1. Principles & Applications
Electronics
Principles & Applications
Sixth Edition
Charles A. Schuler
Chapter 15
Regulated
Power Supplies
(student version)
© Glencoe/McGraw-Hill

2. INTRODUCTION Open-Loop Voltage Regulation
Closed-Loop Voltage Regulation
Current and Voltage Limiting
Switch-Mode Regulators
Troubleshooting

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.

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.

5. The voltage across a conducting zener is relatively constant.
Reverse Bias in Volts 6 4 2 20 I V 40
Reverse
current
in mA 60 80
100
120
140
The voltage across a conducting
zener is relatively constant.

6. Using a Zener Diode as a Voltage Regulator
Load
Unregulated Supply
The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting.

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.

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.

9. 7805 The pass transistor, reference, and
error amplifier are inside IC regulators.

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

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.

12. 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

13. When the drop across R2 reaches 0.7 V, the current limit transistor
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.

14. Conventional Current Limiting
VOUT
Conventional Current Limiting
Constant current
region IL
Short circuit

15. Foldback Current Limiting
VOUT
Foldback Current Limiting
Foldback current
region IL
Short circuit

16. Foldback Current LimitingQ1 R2
+Vin R3 R5 R1 R4 RL Q2 R7 R6 VZ

17. Parallel transistors need emitter swamping
resistors to ensure current sharing.

18. Crowbar Over-voltage Protection Circuit
Fuse
If the zener
conducts, the
SCR turns on and
blows the fuse.
7812
Crowbar Over-voltage Protection Circuit

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

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

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 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.

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

23. Switch-mode Power Supply
VIN
VLOAD
PWM
Step-down configuration
VLOAD < VIN

24. Switch-mode Power Supply
CEMF
VIN
VLOAD
PWM
Step-up configuration
VLOAD > VIN

25. Switch-mode Power Supply
VIN
CEMF
VLOAD
PWM
Inverting configuration

26. Converter-type Switch-mode Supply
OSC. &
PWM
VREF

27. 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

28. 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.

29. Flyback Switching Power Supply

30. Flyback Supply Waveforms

31. 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.

32. 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

33. REVIEW Open-Loop Voltage Regulation Closed-Loop Voltage Regulation
Current and Voltage Limiting
Switch-Mode Regulators
Troubleshooting