

0

The diode is reverse biased by the ohmmeter.

V

mA

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Silicon diode ohmmeter testing

• Low resistance in both directions: the diode is shorted.

• High resistance in both directions: the diode is open.

• Relatively low resistance in the reverse direction: the diode is leaky.

• The ratio of reverse resistance to forward resistance is > 1000: the diode is good.

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0246

20

40

60

80

100

120

140

Reverse bias in Volts

Reversecurrentin mA

A zener diode is designed to break down andconduct backwards at lower voltages.

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0246

20

40

60

80

100

120

140

Reverse bias in Volts

Reversecurrentin mAI

V

V

The voltage across a conductingzener is relatively constant.

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Unregulated SupplyLoad

The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting.

Using a zener diode as a voltage regulator

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VIN VOUT

When VIN < 1.2 VPP

The diodes do not conduct.

This circuit is called a clipper or limiter.

VOUT is not clipped

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VIN VOUT

When VIN > 1.2 VPP

+ 0.6 V

- 0.6 V

VOUT is clipped

The diodes conduct.

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VIN VOUT

When VIN > 1.2 VPP

This circuit is called a clamp or dc restorer.

0.6 VVOUT is clamped.

dc

C is charged.

C

2VOUT(dc) =

VPP - 0.6 V

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The inductive kickcan cause damage.

CEMF

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The coil dischargesthrough the diode and

there is no arc.

Transient suppression diodes can be used with inductive loads.

CEMF

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LED

As the electrons cross the junction, they lose energy in the form of photons.

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Powersupply

VS

RS

The typical voltage drop for most LEDs is from 1.5 to 2.5 V.

LED

IS =VS - VD

RS

LED circuit

VD

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VISIBLE AND/OR INVISIBLELASER RADIATION-AVOID

DIRECT EXPOSURE TO BEAM AVOID EXPOSUREvisible and/or invisible

Laser Radiation isemitted from this

aperture

VISIBLE AND/ORINVISIBLE

LASER DIODE

Ultraviolet LEDs are also dangerous and invisible!

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Powersupply

VS

RS

Photodiodes are reverse biased and conduct in the presence of light.

Photodiode

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Input

Output

Input

Output

Optocoupler

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Step-index multimode fiber

Input pulse Output pulse

The combined shorter and longer path lengthsact to stretch the output pulse.

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Step-index multimode fiber

Input

Output

Due to pulse stretching, high speed data transmission is not possible.

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Input

Output

Single mode fiber

A single path means no pulse stretching andhigh speed data transmission is possible.

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L C ZERO BIAS

C is maximum and fR is minimum.

Tuning diode

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L C REVERSE BIAS

C is less and fR increases.

Tuning diode

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L C MAX. REVERSE BIAS

C is minimum and fR is maximum.

Tuning diode

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Diode applications quiz

A circuit used to control the amplitude of asignal is the ________. clipper

A circuit used to add a dc component to asignal is the ________. clamp

A device containing an LED and a photo-diode is the ________. optocoupler

A tuning diode shows less capacitance asreverse bias ________. increases

The device that is often used to regulatevoltage is the ________ diode. zener

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Here we see a photon enter a PV cell and liberate an electron.

Then we see the electron move to the

front contact and through the load.

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For a given level of illumination, there is one and only one load that will result in maximum power

transfer to the load.

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The MPPT controller maintains system efficiency for varying levels of illumination.

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Measuring the current output from a PV source is a common troubleshooting task.

A camp type current probe is a safer and more convenient method of measuring output current.

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Photovoltaic quiz

Photovoltaic devices directly convert light energy to ________ energy. electrical

A photon entering a PV cell might liberate a free ________. electron

The ideal (best power transfer) load on a PV source varies with the level of_______. illumination

MPPT improves PV system efficiency by adjusting the ________ resistance. load

PV troubleshooting might involve measuring the voltage and/or the ____ output. current

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REVIEW

• The PN Junction• Characteristic Curves of Diodes• Diode Lead Identification• Diode Types and Applications• Photovoltaic Energy Sources