Texas Instruments | Linear versus switching regulators in industrial applications with a 24-V bus | Application notes | Texas Instruments Linear versus switching regulators in industrial applications with a 24-V bus Application notes

Texas Instruments Linear versus switching regulators in industrial applications with a 24-V bus Application notes
Power Management
Texas Instruments Incorporated
Linear versus switching regulators in
industrial applications with a 24-V bus
By Rich Nowakowski, Product Marketing Manager, Power Management Group,
and Robert Taylor, Applications Engineer and Member, Group Technical Staff
Figure 1. Switching (buck) converter with integrated MOSFETs
0.01 µF
C2
L1
100 µH
U1
TPS54061DRB
TP1
J1
1
24VIN
1
2
GND
2
3
4
C1
1 µF
TP2
BOOT
PH
VIN
GND
EN
COMP
RT/CLK
R1
301 kΩ
PWPD
24VIN
9
VSNS
TP3
8
J2
7
R3
53.6 kΩ
6
5
R2
24.9 kΩ
C4
22 pF
C3
0.022 µF
1
5 V at 100 mA
2
GND
C5
4.7 µF
R4
10.2 kΩ
TP4
Linear regulators have been around for many
Figure 2. Integrated, wide-input-voltage linear regulator
years. Some designers still use linear regulators
that are over 20 years old for new and old proj24VIN
U2
ects. Others have made their own linear regulaTP7
TP5
LM317
J3
J4
tors from discrete components. The simplicity
1
4
1
3
5 V at 100 mA
IN
OUT
24VIN
of a linear regulator is hard to beat for a wide
2
2
2
OUT
GND
GND
1
range of voltage conversions. However, lowGND/ADJ
C10
R5
current applications with a 24-V bus, such as for
243 Ω
4.7 µF
C6
industrial automation or HVAC controls, may
1 µF
have thermal issues if the voltage drop is too
R6
TP6
TP8
732 Ω
large. Fortunately, designers have several
choices now that small, high-efficiency, wideinput-voltage switching regulators are available.
This article compares three different solutions that provide a 5-V output at 100 mA from
circuits shown in Figures 1, 2, and 3 are all built on the
a 24-V bus. A synchronous step-down (buck) converter is
same circuit board and use 1-µF input and 4.7-µF output
compared to an integrated linear regulator and a discrete
ceramic capacitors with the same ratings.
linear regulator. Size, efficiency, thermal performance,
The design in Figure 1 uses a synchronous buck contransient response, noise, complexity, and cost are comverter with integrated MOSFETs, the TPS54061 from
pared to help designers choose the solution that best
Texas Instruments (TI). Note that this circuit does not
meets the constraints of a particular application.
require a catch diode but includes an inductor, five capaciConditions of comparison
tors, and four resistors. The device also employs external
Most industrial applications use a 24-V bus and require 5 V
compensation and is tuned to use the same input and outto power various loads, such as logic and low-current
put capacitors as the linear circuits in Figures 2 and 3.
micro­processors. An output current of 100 mA is chosen
The design in Figure 2 uses an integrated, wide-inputbecause it accommodates many logic and processor loads.
voltage linear regulator, TI’s LM317, which is a popular,
However, the power-dissipation level can affect the deciindustry-standard regulator with a 1.5-A output capability.
sion of whether to use a switching or linear regulator. The
This circuit uses two external resistors and two external
9
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Texas Instruments Incorporated
capacitors. The wide difference
Figure 3. Discrete linear regulator
between the input and output
voltages requires the low thermal
24VIN
resistance of a double-decawatt
Q1
TP9
J5
R9 R10
BCP54TA
R8
package (DDPak).
TP11
J6
1
4
24VIN
1
3
2
Figure 3 shows a discrete linear
5 V at 100 mA
51
Ω
51 Ω 51 Ω
2
GND
2
regulator that employs a transistor
R7
GND
1
4.99 kΩ
and a Zener diode with two exterC8
C7
4.7 µF
nal capacitors and four external
1 µF
D1
TP10
TP12
resistors. The Zener diode breaks
5.6 V
down at 5.6 V, and that voltage is
fed to the base of an NPN transistor. Due to the base-emitter voltage
drop, the output is regulated to
~5 V. The external resistors are
used to help with the power dissipation
Table 1. Summary of board area and component count
in the NPN transistor.
BOARD AREA
NUMBER OF
Table 1 summarizes the board area and
REGULATOR TYPE
(in2)
COMPONENTS
COMPLEXITY
component count of each design.
