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15-1
Electronics
Principles & Applications
Eighth Edition
Charles A. Schuler
Chapter 15
Regulated
Power Supplies
(student version)
©2013
McGraw-Hill
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15-2
INTRODUCTION
• Open-Loop Voltage Regulation
• Closed-Loop Voltage Regulation
• Current and Voltage Limiting
• Switch-Mode Regulators
• Troubleshooting
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15-3
Dear Student:
This presentation is arranged in segments. Each segment
is preceded by a Concept Preview slide and is followed by a
Concept Review slide. When you reach a Concept Review
slide, you can return to the beginning of that segment by
clicking on the Repeat Segment button. This will allow you
to view that segment again, if you want to.
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15-4
Concept Preview
• Conducting zener diodes show a relatively
constant voltage drop.
• Zener shunt regulators are not practical for large
load currents.
• A zener can regulate the base voltage of a series
pass transistor.
• Adding an error amplifier provides much better
voltage regulation (closed loop operation).
• IC voltage regulators contain a pass transistor, a
voltage reference, and an error amplifier.
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15-5
Reverse bias in Volts
4
6
2
0
20
40
60
V
I
80
Reverse
current
in mA
100
120
140
V
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The voltage across a conducting
zener is relatively constant.
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15-6
Using a zener diode as a voltage regulator
Unregulated Supply
Load
The load is in parallel with the zener and will see a
relatively constant voltage as long as the zener is conducting.
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15-7
Series pass transistor
VBE
Unregulated Supply
Load
The amplified zener regulator is used when large
load currents are required. The voltage regulation
of this circuit is fair since VBE is relatively constant.
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15-8
Closed-loop is required for demanding applications.
VOUT
Error
Amplifier
If VOUT decreases, the amplifier
output goes more positive to increase
the drive to the pass transistor.
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15-9
The pass transistor, reference, and
error amplifier are inside IC regulators.
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7805
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15-10
Concept Review
• Conducting zener diodes show a relatively
constant voltage drop.
• Zener shunt regulators are not practical for large
load currents.
• A zener can regulate the base voltage of a series
pass transistor.
• Adding an error amplifier provides much better
voltage regulation (closed loop operation).
• IC voltage regulators contain a pass transistor, a
voltage reference, and an error amplifier.
Repeat Segment
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15-11
Concept Preview
• The current capabilities of an IC regulator can be
extended by adding a boost transistor.
• A boost transistor can be protected from
overcurrent by adding a current limit transistor.
• Conventional current limiting won’t always
protect a pass transistor. Foldback current limiting
offers better protection.
• Parallel pass transistors require emitter swamping
resistors so they will share the load current.
• A crowbar circuit provides overvoltage protection.
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IC voltage regulators have modest current limits.
This circuit extends their capabilities.
15-12
PNP current boost transistor
R
7805
When the drop across R
reaches 0.7 volts, the
boost transistor turns on.
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Load
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15-13
This current limit circuit protects the pass transistor.
R2
Current-limit
transistor
7812
R1
Load
When the drop across R2 reaches
0.7 V, the current limit transistor
turns on and shunts R1.
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VOUT
15-14
Conventional current limiting
Constant current
region
IL
Short circuit
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VOUT
15-15
Foldback current limiting
Foldback current
region
IL
Short circuit
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15-16
Foldback current limiting
Q1
+Vin
R2
R3
R1
R4
R5
RL
R7
Q2
R6
VZ
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Parallel transistors need emitter swamping
resistors to ensure current sharing.
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15-17
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15-18
Fuse
If the zener
conducts, the
SCR turns on and
blows the fuse.
7812
Crowbar over-voltage protection circuit
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15-19
Linear power supply quiz
Pass transistors are connected in _______
with the load.
series
IC regulators can provide more current with
the addition of a ______ transistor. boost
The two types of current limiting are
conventional and __________. foldback
Emitter swamping resistors force parallel
transistors to share ________. current
Crowbar circuits are used to protect a
load from excess __________. voltage
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15-20
Concept Review
• The current capabilities of an IC regulator can be
extended by adding a boost transistor.
• A boost transistor can be protected from
overcurrent by adding a current limit transistor.
• Conventional current limiting won’t always
protect a pass transistor. Foldback current limiting
offers better protection.
• Parallel pass transistors require emitter swamping
resistors so they will share the load current.
• A crowbar circuit provides overvoltage protection.
