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Texas Instruments Simplify current and voltage monitoring with isolated SPI and I2C in your BMS Application notes
Simplify current and voltage monitoring with isolated SPI
and I2C in your battery management systems (BMS)
Neel Seshan, Product Marketing Engineer, Isolation, Interface Group
Introduction
With the growth of Hybrid Electric vehicles (HEV) and
Electric vehicles (EV), the conventional 12 V circuits
now need to communicate with higher voltage circuits.
For the hybrid vehicles, this would be 48 V batteries
whereas for the fully electric vehicles this could be 400
V or even more. These multiple voltage domains in the
car make it mandatory to include isolation devices to
protect the low voltage side components and circuits
from the high voltage battery side. This adds a new
challenge to the designers who were not used to
isolation in the conventional vehicles. This document
provides various options to isolate the ADC's in battery
monitoring circuits.
Battery monitoring and over-current detection
circuit
System Basis
Chip (SBC)
Reverse
Battery
Protection
CAN
Transceiver
VCC
Low-Voltage
Side
VCC1
SCLK
MCU
CS
Safety
Logic
Microcontroller
Battery Safety
Disconnect
Controller
Isolated
DC/DC
Isolator
High Voltage and
Isolation
Diagnosis
OUTA
INB
OUTB
Isolator
ISO7741-Q1 OUTC
OUTD
IND
GND1
GND2
VISO
SCLK_ADC
SDI_ADC
ADC
CS_ADC
SDO_ADC
ISO-GND
Figure 2. SPI Signals Isolated in Battery
Management Systems
To Cells
Passive
Electromechanical
Components
I2C Interface
When the ADC is communicating with only I2C
signals, then the isolator also needs to change to
support the I2C communication. The advantage of the
I2C communication is the reduced number for traces
on the board. SPI interface needs 3 or 4 wires while
I2C communication can be done over two lines (one
for data and one for the clock) as shown in Figure 3.
VCC
Low-Voltage
Side
High-Voltage
Side
VCC1
VCC2
Figure 1. Typical monitoring and Over-current
detection circuit in a BMS
SDA
Typical monitoring circuits consist of a shunt resistor in
series with the system load. The voltage drop across
this shunt resistor in indicative of the load current. The
signal from the shunt resistor gets amplified and
converted to digital signal before being fed to the
microcontroller (MCU). Since the MCU is on the low
voltage side while the measurement circuitry
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VCC2
System GND
Power
Switch
Activation
ADC
Monitoring and Overcurrent Detection
High-Voltage
Side
INA
INC
SDO
Wired Interface
Supply
Contractor
Control
Positive
CAN
Most commonly SPI interface is used for
communication between the ADC and the MCU. The
frequently used configuration for the isolation is 3
channels in one direction (SD, SCLK, CS) and one in
the opposite direction (SDO). This leads to a 3/1 digital
isolator as shown in Figure 2. The ISO7741-Q1 is a 4
channel (3/1) device that is available in the reinforced
rating (5 kVrms) in the wide SOIC-16 (DW) package or
in the basic rating (2.5 kVrms) in the QSOP-16 (DBQ)
package.
SDI
Negative
12-V
Supply
SPI Interface
To
Inverter
and Loads
High-Side and
Low-Side
Switches
DC/DC
Converter
and SBC
Input
Protection
comprising of the amplifier and the ADC is on the high
voltage side, an isolation devices sits between these
two circuits as shown in Figure 1. The type of isolator
to be used depends on the interface used to
communicate between the ADC and the MCU.
MCU
SCL
SDA1 Isolator SDA2
SCL1 ISO1541-Q1 SCL2
GND1
System GND
VISO
SDA_ADC
ADC
SCL_ADC
GND2
ISO-GND
Figure 3. I2C Signals Isolated in Battery
Management Systems
Simplify current and voltage monitoring with isolated SPI and I2C in your
battery management systems (BMS) Neel Seshan, Product Marketing Engineer, Isolation, Interface
Copyright © 2018, Texas Instruments Incorporated
Group
1
www.ti.com
The data is bi-directional but the clock can be either
uni-direction in the master-slave configuration or could
be bi-directional as in the multi-master configuration or
in systems that utilize clock stretching. The ISO1540Q1 has two isolated bidirectional channels for clock
and data lines while the ISO1541-Q1 has a
bidirectional data and a unidirectional clock channel.
Both these devices support 2.5 kVrms basic isolation
and are available in the SOIC-8 (D) package.
There are other ways to implement the I2C
functionality using a standard isolator. More details on
designing a reinforced isolated I2C bus interface by
using digital isolators can be found here.
