ch2 final.pdf

© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
Thomas L. Floyd
Content
▪ Diode Operation
▪ Voltage-Current (V-I) Characteristics of a
Diode
▪ Diode Models
▪ Half & Full -Wave Rectifiers
▪ Power Supply Filters and Regulators
▪ Diode Limiters and Clampers
▪ Voltage Multipliers

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Thomas L. Floyd
Diode Operation
▪ What is Diode?
A diode is a semiconductor device with a single
pn junction and metal connections to leads. It
has the ability to pass current in only one
direction.

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Thomas L. Floyd
Typical diode packages
A
K
K
K
A
K
A
A
K
A
K A
K
A
K
A
K
K
A
K
K
A
Some common configurations are

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Thomas L. Floyd
Forward bias
▪ Forward bias
Forward bias is the condition which allows current in
the diode. The bias voltage must be greater than the
barrier potential.

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Thomas L. Floyd
Forward bias

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Thomas L. Floyd
Reverse bias
▪ Reverse bias
Reverse biasis the condition that
essentially prevents current through the
diode.

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Thomas L. Floyd
Reverse bias

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Thomas L. Floyd
Reverse bias
▪ Reverse Current :
The extremely small current that exists in reverse bias
after the transition current dies out is caused by the
minority carriers in the n and p regions that are produced
by thermally generated electron-hole pairs

Thomas L. Floyd
Thomas L. Floyd
V-I Characteristics of a Diode
R
IF
BIAS
V
–
+
–
+
+
–
F
V
B
0.7 V
C
A
0
0
Knee
VF
IF(mA)
R
BIAS
BIAS
I = 0 A
– +
– +
V
V
0
0
Knee
VR
IR
VBR
Forward bias
Reverse bias

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Thomas L. Floyd
V-I Characteristics of a Diode

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Thomas L. Floyd
Diode Model- (Ideal model)

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Thomas L. Floyd
Diode Model- (Practical model)

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Diode Model- (Complete model)

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Thomas L. Floyd
Summary
Example 2-1
Page 43

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Thomas L. Floyd
HALF-WAV ERECTIFIERS
▪ Electronic devices require a source of constant
dc (battery , dc power supply)
▪ The dc power supply converts the standard 120
V/220V, 60 Hz ac voltage into a constant dc
voltage.

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Thomas L. Floyd
Half-Wave Rectifier Operation
▪ The diode conducts during the positive half
cycle, It does not conduct during the negative
half cycle.

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Thomas L. Floyd
Question ?
What is the output if the diode is reversed?
RL
+
–
+
– Vout
0
t
t 0
0 t
t 1
1
Vin
0
t t2
2
I

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Thomas L. Floyd
▪ Average Value of the Half-Wave Output Voltage
– The average value of the half-wave rectified output
voltage is the value you would measure on a dc
voltmeter

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Thomas L. Floyd
▪ Effect of the Barrier Potential on the Half-Wave Rectifier
Output
– When the practical diode model
– is used with the barrier potential of 0.7 V taken into
account.

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Thomas L. Floyd
▪ Peak Inverse Voltage (PIV)
– It is equal to the peak input voltage and is the
maximum voltage across the diode when it is
not conducting.
-Vp(in)
RL
+
–
–
+
I = 0
V
tp
0
PIV at tp

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Thomas L. Floyd
▪ Transformer Coupling
– a transformer is often used to couple the ac input
voltage from the source to the rectifier.
– It provides two advantages. First, it allows the source
voltage to be stepped down. Second, the ac source is
electrically isolated from the rectifier

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▪ The amount that the voltage is stepped down is
determined by the turns ratio of the transformer.
▪ turns ratio = The number of turns in the secondary (Nsec)
divided by the number of turns in the primary (Npri)

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Thomas L. Floyd

Thomas L. Floyd
Thomas L. Floyd
Full-wave Rectifier
▪ Half-wave rectifiers have some applications, the
full-wave rectifier is the most commonly used
type in dc power supplies.
▪ You will learn about two types of full-wave
rectifiers:
– Center-tapped
– Bridge.

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Thomas L. Floyd
Full-wave Rectifier
▪ The number of positive alternations that make
up the full-wave rectified voltage is twice that of
the half-wave voltage for the same time interval

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Thomas L. Floyd
Center-Tapped Full-Wave Rectifier
▪ A center-tapped rectifier is a type of full-wave
rectifier that uses two diodes connected to the
secondary of a center-tapped transformer

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Thomas L. Floyd
Center-Tapped Full-Wave Rectifier

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Thomas L. Floyd
Full-Wave Rectifiers
Full-wave rectifiers utilize both have
periods of the input sin signal and reduce
power dissipation.
All specifications are the same as for half-
wave rectifiers.
Reversing polarity of the diodes gives a
full-wave rectifier with negative output
voltage.
30
+
-
+
-

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Thomas L. Floyd
Full-Wave Rectifiers
Full-wave rectifiers cut capacitor
discharge time in half and require half the
filter capacitance to achieve a given
ripple voltage.
All specifications are the same as for half-
wave rectifiers.
31
+
-
+
-
0
on
V =

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Thomas L. Floyd
Full-Wave Rectifier Equations
32
,
( )
2
2
( )
1 1
P on
dc P on r
P on r
P
P
V V T
V V V V
R C
V
V V
T
T
V
RC V
 
-
 - 
-
  =

c
=T =
2Vr
VP
I
P
=I
DC
T
T
PIV =2V
P
Vr and discharge time is one half of those in
half-wave rectifier.
Conduction interval and peak current are also
reduced.
PIV rating is the same for each diode.
For negative output voltage – reverse the
polarity of both diodes.
0
on
V =

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Thomas L. Floyd
Full-Wave Bridge Rectifier
The requirement for a center-
tapped transformer in the full-
wave rectifier is eliminated
through use of 2 extra diodes.
All other specifications are the
same as for a half-wave
rectifier except
For negative output voltage –
reverse the polarity of both
diodes.
33
+
-
-
+
PIV P on P
V V V
= - 
http://www.falstad.com/circuit/e-fullrect.html

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34

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36

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