Transcript EENG 3510 Ch 3x

EENG 3510 Chapter 3
Diodes
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Chapter 3 Homework
3.2 (c & d), 3.3 , 3.9, 3.19, 3.23
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3.1.1 Current-Voltage Characteristic
diode circuit symbol
i–v characteristic
equivalent circuit in the reverse direction
equivalent circuit in the forward direction
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3.1.1 Current-Voltage Characteristic
an external circuit to limit the forward current
the reverse voltage
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3.1.2 A Simple Application: The Rectifier
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3.1.3 Another Application: Diode Logic Gates
(In a positive-logic system)
OR gate
Y=A+B+C
AND gate (in a positive-logic system)
Y = ABC
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Example 3.2 Find values of I and V
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Example 3.2a Find values of I and V
1.
2.
3.
4.
Both diodes are conducting.
Voltage at B is zero.
ID2 = 10 V -0 V / 10 k = 1 mA
I + ID2 = (0 – (-10) V ) / 5 k = 2 mA
I + 1 = 2mA
I = 1 mA
5. V = 0
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Example 3.2b Find values of I and V
1.
2.
3.
4.
5.
6.
7.
8.
Assume both diodes are conducting.
VB = 0
ID2 = (10 V – 0 V) / 5k = 2 mA
I + 2 = (0 – (-10)) V / 10 k
I = - 1 A This is not correct.
Assume D1 is off and D2 is on.
ID2 = (10 V – (-10 V) )/ 15k = 1.33 mA
V = VB = -10 V + (10 k X 1.33 mA)
V = -10 V + 13.3 V = 3.3 V
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Exercise 3.4a - Find: I & V
I = 5 V / 2.5 K = 2 mA
V
V = 0, Why ?
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Exercise 3.4b - Find: I & V
I = 0 A, Why?
V
V = 5 V, Why ?
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Exercise 3.4c - Find: I & V
V
I = 0 A, Why?
V = 5 V, Why ?
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Exercise 3.4d - Find: I & V
V
I = 5 V / 2.5 K = 2 mA
V = 0, Why ?
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Exercise 3.4e - Find: I & V
I = 3 V / 1 K = 3 mA
V = 3 V, Why ?
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Exercise 3.4f - Find: I & V
I = 4 V / 1 K = 4mA
V = 1 V, Why ?
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3.2 Terminal Characteristics of Junction Diodes
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3.2 Terminal Characteristics of Junction Diodes
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3.2.1 The Forward-Bias Region
Is: Saturation current, in the order of 10-15A, doubles in value
for every 5°C rise in temperature
n = 1, 2: material and physical structure
V = forward voltage
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3.2.1 The Forward-Bias Region (cont.)
Silicon diodes conduct when the forward voltage = 0.7 volts
Germanium diodes conduct when the forward voltage = 0.3volts
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Example
Given: A forward biased diode, forward voltage drop is 0.7 V at 2 mA,
n = 1 at 0.6 V
Find : the current i2
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3.2.2 The Reverse-Bias Region
If |v|>> |VT|(25mV)
i≅-Is (Saturation current)
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3.2.3 The Breakdown Region
If the power dissipated in the diode is
limited to a “safe” level, the breakdown is
normally not destructive
VZK: Z →Zener, K
→Knee
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3.3 Modeling the Diode Forward Characteristics
3.3.1 The Exponential Model
Graphical Analysis
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3.3 Modeling the Diode Forward Characteristics
3.3.3 Iterative Analysis Using theExponential Model
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3.3 Modeling the Diode Forward Characteristics
3.3.5 The Piecewise-Linear Model
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3.3 Modeling the Diode Forward Characteristics
3.3.5 The Piecewise-Linear Model (cont.)
Piecewise-linear model of the diode forward characteristic
and its equivalent circuit representation
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3.3 Modeling the Diode Forward Characteristics
3.3.6 The Constant-Voltage-Drop Model
Development of the constant-voltage-drop
model of the diode forward characteristics. A
vertical straight line (B) is used to approximate
the fast-rising exponential. Observe that this
simple model predicts VD to within 0.1 V over
the current range of 0.1 mA to 10 mA.
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3.3 Modeling the Diode Forward Characteristics
3.3.6 The Constant-Voltage-Drop Model
The constant-voltage-drop model of the diode forward characteristics and its
equivalent-circuit representation.
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3.3 Modeling the Diode Forward Characteristics
3.3.9 Use of the Diode Forward Drop in Voltage Regulation
• A voltage regulator is a circuit whose purpose is to provide a
constant dc voltage between its output terminals
• The output voltage is required to remain as constant as
possible in spite of
– Changes in the load current drawn from the regulator
output terminal
– Changes in the dc power-supply voltage that feeds the
regulator circuit
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3.3 Modeling the Diode Forward Characteristics
3.3.9 Use of the Diode Forward Drop in Voltage Regulation
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3.4 Operation in the Reverse Breakdown Region –Zener Diodes
3.4.1 Specifying and Modeling the Zener Diode
Circuit symbol for a zener diode.
