MS-2411: Optimizing Power Conversion for Isolated Sensor Interfaces

MS-2411: Optimizing Power Conversion for Isolated Sensor Interfaces
Technical Article
MS-2411
.
Optimizing Power Conversion
for Isolated Sensor Interfaces
The data interfaces have been improving steadily from the
era of optocouplers to the latest high speed low power highly
compact digital isolators. In this article, we will examine one
aspect of isolated sensor interfaces that gets less attention
than it deserves. How do we get isolated power to the ADC
and conditioning circuits while shrinking the size of the
interface and improving performance? In the past, the
analog interface boards did not have high channel counts, so
there was enough room on the board for a modest dc-to-dc
converter to be designed to provide power to the sensor
interface. Power dissipation was not a great concern since
there were only one or two interfaces to a module. Currently,
analog PLC modules, as illustrated in Figure 1, can have
four, eight, or even 16 independent isolated channels.
Multiple copies of a modest dc-to-dc converter take up a lot
of space and create a lot of heat.
by Mark Cantrell, Applications Engineer, Analog
Devices, Inc.
IDEA IN BRIEF
In the world of industrial controls, only a few things are
certain; the next product will have a smaller form factor,
more channels, and have a lower target cost per channel.
The expectation is that technology has improved since the
last design and all of these things are possible. To a large
extent, that is the way things have worked out in the past,
and your luck may be holding.
Figure 1. Typical Multichannel Sensor Interface
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MS-2411
Technical Article
solution is dominated by the transformer so in reasonable
volume, a discrete solution cost is less than $1.00.
A good place to start a discussion of power is with a generic
analog interface as shown in Figure 1. The active circuits
consist of a signal conditioning element like an op amp or
instrumentation amp and an ADC with a serial interface
which can be interfaced with the FPGA through digital
isolator channels. This circuitry typically needs significantly
less than 150 mW.
The price you pay for attaining the low cost is that there can
be significant variability in the output voltage over load and
temperature, making the selection of the analog components
of the analog interface more difficult. All analog components
in the analog interface must have excellent power supply
rejection, and the load must not vary quickly, or significant
supply variation can be induced. This results in higher
component costs or, at a minimum, much more engineering
time to evaluate the solution under extreme conditions. The
unregulated supply can have fairly high efficiency, but the
quality of the power is low.
The basic challenge of providing power to the sensor
interface is optimizing the supply to work well within the
required power range. Operation at 0 mW to 150 mW
means that the fixed quiescent power of the controller and
feedback elements that make up the power supply will be a
large portion of the total power used so the efficiency will be
lower. This can be seen in the quiescent current values in
Table 1 for various supply configurations. Alternately, many
simple power supply designs require a minimum load to
operate properly, so power must be wasted in resistive dead
loads to ensure that the supply functions properly. While it is
very simple to drop a 555 timer and transistor on the board
and get some power, it is difficult to make an efficient and
reliable supply that works at low power levels.
REGULATED SUPPLIES AND MODULES
Regulated supplies offer much better output characteristics.
Figure 3 shows a typical dc-to-dc module in the 1 W power
range.
There are three basic categories of dc-to-dc converters used
for this power range:
1.
2.
3.
Unregulated switching supplies or modules
Regulated switching supplies or modules
Chip-scale power converters
Figure 3. Regulated DC-to-DC Module
The controller switches power into the transformer similar
to the unregulated example above. The power level and turns
ratio of the transformer are chosen to give sufficient voltage
at the maximum load to allow an LDO to regulate the output
voltage to a stable level. This scheme gives good power
efficiency at high loads but rolls off to poor efficiency at low
loads. This is exactly where our analog interface application
runs.
Each of these supply architectures needs increasing
complexity of control circuitry and, in the case of the first
two options, increasing component count and solution size.
UNREGULATED SUPPLIES
The simplest solution is the unregulated dc-to-dc converter
as shown in Figure 2.
