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Texas Instruments Isolated sensing systems with low power consumption Application notes
Analog Applications Journal
Industrial
Isolated sensing systems with
low power consumption
By Jose Duenas
Applications Engineer
Tom Hendrick
Applications Engineer
Current-shunt-monitor (CSM) ICs have been a mainstay in
industrial applications for many years. Designed for either
unidirectional or bidirectional current monitoring, CSMs
offer excellent performance when used in either high-side
or low-side current-shunt applications. However, many
modern applications require some level of insulation to
protect the end user from hazardous voltages.
The level of insulation that a particular circuit needs is
driven mainly by the type of end equipment and where
the end equipment will be deployed. For instance, is the
end equipment a solar inverter to be mounted on a roof
top or is it part of a servo motor drive used on an industrial robot? Global location of the end equipment plays a
part as well. In the United States the Underwriters
Laboratory (UL) maintains safety standards for various
end-equipment. For Canada, it is the Canadian Standards
Association (CSA). Europe has the International
Electromechanical Commission (IEC) and the Association
for Electrical, Electronic and Information Technologies
(referred to as the VDE).
There are four main categories of insulation. The first is
functional, which offers no protection against electric
shock. As the name implies, functional insulation is provided to allow proper operation of a circuit or device.
Think of this as the minimum trace spacing across a
printed circuit board from a shunt resistor to the input
terminals of the monitoring device.
The second level of insulation is basic. Basic insulation
relates to the ability of an isolation device (an optocoupler
or digital isolator, for example) to provide a level of protection against electric shock across an isolation barrier.
Next is supplemental or double insulation. This is an
independent insulation layer that is applied in addition to
basic insulation to ensure protection against electric shock
in the event that the basic insulation fails. This is similar
to adding a section of heat-shrink tubing over an input
wiring harness. The fourth category is reinforced insulation. Reinforced insulation is a single insulation system
that provides a level of protection against electric shock
equal to double insulation.
For a typical insulation example, the AMC1305 is a precision, delta-sigma (ΔΣ) modulator with the output separated from the input circuitry by a capacitive isolation
barrier that is highly resistant to magnetic interference.
This barrier is certified to provide reinforced isolation of
up to 7000 VPK, according to the VDE V 0884-10, UL1577,
Texas Instruments
Figure 1. Example of the dual-capacitor
isolation barrier
Lead
Frame
Epoxy Casing
Modulator Die
Series Caps
500 µm
Lead
Frame
Receiver Die
and CSA standards. As shown in Figure 1, the isolation
barrier of this device is constructed with two series capacitors, each having an equivalent of basic insulation through
a silicon dioxide (SiO2) layer of 13.5 µm (27 µm total).
The surge immunity is rated to ±10,000 V and the working
voltage is 1500 VDC and 1000 VRMS, respectively.
Unlike traditional CSM devices that provide an analog
output, the AMC1305 provides a digital bit stream. The
differential analog input is a switched-capacitor circuit
feeding a second-order delta-sigma modulator stage that
digitizes the input signal into a 1-bit output stream. The
converter’s isolated output (DOUT) provides a digital bitstream of ones and zeroes that are synchronous to an
externally provided clock source at the CLKIN pin. The
output bit-stream can be fed directly to the SD-24B module of an MSP430™ microcontroller (MCU) or the sigmadelta filter module (SDFM) of a C2000™ Delfino™
TMS320F2837x MCU.
In addition to dictating the level of isolation required,
the type of application determines how many currents and
voltages need to be monitored. In many cases, the variables of a polyphase system are monitored. One of the
most common types of polyphase system is the threephase case. Typically, three currents and three voltages
could be measured in three-phase systems, and sometimes
a fourth voltage is measured, primarily in cases where a
connection to neutral or ground is available.
Supplying power to the sensing circuitry is greatly simplified when the variables measured in a polyphase system
have low common-mode voltages with respect to a common reference point. This could be the case when performing low-side current measurements and voltage
measurements using resistive dividers. However, many
systems require measuring currents and voltages that can
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AAJ 4Q 2014
Analog Applications Journal
Industrial
have significantly different common-mode
Figure 2. Example of a polyphase system
components. In such cases, isolated power
with current shunts (RS)
supplies are required and the design
becomes a bit more complex.
Consider the system depicted in Figure 2.
There are seven circuit functions that could
VU
be monitored: Three line currents, three
1
3
5
phase-to-phase voltages and one common-toground voltage. For simplicity, only three
VU – VW
VU – VV
RS
current shunts (RS) are depicted and the
DC +
R
RS
S
divider circuits for voltage measurement are
Link DC
–
not shown.
VW
VV
Depending upon which power transistors
VV – VW
(elements labeled 1 through 6) are conduct2
4
6
ing, the common-mode voltage of the shunt
resistors can be either near the full DC-Link
voltage or near ground potential.
