Electronics Fundamentals 8 th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ...

Electronics Fundamentals 8th edition Floyd/Buchla

© 2010 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

chapter 16

electronics fundamentalscircuits, devices, and applications

THOMAS L. FLOYDDAVID M. BUCHLA


Chapter 16Chapter 16


Summary

Semiconductors

Semiconductors are crystalline materials that are characterized by specific energy bands for electrons.

The last energy band is the conduction band, where electrons are mobile.

Between the bands are gaps; these gaps represent energies that electrons cannot posses.

Nucleus

First band

Second band

Valence band

Conduction band

Energy gap

Energy gap

Energy gap

Energy

The next to the last band is the valence band, which is the energy level associated with electrons involved in bonding.




Summary

Electron and hole current

At room temperature, some electrons have enough energy to jump into the conduction band.

Valence band

Conduction band

Energy gap

Energy

After jumping the gap, these electrons are free to drift throughout the material and form electron current when a voltage is applied.

Heat energy

Electron-hole pairFor every electron

in the conduction band, a hole is left behind in the valence band.




Summary

Electron and hole current

The electrons in the conduction band and the holes in the valence band are the charge carriers. In other words, current in the conduction band is by electrons; current in the valence band is by holes.When an electron jumps to the conduction band, valence electrons move from hole-to-hole in the valence band, effectively creating “hole current” shown by gray arrows.

Si Si Si

Free electron




Summary

Impurities

By adding certain impurities to pure (intrinsic) silicon, more holes or more electrons can be produced within the crystal.

To increase the number of holes, trivalent impurities are added, forming a p-type semiconductor. These are elements to the left of Si on the Periodic Table.

To increase the number of conduction band electrons, pentavalent impurities are added, forming an n-type semiconductor. These are elements to the right of Si on the Periodic Table. Si

B

Al

Ga

P

As

Sb

Ge

C

Sn

N

III IV V

In




Summary

The pn junction diode

When a pn junction is formed, electrons in the n-material diffuse across the junction and recombine with holes in the p-material. This action continues until the voltage of the barrier repels further diffusion. Further diffusion across the barrier requires the application of a voltage.

The pn junction is basically a diode, which is a device that allows current in only one direction. A few typical diodes are shown.




Summary

Forward bias

When a pn junction is forward-biased, current is permitted. The bias voltage pushes conduction-band electrons in the n-region and holes in the p-region toward the junction where they combine.

The barrier potential in the depletion region must be overcome in order for the external source to cause current. For a silicon diode, this is about 0.7 V.

p-region n-region

p n

+

R

VBIAS

The forward-bias causes the depletion region to be narrow.




Summary

Reverse bias

When a pn junction is reverse-biased, the bias voltage moves conduction-band electrons and holes away from the junction, so current is prevented.

The diode effectively acts as an insulator. A relatively few electrons manage to diffuse across the junction, creating only a tiny reverse current.

p-region n-region

p n

+VBIAS

R

The reverse-bias causes the depletion region to widen.




Summary

Diode characteristics

The forward and reverse characteristics are shown on a V-I characteristic curve. In the forward bias region, current increases dramatically after the barrier potential (0.7 V for Si) is reached. The voltage across the diode remains approximately equal to the barrier potential.

VR VF

IF

IR

Reverse bias

Forward bias

0.7 V

Barrier potential

The reverse-biased diode effectively acts as an insulator until breakdown is reached.

VBR (breakdown)




Summary

Diode models

The characteristic curve for a diode can be approximated by various models of diode behavior. The model you will use depends on your requirements.

The ideal model assumes the diode is either an open or closed switch.

VR VF

IF

IR

Reverse bias

Forward bias

The complete model includes the forward resistance of the diode.

The practical model includes the barrier voltage in the approximation.

0.7 V




Summary

Half-wave Rectifier

Rectifiers are circuits that convert ac to dc. Special diodes, called rectifier diodes, are designed to handle the higher current requirements in these circuits.

The half-wave rectifier converts ac to pulsating dc by acting as a closed switch during the positive alteration.

The diode acts as an open switch during the negative alteration.

D

D

RL

RL

+

+




Summary

Full-wave Rectifier

The full-wave rectifier allows unidirectional current on both alterations of the input. The center-tapped full-wave rectifier uses two diodes and a center-tapped transformer.

F D1

D2RL

Vsec2

Vsec2

The ac on each side of the center-tap is ½ of the total secondary voltage. Only one diode will be biased on at a time.




Summary

Bridge Rectifier

The bridge rectifier is a type of full-wave circuit that uses four diodes. The bridge rectifier does not require a center-tapped transformer.

F

D1

D2

RL

At any instant, two of the diodes are conducting and two are off.

D3

D4




Summary

Peak inverse voltage

Diodes must be able to withstand a reverse voltage when they are reverse biased. This is called the peak inverse voltage (PIV). The PIV depends on the type of rectifier circuit and the maximum secondary voltage.For example, in a full-wave circuit, if one diode is conducting (assuming 0 V drop), the other diode has the secondary voltage across it as you can see from applying KVL around the green path.

0 V

Vsec

Notice that Vp(sec) = 2Vp(out) for the full-wave circuit because the output is referenced to the center tap.




Summary

Peak inverse voltage

For the bridge rectifier, KVL can be applied to a loop that includes two of the diodes. Assume the top diode is conducting (ideally, 0 V) and the lower diode is off. The secondary voltage will appear across the non-conducting diode in the loop.

