ANALOG CIRCUIT AND DEVICES
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Transcript of ANALOG CIRCUIT AND DEVICES
ANALOG CIRCUIT ANALOG CIRCUIT AND DEVICESAND DEVICES
21/04/231
Semester I2013/2014
Course Code: EEE 3123
TOPIC:
VOLTAGE VOLTAGE REGULATORSREGULATORS
21/04/232
Power Supply Regulation
An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions.
00 Current
Voltage
Ideal power supply
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.
00 Current
Voltage
Real power supply
VNL VNL
VFL
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
OUT
IN
Line Regulation = 100%V
V
Line regulation can also be expressed in terms of percent change in VOUT per volt change on the VIN (%/V).
OUT OUT
IN
/ 100%Line Regulation =
V V
V
Example;A line regulation of 0.05 %/V means that the output voltage changes 0.05 percent when the input voltage increases or decreases by one volt.
Example 17.1• Page 853
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:
NL FL
FL
Load Regulation = 100%V V
V
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 as illustrated in the next slide for a commercial power supply.
Example;A load regulation of 0.01 %/mA means that the output voltage changes 0.01 percent when the load current increases or decreases by 1 mA.
Example 17.2• Page 854
Load Regulation
Commercial power supplies, such as you have in lab, have excellent line and load regulation specifications.
The BK Precision 1651A is an example of a triple output supply (two 0-24 V outputs and a fixed 5 V output). Voltage regulation specifications for this power supply are: Line regulation: ≤0.01% +3 mV (Main supply)
≤5 mV (Fixed 5 V supply) Load regulation: ≤0.01%+3 mV (Main supply)
≤5 mV (Fixed 5 V supply)
Load Regulation
Sometimes the power supply manufactures use the equivalent Thevenin resistance of a supply in place of a load regulation specification.
RTH = ROUT
RLVTH = VNL
VOUT
Power Supply
In this case, VOUT can be found by applying the voltage divider rule:
LOUT NL
OUT L
RV V
R R
In terms of resistances, load regulation can be expressed as:
OUT
FL
Load regulation 100%R
R
Refer Page 854-855 for derivation.
Load Regulation
A power supply has an output resistance of 25 m and a full load current of 0.50 A to a 10.0 load.
(a) What is the load regulation? (b) What is the no load output voltage?
OUT
FL
0.025 Load regulation 100% 100%
10.0
R
R
= 0.25%(a)
(b) By Ohm’s law, VOUT = 5.0 V.
OUTNL
L
OUT L
5.0 V10.0
0.025 + 10.0
VV
R
R R
= 5.013 V
Series Regulators
Series Regulator block diagram:
Basic series regulator circuit:
VIN VOUT
Errordetector
Samplecircuit
Controlelement
Referencevoltage
VIN VOUT
R1
Control element
Q1
D1
VREF+
– Error detectorR2
R3
Samplecircuit
The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor.
Control element: a pass transistor in series with the load between the input and output. Sample circuit: senses a change in the output voltage. Error detector: compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.
A simple representation of a series type of linear regulator:
Refer Page 856 for full explanation.
Series Regulators
The output voltage for the series regulator circuit is: 2OUT REF
3
1R
V VR
(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?
2OUT REF
3
1
100 k1+ 3.9 V
47 k
RV V
R
(a)
(b)
VIN VOUT
R1Q1
D1
VREF+
– R2
R3
47 k
100 k3.9 V
18 V
4.7 k
= 12.2 V
P = VI = (18 V – 12.2 V)(0.2 A)
= 1.16 W
Series Regulators
Current limiting prevents excessive load current. Q2 will conduct when the current through R4 develops 0.7 V across Q2’s VBE. This reduces base current to Q1, limiting the load current.
VIN VOUT
R1
Q1
R3
R2
Q2
Current limiter
R4
+
–
The current limit is:
L(max)4
0.7 VI
R
For example, a 1.4 resistor, limits current to about 0.5 A.
Example 17.4• Page 859
Series Regulators
5R5 OUT
5 6
RV V
R R
An overload causes VR5 to drop because VOUT drops. This means that less current is needed to maintain conduction in Q2 and the load current drops.
+VIN VOUT
R1
Q1
+
–Q2
R4
D1
R3
R2
R6
R5
Fold-back current limiting drops the load current well below the peak during overload conditions. Q2 conducts when VR5 +VBE = VR4 and begins current limiting. VR5 is found by applying the voltage-divider rule:
Shunt Regulators
Shunt Regulator block diagram:
Basic shunt regulator circuit:
VIN
R1
VOUT
Controlelement(shunt)
Samplecircuit
Errordetector
Referencevoltage
VIN
VOUT
R2
Q1
–
+
D1
R4
R3RL
VREF
Error detectorControlelement
Samplecircuit
R1
The control element maintains a constant output voltage by varying the collector current in the transistor.
