Download Chapter 15

Electronics
Principles & Applications
Fifth Edition
Charles A. Schuler
Chapter 15
Regulated
Power Supplies
©1999
Glencoe/McGraw-Hill
INTRODUCTION
• Open-Loop Voltage Regulation
• Closed-Loop Voltage Regulation
• Current and Voltage Limiting
• Switch-Mode Regulators
• Troubleshooting
Reverse bias in Volts
4
6
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 turn
on the pass transistor harder.
The pass transistor, reference, and
error amplifier are inside IC regulators.
7805
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 comes 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
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.
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.
REVIEW
• Open-Loop Voltage Regulation
• Closed-Loop Voltage Regulation
• Current and Voltage Limiting
• Switch-Mode Regulators
• Troubleshooting