Electronics Principles & Applications Fifth Edition Chapter 15 Regulated Power Supplies ©1999...
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Transcript of Electronics Principles & Applications Fifth Edition Chapter 15 Regulated Power Supplies ©1999...
ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition
Chapter 15Regulated
Power Supplies
©1999 Glencoe/McGraw-Hill
Charles A. Schuler
• Open-Loop Voltage Regulation• Closed-Loop Voltage Regulation• Current and Voltage Limiting• Switch-Mode Regulators• Troubleshooting
INTRODUCTION
0246
20
40
60
80
100
120
140
Reverse bias in Volts
Reversecurrentin mAI
V
V
The voltage across a conductingzener is relatively constant.
Load
The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting.
Using a zener diode as a voltage regulator
Unregulated Supply
LoadUnregulated Supply
Series pass transistor
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.
VBE
ErrorAmplifier
Closed-loop is required for demanding applications.
VOUT
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
7805
PNP Current boost transistor
Load
R
When the drop across R reaches 0.7 volts, the
boost transistor turns on.
IC voltage regulators have modest current limits. This circuit extends their capabilities.
7812
Load
This current limit circuit protects the pass transistor.
R2
When the drop across R2 reaches0.7 V, the current limit transistor
turns on and shunts R1.
R1
Current-limittransistor
VO
UT
IL
Conventional current limiting
Short circuit
Constant currentregion
VO
UT
IL
Foldback current limiting
Short circuit
Foldback currentregion
R1
Foldback current limiting
VZ
R5
R6
RL
+Vin
R2
R7
Q1
Q2
R3
R4
Parallel transistors need emitter swamping resistors to ensure current sharing.
7812
Crowbar over-voltage protection circuit
Fuse
If the zener conducts, the
SCR comes on and blows the fuse.
Linear power supply quiz
Pass transistors are connected in _______with the load. series
IC regulators can provide more current withthe addition of a ______ transistor. boost
The two types of current limiting areconventional and __________. foldback
Emitter swamping resistors force paralleltransistors to share ________. current
Crowbar circuits are used to protect aload from excess __________. voltage
Power supplies with pass transistors are linear and are not as efficient as switch-mode power supplies.
Recall: Pulse width modulation (PWM) is one wayto use a digital approach to an analog problem.
PWM
Averagevalue
PC = IC x VCE (The heat loss in a pass transistor can be significant.)
Step-down configuration
Switch-mode power supply
PWM
VIN
VLOAD
VLOAD < VIN
Step-up configuration
Switch-mode power supply
PWM
VIN
VLOAD
VLOAD > VIN
CEMF
Inverting configuration
Switch-mode power supply
PWM
VIN
VLOADCEMF
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.
• Open-Loop Voltage Regulation• Closed-Loop Voltage Regulation• Current and Voltage Limiting• Switch-Mode Regulators• Troubleshooting
REVIEW