0.14
11
High
Switching (Buck) (TPS54061)
Linear-regulator solutions require more
2.25
5
Low
Integrated Linear (LM317)
board area to provide proper thermal
2.25
8
Medium
Discrete
Linear
(Zener/Transistor)
relief on the circuit board. At full load,
each linear-regulator solution must dissipate about 2 W. As a rule of thumb,
approximately 1 W of dissipation in 1 in2
Table 2. Summary of thermal performance
of board area results in a 100°C temperaMAXIMUM
TEMPERATURE
ture rise. The linear-regulator solutions
TEMPERATURE
RISE
are designed to allow for a 40°C tempera(°C)
PACKAGE
(°C)
REGULATOR TYPE
ture rise. The synchronous buck converter
Switching
11
40.7
3×3-mm VSON
is clearly the design of choice when board
56.2
DDPak
Integrated Linear
27
area is limited, despite the number of
69.1
SOT-23, SOT223
40
external components and the design effort Discrete Linear
required to compensate the feedback loop
and select the inductor.
Thermal performance
The thermal image in Figure 4 shows the temperature rise of each design on the circuit
board. The board is designed in a manner such
that none of the circuits disturb the thermal
performance of an adjacent circuit. Table 2
shows that the switching regulator has the lowest temperature rise, at 11°C. With a large difference between the input and output voltages,
the switching regulator with synchronous rectification excels in efficiency compared to either
Figure 4. Heat generated from each circuit (white indicates
highest temperature)
Switching Regulator
30.5°C min, 40.7°C max
66.4°C
Linear Regulator
31.4°C min, 56.2°C max
Discrete Linear Regulator
30.6°C min, 69.1°C max
19.4°C
10
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Table 3. Summary of efficiency and power loss
MAXIMUM LOAD
REGULATOR TYPE
NO LOAD
EFFICIENCY
(%)
POWER LOSS
(W)
QUIESCENT CURRENT
(mA)
Switching
84.5
0.093
0.5
Integrated Linear
20.0
2.06
5.5
Discrete Linear
20.1
2.02
4
The thermal performance is directly related to
the efficiency of each regulator. Figure 5 shows
an efficiency comparison of all three circuits.
As expected, the switching regulator excels at
both light-load and full-load efficiency. At light
loads, switching losses and quiescent-current
losses become more pronounced, which
explains the reduced efficiency at lighter loads.
At light loads, it is better to view the powerloss graph (Figure 6) than the efficiency graph,
since a 50% difference in efficiency at 10 mA
seems like a large margin. However, the
amount of current consumed by the load is
small. When the input voltage is 24 V and the
output current is 10 mA, the power loss of the
switching regulator is 2.8 mW, and the loss of
the integrated linear regulator is 345 mW. At
full load, the measured power dissipated is
0.093 W for the switching regulator versus
2.06 W for the linear regulator, which shows a
wide margin and a drastic improvement.
Table 3 summarizes the efficiency and power
loss of all three circuits. Note that the quiescent current of the discrete linear circuit is
lower than that of the integrated linear circuit.
The integrated linear regulator has more powerconsuming internal circuitry and incorporates
more features than the discrete linear circuit.
90
80
TPS54061
70
Efficiency (%)
Efficiency comparison
Figure 5. Efficiency versus load current
60
50
40
30
20
X
10
Discrete Linear
X
X
X
LM317
X
X
X
X
X
0
10
20
30
40
50
60
70
80
90
100
Load Current (mA)
Figure 6. Power loss versus load current
2.0
X
1.8
X
1.6
Power Loss ( W)
linear circuit. (See Table 3.) It is interesting to
note that the temperature rise of the integrated linear circuit is different from that of
the discrete linear circuit. Since the integrated
linear regulator’s package (DDPak) is larger,
its dissipated heat is spread over more area.
The discrete linear circuit using the SOT-23
and SOT223 packages is smaller than the
DDPak and has a higher package powerdissipation rating, which makes dissipating
the heat more difficult.
X
1.4
X
1.2
LM317
1.0
X
0.8
X
Discrete Linear
X
0.6
X
0.4
X
0.2
TPS54061
X
0
0
10
20
30
40
50
60
70
80
90
100
Load Current (mA)
11
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Output-voltage characteristics
Analog circuits may be sensitive to voltage
ripple, and digital processors may be sensitive
to the accuracy of the core voltage. It is important to check the power supply’s voltage ripple,
voltage-regulation accuracy, and voltage-peak
deviations during load transients. Linear regulators inherently have low ripple and are used to
remove noise from switching regulators. The
voltage ripple of both the integrated and the
discrete linear-regulator circuits under maximum load is under 10 mV. When expressed as a
percentage of the output voltage, accuracy is
better than 0.2%. On the other hand, the voltage ripple of the switching regulator is 75 mV, or
1.5% of the output voltage. The low equivalent
series resistance of the switching regulator’s
ceramic output capacitor allows for the circuit’s
low ripple, despite the switching regulator’s
inherent noise.