Repeat Segment
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15-21
Concept Preview
• Switch mode power supplies use pulse width
modulation and achieve much better efficiency
than linear power supplies.
• Switchers operate at tens of kilohertz so that
smaller inductors and capacitors can be used.
• The three basic switching configurations are stepdown, step-up, and inverting. All three store
energy in an inductor.
• Converter type switchers use transformers.
• Flyback type switchers use transformers and a
critical conduction mode of operation.
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15-22
Power supplies with pass transistors are linear and
are not as efficient as switch-mode power supplies.
PC = IC x VCE (The heat loss in a
pass transistor can be significant.)
Recall: Pulse width modulation (PWM) is one way
to use a digital approach to an analog problem.
PWM
Average
value
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15-23
Switch-mode power supply
VIN
VLOAD
PWM
Step-down configuration
VLOAD < VIN
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15-24
Switch-mode power supply
VIN
CEMF
VLOAD
PWM
Step-up configuration
VLOAD > VIN
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15-25
Switch-mode power supply
VIN
CEMF
VLOAD
PWM
Inverting configuration
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15-26
Converter type switch-mode supply
OSC. &
PWM
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VREF
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Topology/Type
Linear
(step-down)
Arrangement
Strong points
Limitations
• Inexpensive
• Can be very small
• Low no-load current
• Low-noise/low EMI
• Usually the best
solution for smaller
loads
• VOUT must be less
than VIN
• Inefficient at high-input
voltages and/or large
loads
• Can require a large
heat sink
• Limited output power
• Limited range of
input/output voltage
ratio
15-27
Charge pump
(boost or invert
polarity)
• Inexpensive
• Small
• Can boost or invert
Buck
(step-down)
• Lowest peak current
• Efficient
• Modest cost
• Low-ripple current in
output-filter capacitor
• Simple inductor
• Low switch-stress
voltage
• VOUT must be less
than VIN
• High-side switch
• Low peak current
• Low-side switch
• Simple inductor
• Low switch-stress
voltage
• VOUT must be greater
than VIN
• Output cannot be
completely turned off by
removing drive
• No short-circuit
protection
• Simple inductor
• Negative output only
• High-side switch
• High peak currents
Boost
(step-up)
Buck-boost
(inverts polarity)
Flyback
(step-down, step-up or
inverts polarity)
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• Isolated output
• Transformer instead of
• Can offer multiple
inductor
outputs
• High peak currents
• Steps up/down,
• High switch-stress
inverts
voltage
• Low-side switch © 2013 The McGraw-Hill Companies, Inc. All rights reserved.
15-28
Switch-mode quiz
Switch-mode supplies have better _______
than linear supplies.
efficiency
Switch-mode supplies use pulse width
_____________.
modulation
A higher voltage is provided by the
__________ configuration.
step-up
The opposite polarity is provided by the
__________ configuration.
inverting
A push-pull arrangement is common in
________ type power supplies. converter
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15-29
Linear Supply Troubleshooting
• A shorted pass transistor produces high
output voltage.
• An open pass transistor produces no
output voltage.
• Voltage error can be due to overloads,
the reference voltage, the error amplifier,
etc.
• IC regulators can oscillate. Check bypass
capacitors.
• IC regulators are subject to RF fields.
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15-30
Flyback switching power supply
using critical conduction mode (CCM)
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15-31
Flyback switching power supply waveforms
The
transformer
current falls
to zero
before
increasing
again
(CCM).
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15-32
Switch-Mode Troubleshooting
• May not function unless loaded (use a
dummy load).
• Use oscilloscope to view PWM
waveforms.
• Frequency can be critical.
• May use an optoisolator in the feedback
circuit. Disable and substitute for the
feedback signal.
• Check the reference voltage.
• Check for overloads.
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15-33
Concept Review
• Switch mode power supplies use pulse width
modulation and achieve much better efficiency
than linear power supplies.
• Switchers operate at tens of kilohertz so that
smaller inductors and capacitors can be used.
• The three basic switching configurations are stepdown, step-up, and inverting. All three store
energy in an inductor.
• Converter type switchers use transformers.
• Flyback type switchers use transformers and a
critical conduction mode of operation.
Repeat Segment
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15-34
REVIEW
• Open-Loop Voltage Regulation
• Closed-Loop Voltage Regulation
• Current and Voltage Limiting
• Switch-Mode Regulators
• Troubleshooting
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