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3.4 Operation in the Reverse Breakdown Region –Zener Diodes
3.4.4 A Final Remark
• In recent years, zener diodes are replaced in voltage-regulator
design by specially designed integrated circuits (ICs) that
perform the voltage regulation function much more
effectively and with greater flexibility than zener diodes.
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3.5 Rectifier Circuits
120(N2/N1) V
Coils wound
around an iron core
Remove pulsation
Remove ripple
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3.5.1 The Half-Wave Rectifier
Half-wave rectifier
Transfer characteristic of the rectifier circuit
Equivalent circuit of the half-wave rectifier
with the diode replaced with its batteryplus-resistance model.
Input and output waveforms, assuming that rD ! R.
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3.5.1 The Half-Wave Rectifier (cont.)
• Two important parameters:
1) Current-handling capability: the largest current the diode
is expected to conduct
2) Peak inverse voltage (PIV): the diode must be able to
withstand without break
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3.5.2 The Full-Wave Rectifier
Full-wave rectifier utilizing a transformer
with a center-tapped secondary winding
transfer characteristic assuming a constantvoltage-drop model for the diodes;
PIV = 2Vs-VD
input and output waveforms
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3.5.3 The Bridge Rectifier
Most Popular Rectifier Circuit Configuration
The bridge rectifier
input and output waveforms
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3.5.4 The Rectifier with a Filter Capacitor The
Peak Rectifier
A simple circuit used to illustrate the effect of a filter capacitor.
Note that the circuit provides a dc
voltage equal to the peak of the
input sine wave. The circuit is
therefore known as a peak rectifier
or a peak detector.
Input and output waveforms assuming an ideal diode.
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3.5.4 The Rectifier with a Filter Capacitor The
Peak Rectifier
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3.5.4 The Rectifier with a Filter Capacitor The
Peak Rectifier
Waveforms in the full-wave peak rectifier
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3.7.1 Basic Semiconductor Concepts
Simplified physical structure of the junction diode.
(Actual geometries are given in Appendix A.)
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3.7.1 Basic Semiconductor Concepts (cont.)
Two-dimensional representation of the silicon crystal. The circles represent the inner core
of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the
charge of the four valence electrons. Observe how the covalent bonds are formed by
sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are
available for current conduction.
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3.7.1 Basic Semiconductor Concepts (cont.)
At room temperature, some of the covalent bonds are broken by thermal
ionization. Each broken bond gives rise to a free electron and a hole, both of
which become available for current conduction.
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3.7.1 Basic Semiconductor Concepts (cont.)
The concentration of free electrons n, and the
concentration of holes p
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3.7.1 Basic Semiconductor Concepts (cont.)
Diffusion: electrons (holes) will diffuse from the region of high
concentration to the region of low concentration
Drift:when an electric field is applied across a piece of silicon, free
electrons and holes are accelerated by the electric field E. The
positively charged holeswill drift in the direction of E, while the
negatively charged electronswill drift in a direction oppositeto that of
electric field.
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3.7.1 Basic Semiconductor Concepts (cont.)
Doping of a silicon crystal to turn it into n type or p type is
achieved by introducing a small number of impurity atoms.
Ex: phosphorus
A silicon crystal doped by a pentavalent element. Each
dopant atom donates a free electron and is thus called a
donor. The doped semiconductor becomes n type.
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3.7.1 Basic Semiconductor Concepts (cont.)
Doping of a silicon crystal to turn it into n type or p type is
achieved by introducing a small number of impurity atoms.
Ex: boron
A silicon crystal doped with a trivalent impurity. Each dopant
atom gives rise to a hole, and the semiconductor becomes p type.
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3.7.2 The pn Junction Under Open-Circuit Conditions
Equilibium: Is= ID,
Maintained by the barrier voltage V0
(a) The pn junction with no applied voltage (open-circuited terminals). (b) The
potential distribution along an axis perpendicular to the junction.
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3.7.2 The pn Junction Under Open-Circuit Conditions
Barrier Voltage:
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3.7.2 The pn Junction Under Open-Circuit Conditions
Width of the Depletion Region
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3.7.3 The pn Junction Under Reverse-Bias Conditions
Anode
Depletion width: increases
Barrier voltage v0: increase
Cathode
I = IS-ID
The pn junction excited by a constant-current source I in the reverse direction. To avoid
breakdown, I is kept smaller than IS. Note that the depletion layer widens and the barrier
voltage increases by VR volts, which appears between the terminals as a reverse voltage.
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3.7.3 The pn Junction Under Reverse-Bias Conditions
Anode
Depletion width: decrease
Barrier voltage v0: decrease
Cathode
I = ID -IS
The pn junction excited by a constant-current source supplying a current I in the forward
direction. The depletion layer narrows and the barrier voltage decreases by V volts, which
appears as an external voltage in the forward direction.
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