0 V

Notice that Vp(sec) = Vp(out) for the bridge because the output is across the entire secondary.

Vsec




Summary

Power supplies

By adding a filter and regulator to the basic rectifier, a basic power supply is formed.

7805

FD1

D2C1

D3

D4C2

Typically, a large electrolytic capacitor is used as a filter before the regulator, with a smaller one following the regulator to complete filtering action.

1000 F 1 F

IC regulator




Summary

Special-purpose diodes

Special purpose diodes include Zener diodes – used for establishing a reference voltage

Varactor diodes – used as variable capacitors

Light-emitting diodes – used in displays

Photodiodes – used as light sensors




Summary

Troubleshooting power supplies

Begin troubleshooting by analyzing the symptoms and how it failed. Try to focus on the most likely causes of failure.

7805

FD1

D2C1

D3

D4C2

1000 F 1 F

IC regulator

A power supply has no output, but was working until a newly manufactured PC board was connected to it. (a) Analyze possible failures. (b) Form a plan for troubleshooting.




Summary


7805

FD1

D2C1

D3

D4C2

1000 F 1 F

IC regulator

The supply had been working, so the problem is not likely to be an incorrect part or wiring problem. The failure was linked to the fact that a new PC board was connected to it, which points to a possible overloading problem. If the load was too much for the supply, it is likely a fuse would have blown, or a part would likely have overheated, accounting for the lack of output.




Summary


1. Disconnect power and check the fuse. If it is bad, replace it. Before reapplying power, remove the load, open the power supply case, and look for evidence of overheating (such as discolored parts or boards). If no evidence of overheating proceed.2. Check the new pc board (the load) for a short or overloading of the power supply that would cause the fuse to blow. Look for evidence of overheating.

3. Verify operation of the supply with measurements (see next slide).

Based on the analysis, a sample plan is as follows. (It can be modified as circumstances warrant.)




Summary


Reapply power to the supply but with no load. If the output is okay, put a resistive test load on the power supply and measure the output to verify it is operational. If the output is correct, the problem is probably with the new pc board. If not, you will need to further refine the analysis and plan, looking for an internal problem.

The analysis showed that a likely cause of failure was due to an overload. For the measurement step, it may be as simple as replacing the fuse and confirming that the supply works. After replacing the fuse:




Majority carrier

Minority carrier

PN junction

Diode

The most numerous charge carrier in a doped semiconductor material (either free electrons or holes.

Selected Key Terms

The boundary between n-type and p-type semiconductive materials.

An electronic device that permits current in only one direction.

The least numerous charge carrier in a doped semiconductor material (either free electrons or holes.




Barrier potential

Forward bias

Reverse bias

Full-wave rectifier

A circuit that converts an alternating sine-wave into a pulsating dc consisting of both halves of a sine wave for each input cycle.

The condition in which a diode conducts current.

The inherent voltage across the depletion region of a pn junction diode.

Selected Key Terms

The condition in which a diode prevents current.




Bridge rectifier

Zener diode

Varactor

Photodiode A diode whose reverse resistance changes with incident light.

A type of diode that operates in reverse breakdown (called zener breakdown) to provide a voltage reference.

A type of full-wave rectifier consisting of diodes arranged in a four corner configuration.

Selected Key Terms

A diode used as a voltage-variable capacitor.




Quiz

1. An energy level in a semiconductor crystal in which electrons are mobile is called the

a. barrier potential.

b. energy band.

c. conduction band.

d. valence band.




Quiz

2. A intrinsic silicon crystal is

a. a poor conductor of electricity.

b. an n-type of material.

c. a p-type of material.

d. an excellent conductor of electricity.




Quiz

3. A small portion of the Periodic Table is shown. The elements highlighted in yellow are

a. majority carriers.

b. minority carriers.

c. trivalent elements.

d. pentavalent elements.

Si

B

Al

Ga

P

As

Sb

Ge

C

Sn

N

III IV V

In




Quiz

4. At room temperature, free electrons in a p-material

a. are the majority carrier.

b. are the minority carrier.

c. are in the valence band.

d. do not exist.




Quiz

5. The breakdown voltage for a silicon diode is reached when

a. the forward bias is 0.7 V.

b. the forward current is greater than 1 A.

c. the reverse bias is 0.7 V.

d. none of the above.




Quiz

6. The circuit shown is a

a. half-wave rectifier.

b. full-wave rectifier.

c. bridge rectifier.

d. zener regulator.




Quiz

7. PIV stands for

a. Positive Ion Value.

b. Programmable Input Varactor.

c. Peak Inverse Voltage.

d. Primary Input Voltage.




Quiz

8. A type of diode used a a voltage-variable capacitor is a

a. varactor.

b. zener.

c. rectifier.

d. LED.




Quiz

9. If one of the four diodes in a bridge rectifier is open, the output will

a. be zero.

b. have ½ as many pulses as normal.

c. have ¼ as many pulses as normal.

d. be unaffected.




Quiz

10. When troubleshooting a power supply that has a bridge rectifier, begin by

a. replacing the bridge rectifier.

b. replacing the transformer.

c. making measurements.

d. analyzing the symptoms and how it failed.




Quiz

Answers:

1. c

2. a

3. c

4. b

5. d

6. b

7. c

8. a

9. b

10. d

Electronics Fundamentals 8 th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ...

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Transcript of Electronics Fundamentals 8 th edition Floyd/Buchla © 2010 Pearson Education, Upper Saddle River, NJ...