Texas Instruments | Shunt-based current-sensing solutions for BMS applications in HEVs and EVs (Rev. A) | Application notes | Texas Instruments Shunt-based current-sensing solutions for BMS applications in HEVs and EVs (Rev. A) Application notes

Texas Instruments Shunt-based current-sensing solutions for BMS applications in HEVs and EVs (Rev. A) Application notes
Shunt-based current-sensing solutions for BMS
applications in HEVs and EVs
Guang Zhou
Introduction
Low Voltage (12-V to 48-V) BMS Current Sensing
Hybrid electric vehicles (HEV) and electric vehicles
(EV) continue to gain share in the overall global
automotive market. The battery management system
(BMS) for these vehicles carries out the important
tasks of keeping the battery operating inside the safe
operating area (SOA), monitoring power distribution,
and keeping track of the state of charge (SoC).
The advantages of nonisolated shunt-based current
sensing include simplicity, low cost, excellent linearity,
and accuracy. On the other hand, limited commonmode range can restrict application in a high-side
current-sensing configuration.
400-V and 800-V
Charger
48-V Conversion
To Battery Array
12-V Conversion
Non-isolated
CSA
Non-isolated
CSA
12-V Load
Non-isolated
CSA
48-V Load
High-Voltage Load
ISOLATION
Low side for HV;
High side possible for LV
Isolated
CSA
PFC and DC/
DC Charger
CSA
In a typical HEV and EV, both high- and low-voltage
subsystems are present. The high-voltage subsystem
operates at several hundred volts, and interfaces
directly with utility grid or high-voltage dc sources. The
low-voltage subsystem generally operates at 48 V and
12 V.
Another drawback of shunt-based current sensing is
that at high-current levels, power dissipation by the
shunt can potentially be significant.
Non-isolated
CSA
Non-isolated
CSA
Figure 1. Topologies of Current Sensing in BMS
TI offers a variety of isolated current sensing devices
that can be used in high-voltage BMS systems. Among
them is the DRV425, which is fluxgate technology
based. The TIPD205, ±100-A bus bar current sensor
using open-loop fluxgate sensors reference design
illustrates how this design is achieved. A summary of
other examples of isolated current sensing technology
can be found in the Comparing shunt- and hall-based
isolated current-sensing solutions in HEV/EV
application note. Here, however, the focus is solely on
a nonisolated, high-side, shunt-based current-sensing
amplifier (CSA), also called a current shunt monitor
(CSM), in 12-V to 48-V BMS subsystems.
SBAA324A – December 2018 – Revised January 2019
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CSA
Figure 2. Current-Sensing Amplifiers in an HEV or
EV Charger
Battery array is an important component of any HEV or
EV. There are mainly two types of rechargeable
batteries: The lead acid battery that has been around
for over 100 years, and the Li-Ion battery that has only
been put into practical use since the 1980s. At the
time of this publication, there is a continued,
tremendous research effort to introduce new types of
batteries, such as aluminum air and zinc air batteries.
The ultimate goal is to commercialize the next game
changer; a battery that is safer, longer lasting, and
lower maintenance with higher power density. When it
comes to battery management, there are many
differences between lead acid and Li-Ion batteries.
However, there are also many similarities. Both types
follow a certain constant voltage-constant current (CVCI) charging profile. The CSA plays an important role
in making sure the battery remains within the SOA.
Charging current can be quite high, and can reach
hundreds of amps. Historically, measuring this current
with shunt-based topologies has been challenging.
However, with the availability of ultra-low resistance
shunts, the option is now viable.
On the other hand, a BMS system must monitor the
power distribution as accurately as possible during
normal operation in order to provide overall system
health and safety information. State of charge (SoC),
which is the equivalent of a fuel gauge for the battery
Shunt-based current-sensing solutions for BMS applications in HEVs and
EVs Guang Zhou
Copyright © 2018–2019, Texas Instruments Incorporated
1
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pack in an HEV or EV, correlates to driving range.
Current sensing and integration is one of the important
methods to determine SoC. Even when the engine is
shut off, not all onboard electronics are completely
turned off. These off-state currents contribute to the
overall leakage current, and there is a strong desire to
have the leakage current monitored and accounted for.
