Texas Instruments | Current sensing with INA226-Q1 in HEV and EV BMS subsystems (Rev. A) | Application notes | Texas Instruments Current sensing with INA226-Q1 in HEV and EV BMS subsystems (Rev. A) Application notes

Texas Instruments Current sensing with INA226-Q1 in HEV and EV BMS subsystems (Rev. A) Application notes
Current sensing with INA226-Q1 in HEV and EV BMS
subsystems
Guang Zhou
Introduction
Power Supply
(0 V to 36 V)
A typical HEV and EV battery management system
(BMS) can have both high-voltage and low-voltage
subsystems. The low-voltage subsystem generally
refers to the 12-V to 48-V systems that are responsible
for managing power train, lighting, air-conditioning, and
infotainment systems. The load current can vary
widely depending on the operating condition of the
vehicle. In order to measure current accurately over a
wide dynamic range, precision current sense amplifiers
(CSA) that are stable over temperature are highly
desirable. The CSA can be configured as either high
side or low side in a BMS system.
CSA
High Side
12-48V
Battery
HighSide
Shunt
VS
(Supply Voltage)
VBUS
SDA
SCL
´
Load
Power Register
V
2
VIN+
LowSide
Shunt
Current Register
ADC
I
VIN–
Voltage Register
I C or SMBus
Compatible
Interface
Alert
A0
Alert Register
A1
GND
Figure 2. INA226-Q1 Block Diagram
The INA226-Q1 features an ultra-low input shunt offset
of 10 µV; and shunt voltage resolution of 2.5 µV. The
exceptional accuracy and resolution make the INA226Q1 an excellent choice for current sensing applications
where wide dynamic range is required. Ultra low drift
makes one-time calibration a possibility thereby
enables accurate measurements at low current levels.
Optimize for Speed and Accuracy of the INA226-Q1
Load
CSA
Low Side
Figure 1. Typical Current-Sensing Configurations
in a Low-Voltage BMS Subsystem
INA226-Q1 Features
The INA226-Q1 is a current shunt and power monitor
with an I2C- or SMBUS-compatible interface featuring
up to 16 programmable addresses. The INA226-Q1
senses bus voltages that can vary from 0 V to 36 V,
and shunt voltage that can vary from –81.9175 mV to
+81.9175 mV, independent of the supply voltage. The
differential shunt voltage is measured with respect to
the IN– pin, whereas the bus voltage is measured with
respect to ground. The device operates from a single
2.7-V to 5.5-V supply, drawing a typical quiescent
current of 330 µA during active conversion.
With an integrated high-performance analog-to-digital
converter (ADC), the INA226-Q1 eliminates many of
the error sources otherwise found in a discrete solution
consisting of op amp, resistor network, and ADC.
SBAA325A – December 2018 – Revised January 2019
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CBYPASS
0.1 mF
Great flexibility is built into the INA226-Q1 ADC
architecture. Conversion time and averaging can be
optimized to achieve the best conversion speed and
accuracy. Also, the conversion time and averaging can
be independently configured for bus voltage and shunt
voltage. Therefore, the bus voltage and shunt voltage
can be sampled at different speeds. This design
further increases the flexibility in designing an optimal
sampling scheme for a given application.
For example, suppose the voltage, current, and power
information is desired every 5 ms. The device can be
configured with the conversion times set to 588 μs for
both shunt- and bus-voltage measurements, and the
averaging mode set to 4. This configuration results in
the data updating approximately every 4.7 ms.
The device could also be configured with a different
conversion time setting for the shunt- and bus-voltage
measurements. This configuration allows for time to be
focused on the more challenging measurement. In the
BMS system, it is usually at the lowest current range
level, which translates into the lowest shunt voltage.
Bus voltage measurement can possibly be obtained
with a reduced sampling time relative to the shunt
voltage measurement. Using the same example, the
shunt voltage conversion time can be set to 4.156 ms,
the bus voltage conversion time set to 588 μs, and the
averaging mode set to 1. This configuration also
results in data updating approximately every 4.7 ms.
Current sensing with INA226-Q1 in HEV and EV BMS subsystems Guang Zhou
Copyright © 2018–2019, Texas Instruments Incorporated
1
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Comparative Study of Speed and Accuracy for the
INA226-Q1
Increasing the conversion time or averaging effectively
filters the input noise. The averaging feature can
significantly improve the measurement accuracy. This
approach allows the device to reduce noise coupled to
the signal during measurements. A greater number of
averages enables the device to be more effective in
reducing the noise component of the measurement.
The internal ΔΣ ADC of the INA226-Q1 provides
inherently good noise rejection. The conversion times
selected can have an impact on the measurement
accuracy. Longer conversion time allows averaging on
a larger data stream, which in turn is a more precise
representation of the analog input signal. In order to
achieve the highest accuracy measurement possible,
use a combination of the longest allowable conversion
times and highest number of averages based on the
timing requirements of the system.
Figure 3. Noise Output (V) at Three Conversion
Times – 140 µs, 1.1 ms, and 8.244 ms –
With VCM = 0 V
The following graphs provide a visual contrast of the
effects of different choices of conversion times and
different number of averaging.
Figure 3 shows the effects of varying conversion time.
In this test, both the shunt voltage and bus voltage
(VCM) are set to 0 V. Averaging is disabled; that is, the
number of averaging is set to 1. The conversion time
is set to 140 µs, 1.1 ms, and 8.244 ms, which
correspond to each of the first, middle, and last third of
the graph relative to the horizontal axis. As expected,
the noise floor drops visibly as conversion time
increases.
Figure 4 shows the effects of averaging when the
conversion time is fixed. In this test, the conversion
time is fixed at 140 µs. The number of averaging is set
to 1, 28, and 1024, which correspond to the first,
middle, and last third of the graph relative to the
horizontal axis. Similarly, the noise floor drops
dramatically as the number of averaging increases.
Figure 4. Noise Output (V) at Three Averaging
Levels – 1, 128, and 1024 – With VCM = 0 V
Figure 5 is obtained with the same condition as in
Figure 4, except that the common-mode voltage is set
to 25 V instead of 0 V. The change in common-mode
voltage has an effect on the INA226-Q1 output;
however, this effect is bound by the CMRR spec,
which is typically 140 dB.
Conclusion
Current sensing in HEV and EV BMS subsystems
faces the challenge of wide dynamic range. Digital
precision current-sense amplifiers with ultra-low drift
can greatly improve the overall performance while
reducing system complexity.
2
Figure 5. Noise Output (V) at Three Averaging
Levels – 1,128, and 1024 – With VCM = 25 V
Current sensing with INA226-Q1 in HEV and EV BMS subsystems Guang Zhou
SBAA325A – December 2018 – Revised January 2019
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