In order to take advantage of a design
using isolated delta-sigma modulators, each
of the seven monitoring circuits require a
start by producing 3.3 VDC from the 48-VDC source with
separate isolated power supply for the high side of the
a buck-bias, step-down switching regulator (Figure 3).
delta-sigma modulators. The term “high side” is often used
Figure 4 shows how a second stage could generate an isoto refer to the analog input side of the galvanic isolation
lated 5-VDC supply from the 3.3-VDC supply with a small
barrier.
isolation transformer in conjunction with a transformer
For example, in a system with a 48-V DC-Link voltage,
driver.
one approach to design the required power supply could
Figure 3. Step-down switching regulator design
10 nF
VIN_MIN = 45 V
VIN_MAX = 48 V
L1
Vin Boost
VIN
330 µH
SD/Ron
FB
Rcl GND Vcc
124 kΩ
+
–
SW
LM5008
R ON
100 nF
169 kΩ
22 µF
VOUT = 3.3 V
IOUT = 0.155 A
4.7 µF
3 nF
D1
0.79 V
316 Ω
Rseries
0.910 Ω
1 kΩ
10 nF
IOUT
Figure 4. Isolated 5-VDC supply from 3.3 VDC
VIN (3.3 V)
10 µF
SN6501
4
GND D2
VCC
5
GND D1
3
T1
2
2
3
1
TP2
LDO
1
10 µF
IN OUT
5
GND
EN
NC
4
VOUT (5 V)
10 µF
0.1 µF
0.1 µF
Texas Instruments
TP1
MBR0520L
MBR0520L
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AAJ 4Q 2014
Analog Applications Journal
Industrial
Table 1. A comparison between two acquisition systems based on isolated delta-sigma modulators
CURRENT
REQUIRED FROM
THE 3.3-V BUS
(A)*
POWER DRAWN
FROM THE
48-VDC BUS
(W)
0.45
0.155
0.69
1.7
0.57
2.27
EFFICIENCY OF
THE 3.3-VDC TO POWER REQUIRED
ON THE 3.3-V BUS
5-VDC STAGE
(W)
(%)
IAVDD
(max)
(mA)
UNITS PER
SYSTEM
SUM OF CURRENTS
REQUIRED IN THE
5-VDC BUSES
(mA)
AMC1305
7
7
49
54
Alternative
Device
36
7
252
74
ISOLATED
DELTA-SIGMA
MODULATOR
* An additional 10% to 12% margin has been added to the current requirement.
Table 1 compares two scenarios. In one scenario, seven
AMC1305 units were used for monitoring. Figures 3 and 4
show the circuits that fulfilled the power requirements for
the design with seven AMC1305 devices. The second scenario used an alternative device for the delta-sigma modulator and different components were used for the 48-V to
3.3-V power section.
The alternative-device scenario shows the implications
of using seven units of a device that has higher power consumption on its analog input side (high side).
TI’s family of isolated delta-sigma modulators includes
some components with a specified input range of ±250 mV
and others of ±50 mV. Compared to devices with a higher
input range, devices with a lower input range allow system
designers to reduce power dissipation in the sensing-­
current shunt by 80%.
Using a low-power, isolated-sensing solution brings
about more efficient acquisition systems (from an energy
point of view) as well as better performance. The greatest
impact that higher power consumption can have in the
acquisition system’s performance is in gain-error drift and
offset-error drift. An isolated delta-sigma modulator with
higher power consumption is bound to experience a
higher internal temperature rise during normal operation.
Moreover, the ambient temperature of the isolated deltasigma modulator is bound to be higher for systems with
power-management circuitry that is tasked to deliver more
than three times more power. The combination of higher
internal and ambient temperatures in systems with higher
power consumption yields solutions with more errors and
poorer signal-to-noise ratio (SNR).
Texas Instruments
The best-in-class drift performance provided by the
AMC1305 reduces temperature dependency and yields
higher system performance over a wider temperature
range. Also, gain-error drift is cut by as much as 58% and
offset drift by 74% when compared to the closest
competitor.
Conclusion
Many modern applications require isolation. The specific
isolation level needed is driven by the type of end equipment in question and the regulatory body certifying the
equipment.
Although power consumption is sometimes neglected as
a key design criterion, the performance and efficiency of
isolated sensing systems can be greatly improved by carefully selecting devices that have high-precision, isolated
front-ends with optimized power-consumption specifications, such as the TI family of AMC1305 products.
Related Web sites
www.ti.com/4q14-AMC1305L25
www.ti.com/4q14-AMC1305M25
www.ti.com/4q14-AMC1305M05
C2000™ Delfino™ MCU
www.ti.com/delfino
Subscribe to the AAJ:
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Analog Applications Journal
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