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

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

3. 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.

4. 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.

5. 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.

6. 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.

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

8. PNP Current boost transistor
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

9. 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.

10. Conventional current limiting
VOUT
Conventional current limiting
Constant current
region IL
Short circuit

11. Foldback current limiting
VOUT
Foldback current limiting
Foldback current
region IL
Short circuit

12. Foldback current limitingQ1 R2
+Vin R3 R5 R1 R4 RL Q2 R7 R6 VZ

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

14. Crowbar over-voltage protection circuit
Fuse
If the zener
conducts, the
SCR comes on and
blows the fuse.
7812
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.
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

17. Switch-mode power supply
VIN
VLOAD
PWM
Step-down configuration
VLOAD < VIN

18. Switch-mode power supply
CEMF
VIN
VLOAD
PWM
Step-up configuration
VLOAD > VIN

19. Switch-mode power supply
VIN
CEMF
VLOAD
PWM
Inverting configuration

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. 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.

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