Ideally, a single current-sense amplifier must monitor
the entire current range, from several hundred amps
down to a few amps, possibly even to milliamps.
Maintaining accuracy within such a wide dynamic
range is often one of the greatest challenges in
designing for BMS current sensing.
Sizing the Shunt Resistor
The INA240-Q1 is an excellent choice for 48-V
systems because of its 80-V common-mode
specification. The INA226-Q1 is a digital-output
current-sense amplifier designed for up to a 36-V
common-mode voltage.
The device integrates a high-performance ADC within
the same chip, offering an exceptional 10-µV max
offset specification. Both devices are manufactured
with TI proprietary Zero-Drift technology, which makes
single temperature calibration possible.
Table 1. Comparison Between INA240A1 and
INA226
Key Specifications
The maximum current and power rating of the shunt
resistor often determines the highest shunt value that
can be used. The higher the shunt resistance, the
bigger the shunt voltage, and the smaller the relative
error due to system nonidealities, such as amplifier
offset, gain error, and drift. However, the higher the
shunt voltage, the higher the power dissipation.
Excessive power dissipation causes temperature rise,
which not only degrades system performance, but also
can potentially be destructive when not properly
controlled. On the other hand, the lowest shunt value
is determined by the minimum current and accuracy of
the current-sense amplifier.
As an example, suppose the CSA offset is 10 µV,
while all other error sources are negligible, and the
shunt resistance is 100 µΩ. Without calibration, for a
100-mA current, the reported current could be
anywhere between 0 mA and 200 mA. If the shunt is
changed to 1 mΩ, the same current is reported
anywhere between 90 mA and 110 mA. In practice, a
shunt resistor is often chosen to be between the two
extreme values.
INA240A1
INA226
Analog Out
I2C
Maximum VCM
80 V
36 V
Minimum VCM
–4 V
0V
2.7 V to 5.5 V
2.7 V to 5.5 V
Output
Supply voltage (VS)
Shunt voltage (VS= 5 V)
VOS at 12 V
VOS drift
Gain error
Noise density
Zero-drift current-sense amplifiers enable single-point
calibration, and make such challenging designs
possible by offering stable performance over
temperature.
2
±81.9175 mV
±10 µV, max
0.25 µV/°C
0.1 µV/°C
0.20%
0.10%
40 nV/√Hz
NA
Automotive Digital Output CSA Recommendations
In addition to the INA226-Q1, TI offers other digital
output current, voltage, and power monitors. Some
example products and adjacent technical documents
are compiled in Table 2 and Table 3.
Table 2. Alternative Device Recommendations
Device
Digital
Interface
INA220-Q1
I2C, SMBUS
26-V, Bidirectional, Zero-Drift, Low- or
High-Side, I2C Current/Power Monitor
INA3221-Q1
I2C, SMBUS
26-V, Triple-Channel, Bidirectional, ZeroDrift, Low- or High-Side, I2C, Current and
Voltage Monitor w/Alerts
Choose the Correct Current Sense Amplifier
TI’s precision, nonisolated current sense amplifiers
offer a wide choice in terms of key parameters, such
as common-mode voltage, bandwidth, offset, drift, and
power consumption. Sensing current accurately over a
wide dynamic range is a great challenge. The problem
is especially acute at the lower end, where system
error can easily overwhelm the useful signal. A system
calibration becomes necessary in order to be able to
subtract system error from the measurements.
±125 mV
±25 µV, max
Description
Table 3. Adjacent Tech Notes
Literature Number
Literature Title
SBAA325
Current Sensing with INA226-Q1 in HEV/EV Low
Voltage BMS Subsystems
SBOA295
High Voltage, High-Side Floating Current Sensing
Circuit Using Current Output Current Sense Amplifier
Conclusion
For current sensing in HEV and EV low-voltage BMS
subsystems, in addition to low-side, a high-side shuntbased solution is a viable option. Zero-Drift technology
enables one-time calibration, which makes low-current
measurement possible. Digital output devices can
further simplify the design by taking advantage of the
existing communication bus.
Shunt-based current-sensing solutions for BMS applications in HEVs and
EVs Guang Zhou
SBAA324A – December 2018 – Revised January 2019
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