Diddes lecture

7/30/2019 Diddes lecture

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Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Load-Line Analysis

2

The load line plots all possible

combinations of diode current (ID)

and voltage (VD) for a given circuit.

The maximumID equalsE/R, and

the maximum VD equalsE.

The point where the load line and

the characteristic curve intersect is

the Q-point, which identifiesID and

VD for a particular diode in a given

circuit.


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Series Diode Configurations

Constants

Silicon Diode: VD = 0.7 V

Germanium Diode: VD = 0.3 V

Analysis (for silicon)

VD = 0.7 V (or VD =E ifE < 0.7 V)

VR = EVD

ID = IR = IT= VR / R

3

Forward Bias


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Series Diode Configurations

Diodes ideally behave as open circuits

Analysis

VD = E

VR = 0 V

ID= 0 A

4

Reverse Bias


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Parallel Configurations

5

mA14

2

mA28

D2I

D1I

mA28

.33k

V.7V10

R

DVE

RI

V9.3R

V

V0.7O

VD2

VD1

V

V0.7D

V


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Half-Wave Rectification

6

The diode only

conducts when it is

forward biased,

therefore only half

of the AC cycle

passes through the

diode to the

output.

The DC output voltage is 0.318Vm, where Vm = the peak AC voltage.


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PIV (PRV)

7

Because the diode is only forward biased for one-half of the AC cycle, it is

also reverse biased for one-half cycle.

It is important that the reverse breakdown voltage rating of the diode be

high enough to withstand the peak, reverse-biasing AC voltage.

PIV (or PRV) > Vm

PIV = Peak inverse voltage

PRV = Peak reverse voltage

Vm

= Peak AC voltage


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Full-Wave Rectification

8

Half-wave: Vdc = 0.318Vm

Full-wave: Vdc = 0.636Vm

The rectification process can be improved by

using a full-wave rectifier circuit.

Full-wave rectification produces a greaterDC output:


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9

Bridge Rectifier

Four diodes are connected in a

bridge configuration VDC = 0.636Vm


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10

Center-Tapped Transformer

Rectifier

Requires

Two diodes

Center-tapped transformer

VDC = 0.636Vm


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Summary of Rectifier Circuits

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Vm= peak of the AC voltage.

In the center tapped transformer rectifier circuit, the peak AC voltageis the transformer secondary voltage to the tap.

Rectifier Ideal VDC Realistic VDC

Half Wave Rectifier VDC = 0.318Vm VDC = 0.318Vm0.7

Bridge Rectifier VDC = 0.636Vm VDC = 0.636Vm2(0.7 V)

Center-Tapped Transformer

RectifierVDC = 0.636Vm VDC = 0.636Vm0.7 V


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Diode Clippers

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The diode in a series clipperclipsany voltage that does not forward

bias it:A reverse-biasing polarity

A forward-biasing polarity less than0.7 V (for a silicon diode)


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Biased Clippers

13

Adding a DC source in

series with the clipping

diode changes the

effective forward bias of

the diode.


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Parallel Clippers

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The diode in a parallel clipper

circuit clips any voltage that

forward bias it.

DC biasing can be added inseries with the diode to change

the clipping level.


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Summary of Clipper Circuits

15

more


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Summary of Clipper Circuits

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Biased Clamper Circuits

18

The input signal can be any type

of waveform such as sine, square,

and triangle waves.

The DC source lets you adjust

the DC camping level.


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Summary of Clamper Circuits

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Zener Diodes

20

The Zener is a diode operatedin reverse bias at the Zener

Voltage (Vz).

When Vi VZ

The Zener is on Voltage across the Zener is VZ

Zener current:IZ=IRIRL The Zener Power:PZ = VZIZ

When Vi< VZ The Zener is off

The Zener acts as an open circuit


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Zener Resistor Values

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ZKRLIII

min

min

max

L

Z

L

I

VR

min

max

L

Z

L

L

L

R

V

R

VI

Zi

Z

LVV

RVR

min

IfR is too large, the Zener diode cannot conduct because the available amount of

current is less than the minimum current rating,IZK. The minimum current is

given by:

Themaximum value of resistance is:

IfR is too small, the Zener current exceeds the maximum current

rating,IZM. The maximum current for the circuit is given by:

The minimum value of resistance is:


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Voltage-Multiplier Circuits

Voltage Doubler

Voltage Tripler

Voltage Quadrupler

22

Voltage multiplier circuits use a combination of diodes and

capacitors to step up the output voltage of rectifier circuits.


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Voltage Doubler

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This half-wave voltage doublers output can be calculated by:

Vout = VC2 = 2Vm

where Vm = peak secondary voltage of the transformer


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Voltage Doubler

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Positive Half-Cycle

o D1 conductso D2 is switched off

o Capacitor C1 charges to Vm

Negative Half-Cycle

o D1 is switched off

o D2 conductso Capacitor C2 charges to Vm

Vout = VC2 = 2Vm


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Voltage Tripler and Quadrupler

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Copyright 2009 by Pearson Education, Inc.U S ddl Ri N J 07458 All i ht d

Electronic Devices and Circuit Theory, 10/eRobert L Boylestad and Louis Nashelsky

Practical Applications

Rectifier Circuits Conversions of AC to DC for DC operated circuits

Battery Charging Circuits

Simple Diode Circuits

Protective Circuits against

Overcurrent

Polarity Reversal

Currents caused by an inductive kick in a relay circuit

Zener Circuits

Overvoltage Protection

Setting Reference Voltages

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