Comparing the output-voltage accuracy of the
switching and linear regulators from no load to
full load shows that the switching regulator has
better performance. Further inspection of the
product specification tables reveals that the reference voltage of the switching regulator is the
most accurate of the three circuits. The switching regulator is a relatively new integrated
circuit, and DC/DC converters are trending
towards higher reference-voltage accuracies.
The discrete linear circuit, which uses a simpler
method for regulating the output voltage, has
the worst performance. In many cases, applications do not need high voltage accuracy since
the 5-V output may be postregulated.
The load-transient plots can be seen in
Figures 7 through 9. Although the switching
regulator has high output-voltage accuracy, its
measured peak-to-peak voltage during a load
transient is not as competitive as that of the linear circuits. The switching regulator’s measured
peak-to-peak voltage during a 50- to 100-mA
load step is 250 mV, or 5% of the output voltage, compared to 40 mV for the linear circuits.
Additional output capacitance can be added to
the switching regulator to reduce the voltage
peaks, but with penalties in cost and size. Note
that the discrete linear circuit is not designed to
attempt recovery of the output voltage during a
load transient. Also, the simplicity of the circuit
does not allow for current limiting or thermalshutdown protection!
Texas Instruments Incorporated
Figure 7. Switching regulator during load transient
Output Voltage (AC, 200 mV/div)
3
Load Current (100 mA/div)
4
Time (1 ms/div)
Figure 8. Integrated linear regulator during load transient
Output Voltage (AC, 200 mV/div)
3
Load Current (100 mA/div)
4
Time (1 ms/div)
Figure 9. Discrete linear regulator during load transient
Output Voltage (AC, 50 mV/div)
3
Load Current (100 mA/div)
4
Time (1 ms/div)
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Table 4 summarizes the output-voltage characteristics of
the three regulator designs.
offers drastic improvements in efficiency and board space
compared to either linear circuit. If a design must have the
absolute lowest cost, a discrete linear circuit can help, but
the trade-off is worse performance with potential penalties, such as the additional cost of heat sinking and the
lack of protection features.
Table 6 summarizes the characteristics of all three regulator designs to aid the designer in choosing the best solution for a given application.
Cost comparison
Most of the external components used in these circuits are
small, passive resistors and capacitors that cost well below
$0.01. The highest-cost component of the three circuits is
the silicon. Costs for all three bills of materials (BOMs),
shown in Table 5, were collected from U.S. distribution
channels at 10,000-unit suggested resale pricing. As can
be seen, both linear-regulator solutions cost much less
than the switching regulator. Unfortunately, the switching
regulator requires an external inductor, which can cost
about $0.10; but the improvement in efficiency and size
may be worth the additional cost. The cost difference
between the integrated and discrete linear circuits is only
$0.06! The protection features alone may prove the value
of the integrated over the discrete linear regulator.
References
1. “3-terminal adjustable regulator,” LM317 Datasheet.
Available: www.ti.com/slvs044-aaj
2. “Wide input 60V, 200mA synchronous step-down DC-DC
converter with low IQ,” TPS54061 Datasheet. Available:
www.ti.com/slvsbb7-aaj
Related Web sites
Power Management:
www.ti.com/power-aaj
Conclusion
There are many power-management solutions available to
designers, and the best solution depends on the particular
needs of the application. Power-management solutions that
reduce energy consumption and save board space allow
designers to make their products more differentiated and
attractive on the market. A synchronous buck converter
www.ti.com/lm317-aaj
www.ti.com/tps54061-aaj
Subscribe to the AAJ:
www.ti.com/subscribe-aaj
Table 4. Summary of output-voltage characteristics
MAXIMUM
LOAD RIPPLE (mV)
OUTPUT TRANSIENT WITH
50- TO 100-mA LOAD STEP
(mV)
REGULATION ERROR WITH
0- TO 100-mA LOAD STEP
(mV)
Switching
75
250
1.5
Integrated Linear
<10
40
0.7
Discrete Linear
<10
40
21.8
REGULATOR TYPE
Table 5. Summary of BOM cost
REGULATOR TYPE
BOM COST AT 10-ku RESALE PRICE
(U.S. DOLLARS)
Switching
1.80
Integrated Linear
0.32
Discrete Linear
0.26
Table 6. Characteristics of 5-V/100-mA regulators with a 24-V input
BOM COST AT 10-ku RESALE PRICE
(U.S. DOLLARS)
VOUT RIPPLE
(mV)
FULL-LOAD
EFFICIENCY
(%)
BOARD AREA
(in2)
COMPLEXITY
Switching
1.80
75
84.5
0.14
High
Integrated Linear
0.32
<10
20.0
2.25
Low
Discrete Linear
0.26
<10
20.1
2.25
Medium
REGULATOR TYPE
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