There are many active regulation schemes that could allow
better efficiency over the full load range, but they require
much more complex control circuitry, and most require a
feedback channel across the isolation barrier. This adds
significant cost and size to the design and is typically not
done for modules in this power range.
Integration of these power supplies has not progressed past
the potted module or PCB daughter card because of difficulty
incorporating the transformer into the assembly. Manufacturers
have had limited success reducing the size of these devices.
Figure 2. Unregulated DC-to-DC Module
This design uses fixed frequency fixed duty cycle input
switching to create a secondary side power that is rectified
and filtered. The transformer chosen will need to be rated
for the isolation voltage required by the application. The
higher the isolation requirement, the larger the transformer
will be, in both PCB footprint and height. The cost of this
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Technical Article
MS-2411
CHIP-SCALE CONVERTERS
The development of chip-scale transformer technology by
Analog Devices for the iCoupler® digital isolator products
has created a new class of dc-to-dc converters. The technology
lends itself well to low power high functionality power
supply designs. The transformers are “air core” meaning that
there are no magnetic materials present in the transformer.
This means that these tiny transformers have their highest
Q at about 125 MHz. The switching frequency is so high it is
not practical to alter the duty factor of the switch signal to
control power. Instead, the control circuitry gates the entire
oscillator of and on to regulate voltage at the secondary.
Figure 4. ADuM5010 Chip-Scale Converter
COMPARISONS
Let’s look at some practical examples to illustrate the differences
between the designs we have discussed. Table 1 shows a
comparison of the properties of two power modules and a
chip-scale converter. The TI modules chosen were the
closest commonly available module in power to the 0 mW to
150 mW range identified in the sensor interface
requirement.
The transformers are small enough to be integrated into a
standard IC package with an internal split lead frame. All of
the components from both sides of the isolation barrier
required for forward power and output feedback can be
integrated into a pair of silicon die eliminating the need for
discrete external components and allowing advanced features
to be implemented. The chip-scale power converter can contain
all of the functionality of a fully regulated dc-to-dc power
supply providing tight regulation and good efficiency at low
load conditions.
Most designers need to make a power efficient design. What
jumps out of Table 1 is the efficiency of the unregulated
solution, but there are drawbacks to choosing that solution.
This module is rated at 1 W, and its data sheet does not even
rate its performance below 100 mW. It is likely that the
output voltage is significantly higher than rated and the
efficiency falls off rapidly.
Table 1. Technology Comparison
Type
P/N
Peak Efficiency
10 mA
Efficiency
Quiescent
Current
Max Power
Load Regulator
Size
Cost
Chip-Scale Converter
ADuM5010
30%
27%
6.8 mA
150 mW
1.3%
7.4 × 7.4 × 2
$1.50
Regulated Module
DCR010505
50%
21%
18 mA
1W
3%
18 × 10 × 2.5
$5.95
Unregulated Module
DCH010505
72%
N/A
60 mA
1W
10%
20 × 8 × 10
$4.25
Page 3 of 5
MS-2411
Technical Article
its primary power oscillator at 125 MHz well above the
sampling frequency of most industrial sensor ADCs. There
is still ripple due to the PWM control of the power oscillator,
but the biggest noise source is above the bandwidth of the
ADC and easily filtered.
Efficiency (%)
The next highest efficiency is with the regulated module. It is
rated for use at light loads and is well behaved. However, if
we actually look at the efficiency of the regulated module
compared to the chip-scale converter, Figure 5 shows that
since the chip-scale converter has active feedback regulation,
its efficiency rises much faster to its final value, so between
0 mA and 15 mA of load the chip-scale solution is actually
more efficient. This is most of the target range identified in
the original analog interface definition. So, the chip-scale
solution is a better choice even though it has the lowest
maximum efficiency.
50
45
40
35
30
25
20
15
10
5
0
ADDITIONAL BENEFITS OF CHIP-SCALE
CONVERTERS
Just on the basis of the size efficiency, the chip-scale
converter is a good choice for this application. However,
there are many other advantages to the technology. Let’s look
at the new ADuM5010 isolated power converter in detail.