A simple representation of a shunt type of linear regulator;
Shunt Regulators
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.
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 VB and an increase in VC. VIN
VOUT
R2
Q1
–
+
D1
R4
R3RL
VREF
Error detectorControlelement
Samplecircuit
R1
Q1 in parallel with the load.R1 in series with the load.Regulation is achieved by controlling the current through the parallel transistor Q1.
Example 17.5• Page 862
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.
VC
ton toff ton toff ton toff tonton toff ton toff ton toff ton
VC
VC
ton toff ton toff ton toff tonon/off control
VOUT
An increase in the duty cycle increases the output voltage.A decrease in the duty cycle decreases the output voltage.
Switching Regulators
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.
VIN
VOUT
R1
Q1
R3
R2
RLD1
–
+
D2
Variablepulse-widthoscillator
VREF
L
C
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.
on
C charges
+ VIN
VOUT
R1
Q1
R3
R2
RLD1
–
+
D2
Variablepulse-widthoscillator
VREF
L
C+
L reverses polarityoff
VINVOUT
R1Q1
R3
R2
RL
D1 C
D2
Variablepulse-widthoscillator
L
++
+–
C discharges
+
on
offL field builds
Switching Regulators
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.
VINVOUT
R1Q1
R3
R2
RL
D1 C
D2
Variablepulse-widthoscillator
L
++
+–
on
off
C charges
+
L field collapsesVIN
VOUT
R1Q1
R3
R2
RL
D1 C
D2
Variablepulse-widthoscillator
L
++
+–
+VIN
–VOUT
R1
Q1
R3
R2 RL
D1
D2
Variablepulse-widthoscillator
L
+
–
C+
on
off
L field builds
C discharges
Switching Regulators
In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source.
Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input.
+VIN
–VOUT
R1
Q1
R3
R2 RL
D1
D2
Variablepulse-widthoscillator
L
+
–
C
on
L field collapses
+ C charges
off+VIN
–VOUT
R1
Q1
R3
R2 RL
D1
D2
Variablepulse-widthoscillator
L
+
–
C
IC Voltage Regulators
Integrated circuit voltage regulators are available as series regulators or as switching regulators. The popular three-terminal regulators are often used on separate pc boards within a system because they are inexpensive and avoid problems associated with large power distribution systems (such as noise pickup).
12
3
1 23
4
Type number Output voltage
78057806780878097812781578187824
+5.0 V+6.0 V+8.0 V+9.0 V
+12.0 V+15.0 V+18.0 V+24.0 V
78XX Regulators
The 78XX series is a fixed positive output regulator available in various packages and with standard voltage outputs.
TO-220 case
D-PAK
IC Voltage Regulators
The only external components required with the 78XX series are input and output capacitors and some form of heat sink. These IC’s include thermal shutdown protection and internal current limiting.
Positiveinput
Gnd
Positiveoutput78XX
(1)
(2)
(3)The 78XX series are primarily used for fixed output voltages, but with additional components, they can be set up for variable voltages or currents.
12
3
1 23
4
Heat-sink surface
IC Voltage Regulators
The 79XX series is the negative output counterpart to the 78XX series, however the pin assignments are different on this series. Other specifications are basically the same.
Negativeinput
Gnd
Negativeoutput
79XX
(1)
(2) (3) 79057905.2790679087912791579187924
–5.0 V–5.2 V–6.0 V–8.0 V
–12.0 V–15.0 V–18.0 V–24.0 V
Type number Output voltage
IC Voltage Regulators
The LM317 is a adjustable positive output IC regulator. There is a fixed reference voltage of +1.25 V between the output and adjustment terminals. There is no ground pin.
The output voltage is calculated by: 2OUT REF ADJ 2
1
1R
V V I RR
AdjustmentR1
R2
C1
C2
C3
Positiveinput
Positiveoutput
(1)
(3) (2)LM317
What is VOUT? (Assume IADJ = 50 A.)
= 16.8 V
R1
R2
C1
C2
C3
(1)
(3) (2)LM317
OUT
2 kΩ1.25 V 1 50 μA 2 k
150 V
2 k
150
+20 V +16.8 V
IC Voltage Regulators
IC regulators are limited to a maximum allowable current before shutting down. The circuit shown is uses an external pass transistor to increase the maximum available load current.
