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Lecture 9:
Power Supplies
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Power Supply Regulation
An ideal power supply provides a constant dc voltage despite
changes to the input voltage or load conditions.
The output voltage of a real power supply changes under load as shown
in the second plot. The output is also sensitive to input voltage changes.
Voltage
Voltage
VNL
VNL
VFL
Ideal power supply
0
Real power supply
Current
0
Electronic Devices, 9th edition
Thomas L. Floyd
0
Current
0
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Line Regulation
Line regulation is a measure of how well a power supply is able to
maintain the dc output voltage for a change in the ac input line voltage.
The formula for line regulation is
 VOUT 
Line Regulation = 
100%
 VIN 
VOUT / VOUT 100%

Line Regulation =
VIN
Line regulation can also be
expressed in terms of
percent change in VOUT per
volt change on the VIN
(%/V).
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Load Regulation
When the amount of current through a load changes due to a
varying load resistance, the voltage regulator must maintain a
nearly constant output voltage across the load.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Load Regulation
Load regulation is a measure of how well a power supply is
able to maintain the dc output voltage between no load and
full load with the input voltage constant. It can be expressed as
a percentage change in load voltage:
 VNL  VFL 
Load Regulation = 
100%
 VFL 
Load regulation can also be expressed in terms of percent
change in the output per mA change in load current (%/mA).
Sometimes a maximum error voltage is given in the specification.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Load Regulation
Sometimes the equivalent Thevenin resistance of a supply is
specified in place of a load regulation specification.
In this case, VOUT can be found by
applying the voltage divider rule:


RL
VOUT  VNL 

R

R
L 
 OUT
In terms of resistances, load
regulation can be expressed as:
Power Supply
RTH = ROUT
VOUT
VTH = VNL
RL
 ROUT 
Load regulation  
100%
 RFL 
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Load Regulation
A power supply has an output resistance of 25 mW and a full
load current of 0.50 A to a 10.0 W load.
(a) What is the load regulation?
(b) What is the no load output voltage?
 ROUT 
 0.025 W 
Load
regulation

100%

(a)



100% = 0.25%
 10.0 W 
 RFL 
(b) By Ohm’s law, VOUT = 5.0 V.
VNL 
Electronic Devices, 9th edition
Thomas L. Floyd
VOUT


RL


R

R
L 
 OUT

5.0 V
= 5.013 V
10.0 W




 0.025 W + 10.0 W 
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Regulators
The fundamental classes of voltage regulators are linear
regulators and switching regulators.
Both of these are available in integrated circuit form.
Two basic types of linear regulator are:
the series regulator
the shunt regulator.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Series Regulators
Series Regulator block diagram:
Control
element
VIN
VOUT
Basic series regulator circuit:
Reference
voltage
Error
detector
Sample
circuit
Control element
VIN
VOUT
Q1
R1
+
VREF
The control element maintains
a constant output voltage by
varying the collector-emitter
voltage across the transistor.
Electronic Devices, 9th edition
Thomas L. Floyd
–
D1
Error detector
R2
Sample
circuit
R3
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Series Regulators
The op-amp in the series regulator is actually connected as a noninverting
amplifier where the reference voltage VREF is the input at the noninverting
terminal, and the voltage divider R2/R3 forms the negative feedback
circuit. The closed-loop voltage gain is
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Series Regulators
 R 
The output voltage for the series regulator circuit is: VOUT  1  2 VREF
 R3 
(a) What is the output voltage for the series regulator?
(b) If the load current is 200 mA, what is the power
dissipated by Q1?
 R 
(a) VOUT  1  2  VREF
 R3 
 100 kW 
 1+
 3.9 V
 47 kW 
= 12.2 V
(b) P = VI
= (18 V – 12.2 V)(0.2 A)
= 1.16 W
Electronic Devices, 9th edition
Thomas L. Floyd
VIN
VOUT
18 V
Q1
R1
4.7 kW
VREF
+
–
3.9 V
D1
R2
100 kW
R3
47 kW
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Shunt Regulators
Shunt Regulator block diagram:
R1
VIN
VOUT
Reference
voltage
Error
detector
Control
element
(shunt)
Basic shunt regulator circuit:
Sample
circuit
VOUT
VIN
R1
R2
–
VREF
The control element maintains
a constant output voltage by
varying the collector current in
the transistor.
Electronic Devices, 9th edition
Thomas L. Floyd
Error detector
Control
element
Q1
+
RL
R3
D1
Sample
circuit
R4
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Summary
Shunt Regulators
Shunt regulators use a parallel transistor for the control
element. If the output voltage changes, the op-amp senses
the change and corrects the bias on Q1 to follow. For
example, a decrease in output voltage causes a decrease in
V
VB and an increase in VC. V
OUT
IN
R1
Although it is less efficient
than the series regulator, the
shunt regulator has inherent
short-circuit protection. The
maximum current when the
output is shorted is VIN/R1.
Electronic Devices, 9th edition
Thomas L. Floyd
R2
Error detector
–
VREF
Control
element
Q1
+
RL
R3
D1
Sample
circuit
R4
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Shunt Regulators
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Switching Regulators
All switching regulators control the output voltage by
rapidly switching the input voltage on and off with a duty
cycle that depends on the load. Because they use high
frequency switching, they tend to be electrically noisy.
Andecrease
A
increaseininthe
theduty
dutycycle
cycledecreases
increasesthe
theoutput
outputvoltage.
voltage.
on/off
control
tonton ton
tofftoff toff
tonton ton
tofftoff toff
tonton ton
tofftoff toff
tonton ton
VC
VC
VC
VOUT
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Buck Converter
A step-down switching regulator controls the output
voltage by controlling the duty cycle to a series transistor.
The duty cycle changes depending on the load requirement.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Buck Converter
VIN
Q1
C Lcharges
reversesLpolarity
off
on
+

+

RL
C
D1
R1
VOUT
Variable
pulse-width
oscillator
R2
–
+
R3
D2
VREF
Because the transistor is either ON or OFF on all switching regulators,
the power dissipated in the transistor is very small and the regulator is
very efficient. The pulses are smoothed by an LC filter.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Buck Converter
Andecrease
A
increaseininthe
theduty
dutycycle
cycledecreases
increasesthe
theoutput
outputvoltage.
voltage.
on/off
control
tonton ton
tofftoff toff
tonton ton
tofftoff toff
tonton ton
tofftoff toff
tonton ton
VC
VC
VC
VOUT
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Buck Converter
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Boost Converter
In a step-up switching regulator, the control element
operates as a rapidly pulsing switch to ground. The switch
on and off times are controlled by the output voltage.
Step-up action is due to
the fact the inductor
changes polarity during
switching and adds to
VIN. Thus, the output
voltage is larger than
the input voltage.
VIN
+
L field
L field
collapses
builds
OUT
+
R1
L
+
Variable
pulse-width
oscillator
–
D2
Electronic Devices, 9th edition
Thomas L. Floyd
charges
discharges
on
off C C
+V
D1
C
on
off
Q1
RL
R2
+
R3
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
Boost Converter
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.