This device can give the performance of a telecom dc-to-dc
converter at the low power range required for analog
interfaces.
1.
DCR01
5V/5V
ADuM5010
5V/5V
0
10
20
30
2.
40
Load Current (mA)
Figure 5. Efficiency of DC-to-DC Regulated Module Compared to Chip-Scale
Converter
The solution size is the next point of comparison. The
modular solutions are both 180 mm2 on the PCB, and the
unregulated module is actually 10 mm tall, making it not
only take up board space, but it is likely the tallest item on
the board, determining the case size for our theoretical
module. The clear choice again is the chip-scale module in a
low profile SSOP20 JEDEC standard package, at 55 mm2, plus
some bypass capacitors and two resistors.
3.
The advantage of choosing a regulated versus an unregulated
solution is related to the power supply rejection of the ADC
and amplifier in the analog front end. Better regulation
allows much more flexibility in choosing components that
do the measurement job required rather than limiting the
choices to the parts with the best power supply rejection
numbers.
4.
5.
The final differentiating factor between modular and
discrete solutions and chip-scale solutions is the operating
frequency. The switching currents create noise and ripple on
the power supply. In many cases, the modules operate in the
200 kHz to 1 Mhz frequency range, which corresponds to
the frequency sample rates for many sensor applications.
Care must be taken to properly filter or antialias the data
from the power supply noise. The chip-scale solution runs
6.
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Infinitely adjustable output voltage. The ADuM5010
sets its output voltage via a voltage divider on the
secondary side. It can range from 3.15 Vto 5.5 V. Many
analog ADCs and op amps operate with nonstandard
supply rails, so the voltage can be adjusted to take
advantage of optimum supply conditions.
Thermal shutdown protects the supply during short
circuit overload conditions, especially at high ambient
temperatures where the maximum die temperature
could be exceeded. The thermal shutdown trips at
154°C, and the die must cool by 10°C before the part
will automatically restart. No external processor
intervention is required to restart the supply.
Softstart is implemented through primary side control
of the PWM as power is applied. This allows the part to
start with negligible inrush current. When multiple
parts are starting simultaneously, inrush current can
overwhelm a weak dc input supply rail and cause
unpredictable operation.
Primary side power disable allows the converter to be
shut down to a very low standby state. This feature
combined with softstart can allow power saving
schemes which turn power off to a sensor between
measurements.
Under voltage lockout (UVLO) on the primary side
input supply. This feature prevents the converter from
starting at low input supply rails. This allows the input
supplies to charge significantly before the downstream
ADuM5010 tries to draw power.
Fully certified isolation. This can allow reduced type
testing of modules and elimination of in-line test during
production.
Technical Article
MS-2411
CONCLUSION
ABOUT THE AUTHOR
The analog sensor interface application, as designed for most
PLC applications, requires isolation of both digital communications and power. The power levels are very low, lower
than most dc-to-dc converters can function efficiently and
predictably. However, the interface benefits greatly from
having a well regulated and well behaved power supply. The
ADuM5010 isolated chip-scale converter fits the requirements
of the isolated analog input very well, with 150 mW of power
and a set of features normally only available in high power
dc-to-dc converters. This part is the power-only version of
a family of devices that combine power with isolated data
channels. The ADuM521x dual data channel devices will
allow the data interface to be combined saving even more
space. Higher channel count devices will fill out the line as it
develops. This allows power to be applied safely and simply
with a minimum of design effort.
Mark Cantrell is an applications engineer for the iCoupler
Digital Isolator Group at Analog Devices, Inc. (ADI). His
area of expertise is iCoupler digital isolation products,
including isoPower® isolated power supply devices and
communication bus devices such as I2C and USB isolators.
He is also responsible for agency safety certifications for all
iCoupler digital isolator products. Mark received his MS in
physics from Indiana University. He can be reached via
email at [email protected]
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