power supplies

1. Electronics
Principles & Applications
Sixth Edition
Chapter 15
Regulated
Power Supplies
©2003 Glencoe/McGraw-Hill
Charles A. Schuler

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

3. 0
2
4
6
20
40
60
80
100
120
140
Reverse
current
in mA
I
V
V
Reverse Bias
in Volts
The voltage across a conducting
zener is relatively constant.

4. 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.
Using a Zener Diode as a Voltage Regulator

5. Load
Unregulated 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

6. Error
Amplifier
Closed-loop is required for demanding applications.
VOUT
If VOUT decreases, the amplifier
output goes more positive to increase
the drive to the pass transistor.

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

8. 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.

9. 7812
Load
This current limit circuit protects the pass transistor.
R2
When the drop across R2 reaches
0.7 V, the current limit transistor
turns on and shunts R1.
R1
Current-limit
Transistor

10. V
OUT
IL
Conventional Current Limiting
Short circuit
Constant current
region

11. V
OUT
IL
Foldback Current Limiting
Short circuit
Foldback current
region

12. R1
Foldback Current Limiting
VZ
R5
R6
RL
+Vin
R2
R7
Q1
Q2
R3
R4

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

14. 7812
Fuse
If the zener
conducts, the
SCR turns on and
blows the fuse.
Crowbar Over-voltage Protection Circuit

15. 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

16. Power supplies with pass transistors are linear and
are not as efficient as switch-mode power supplies.
Recall: Pulse width modulation (PWM) is one way
to use a digital approach to an analog problem.
PWM
Average
value
PC = IC x VCE (The heat loss in a
pass transistor can be significant.)

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

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

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

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

21. 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

22. 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.

23. Flyback Switching Power Supply

24. Flyback Supply Waveforms

25. 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.

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