Transcript Electronics Principles & Applications Sixth Edition Charles A. Schuler Chapter 15 Regulated Power Supplies (student version) ©2003 Glencoe/McGraw-Hill INTRODUCTION • Open-Loop Voltage Regulation • Closed-Loop Voltage Regulation • Current and Voltage Limiting •

Electronics
Principles & Applications
Sixth Edition
Charles A. Schuler
Chapter 15
Regulated
Power Supplies
(student version)
©2003
Glencoe/McGraw-Hill
INTRODUCTION
• Open-Loop Voltage Regulation
• Closed-Loop Voltage Regulation
• Current and Voltage Limiting
• Switch-Mode Regulators
• Troubleshooting
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.
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.
Reverse Bias
in Volts 6
4
2
0
20
40
60
V
I
80
Reverse
current
in mA
100
120
140
V
The voltage across a conducting
zener is relatively constant.
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.
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.
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.
The pass transistor, reference, and
error amplifier are inside IC regulators.
7805
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
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.
IC voltage regulators have modest current limits.
This circuit extends their capabilities.
PNP Current Boost Transistor
R
7805
When the drop across R
reaches 0.7 volts, the
boost transistor turns on.
Load
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.
VOUT
Conventional Current Limiting
Constant current
region
IL
Short circuit
VOUT
Foldback Current Limiting
Foldback current
region
IL
Short circuit
Foldback Current Limiting
Q1
+Vin
R2
R3
R1
R4
R5
RL
R7
VZ
Q2
R6
Parallel transistors need emitter swamping
resistors to ensure current sharing.
Fuse
If the zener
conducts, the
SCR turns on and
blows the fuse.
7812
Crowbar Over-voltage Protection Circuit
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
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
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
step-down, 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.
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
Switch-mode Power Supply
VIN
VLOAD
PWM
Step-down configuration
VLOAD < VIN
Switch-mode Power Supply
VIN
CEMF
VLOAD
PWM
Step-up configuration
VLOAD > VIN
Switch-mode Power Supply
VIN
CEMF
VLOAD
PWM
Inverting configuration
Converter-type Switch-mode Supply
OSC. &
PWM
VREF
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
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.
Flyback Switching Power Supply
Flyback Supply Waveforms
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.
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
step-down, 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
REVIEW
• Open-Loop Voltage Regulation
• Closed-Loop Voltage Regulation
• Current and Voltage Limiting
• Switch-Mode Regulators
• Troubleshooting