VIN
RL
Rext
Qext
VOUTC1
C2
78XX
Rext sets the point where Qext begins to conduct:
extmax
0.7 VR
I
For example, if Imax is 1.0 A, Rext = 0.7
What minimum power rating is required for Rext?P = I2R = (1 A)2(0.7 ) = 0.7 W
IC Voltage Regulators
The 78S40 is an IC containing all of the elements needed to configure a switching regulator, using a few external parts.It is a universal switching regulator subsystem because it can be configured as a step-down, step-up, or inverting regulator by the user. The data sheet shows typical circuits for these configurations.
–
+
10 11 12 13 14 15 16
7 6 5 4 3 2 1
–
+
Q1
Q2
SOscillator
R
Q
Comp.
Flip-flop
VOUT
R1
L
CO
R2
VINCT
RCS
VCC
1.25 Vreference
D1
8
9
–
+
9 10 11 12 13 14 15 16
Noninvertinput
Invertinput
GndTiming
capVCC
Ipk
senseDriver
collectorSwitch
collector
8 7 6 5 4 3 2 1
Referencevoltage
Invertinput
Noninvertinput
VCC
op-ampOutput
Switchemitter
Anode Cathode
–
+
Q1
Q2
SOscillator
R
Q
Comp.
Flip-flop
1.25 Vreference
D1
Here is the step-down configuration.
Selected Key TermsSelected Key Terms
Regulator
Line regulation
Load regulation
Linear regulator
Switching regulator
A electronic circuit that maintains an essentially constant output voltage with changing input voltage or load current.
The percentage change in output voltage for a given change in input (line) voltage.
The percentage change in output voltage for a given change in load current.
A voltage regulator in which the control element operates in the linear region.
A voltage regulator in which the control element operates as a switch.
QuizQuiz
1. The load regulation of an ideal power supply is
a. 0%
b. 25%
c. 50%
d. 100%
QuizQuiz
2. A correct formula for load regulation is
a.
b.
c.
d.
OUT
IN
Load Regulation = 100%V
V
NL FL
FL
Load Regulation = 100%V V
V
FL
NL FL
Load Regulation = 100%V
V V
OUT OUT
IN
/ 100%Load Regulation =
V V
V
QuizQuiz
3. An alternate way to express load regulation is in terms of the
a. output resistance and the full-load resistance
b. output resistance and the shorted-load resistance
c. input resistance and the full-load resistance
d. input resistance and the shorted-load resistance
QuizQuiz
4. In the circuit shown, R4 = 0.7 . The output current will be limited to
a. 0.5 A
b. 0.7 A
c. 1.0 A
d. 1.4 A
VIN VOUT
R1
Q1
R3
R2
Q2
Current limiter
R4
+
–
0.7
QuizQuiz
5. The block diagram for a series voltage regulator is shown. The yellow box represents a
a. control element
b. sample circuit
c. error detector
d. reference voltage
VIN VOUT
?
QuizQuiz
6. The block diagram for a shunt voltage regulator is shown. The yellow box represents a
a. control element
b. sample circuit
c. error detector
d. reference voltage
VIN
R1
VOUT
?
QuizQuiz
7. The circuit in the blue shaded area is a
a. high speed switching circuit
b. fold-back current limiter
c. reference source
d. shunt regulator
+VIN VOUT
R1
Q1
+
–Q2
R4
D1
R3
R2
R6
R5
QuizQuiz
8. A major advantage of all switching regulators is
a. low noise
b. high output impedance
c. high efficiency
d. all of the above
QuizQuiz
9. The type of regulator circuit shown is a
a. series linear
b. series switching
c. shunt switching
d. none of the above
+VIN
–VOUT
R1
Q1
R3
R2 RL
D1
D2
Variablepulse-widthoscillator
L
+
–
C+
on
off
L field builds
C discharges
+VIN
–VOUT
R1
Q1
R3
R2 RL
D1
D2
Variablepulse-widthoscillator
L
+
–
C
on
L field collapses
+ C charges
off+VIN
–VOUT
R1
Q1
R3
R2 RL
D1
D2
Variablepulse-widthoscillator
L
+
–
C
QuizQuiz
10. The output voltage from a 7912 is a regulated
a. +5 V
b. +12 V
c. 5 V
d. 12 V
References• Thomas L. Floyd, “Electronic Devices Conventional
Current Version, Ninth Edition (International Edition)”, Pearson Prentice Hall, 2012
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