Discrete Isolated Power
In addition to isolating the signals, the power supply
between the ADC and the MCU also needs to be
isolated. In some systems there are step down
regulators to provide the power supply needed by
each side and in those cases there is no need to
generate the isolated power supply separately. But in
case there is a need to provide the isolated power
supply from the MCU side to power up the ADC and
the amplifier, then SN6501-Q1 provides a push pull
driver to drive the primary of the isolation transformer
that provides the required load current and voltage on
the secondary side. Simple rectifying passives and an
LDO (optional) complete the isolated power circuitry as
depicted in Figure 4 and Figure 5.
D2
4
8
3
2
7
6
1
OUT
IN
LDO
TPS76350-Q1
3
EN
GND
VCC
EN
Transformer
Driver VCC
SN6501-Q1
CLK
VCC
2
1
D1
MCU
VCC2
INA
SDI
OUTA
Isolator
OUTB
ISO7741-Q1
OUTC
INB
CS
INC
SDO
NC
4
5
VCC1
SCLK
GND
5
OUTD
IND
GND1
GND2
System GND
SCLK_ADC
SDI_ADC
CS_ADC
ADC
SDO_ADC
Discrete isolated power solution consumes larger
board space due to the external transformer size, not
just in the X and Y dimension but also in the height (Z)
dimension. Figure 6 shows an alternate solution that
uses a digital isolator with integrated power. In this
solution, the isolated power circuitry is included inside
the isolator device thereby making this solution
compact and system certifications easier with only a
single device to certify. With discrete isolated power
solution, the external transformer can be larger so the
switching frequency is lower and so are the emissions.
With integrated power solution, since the transformers
are smaller in size, the switching frequencies are
higher leading to higher emissions as compared to the
discrete solution. But these emissions can be reduced
by using stitching capacitors as demonstrated in this
application note.
VCC
SCLK
SDI
MCU
CS
D2
Transformer
Driver VCC
SN6501-Q1
CLK
VCC
4
8
3
2
7
6
1
OUT
LDO
TPS76350-Q1
3
EN
2
1
D1
VCC1
SDA
MCU
SCL
OUTA
INB
Isolated Data OUTB
and Power
ISOW7841-Q1OUTC
OUTD
IND
GND1
GND2
System GND
SCLK_ADC
SDI_ADC
CS_ADC
ADC
SDO_ADC
ISO-GND
Figure 6. Integrated Isolated Power Supply for
Isolated SPI
Conclusions
Digital isolators or isolated I2C devices can be used
for protection of the low voltage side from the high
voltage side in battery monitoring sub-circuit of a BMS.
The selection of the appropriate isolation depends on
the interface (SPI or I2C) used. Discrete or integrated
isolated power supplies can be selected depending on
the trade-offs of emissions, board space and design
simplicity.
ISO-GND
GND
EN
VISO
INA
INC
SDO
Table 1. Device Features
Figure 4. Discrete isolated Power Supply for
Isolated SPI
VCC
VCC1
GND
NC
5
Description
Performance
parameters
Package
ISO1540-Q1
Low power Isolated Bidirectional Clock,
Bidirectional I2C Isolators
2.5kVrms, 1MHz
SOIC-8
ISO1541-Q1
Low Power Isolated Unidirectional
Clock, Bidirectional I2C Isolators
2.5kVrms, 1MHz
SOIC-8
ISO7741-Q1
High-Speed, Low-Power, Robust EMC
Quad-Channel (3/1) Digital Isolator
5kVrms, 100Mbps
SSOP16,
SOIC-16
ISO7742-Q1
High-Speed, Low-Power, Robust EMC
Quad-Channel (2/2) Digital Isolator
5kVrms, 100Mbps
SSOP16,
SOIC-16
SN6501-Q1
Low-Noise 350 mA, 410 kHz
Transformer Driver
High Primary-side
Current Drive: 350
mA (5V) / 150 mA
(3.3V)
SOT23-5
ISOW7841
High-Efficiency, Low-Emissions,
Reinforced Digital Isolator With
Integrated Power
0.65W Output
Power, 5kVrms,
100Mbps
SOIC-16
4
5
VCC2
SDA1
SDA2
Isolator
SCL1 ISO1541-Q1 SCL2
GND1
IN
Device
SDA_ADC
SCL_ADC
ADC
GND2
System GND
ISO-GND
Figure 5. Discrete Isolated Power Supply for
Isolated I2C
Integrated Isolated Power
2
Simplify current and voltage monitoring with isolated SPI and I2C in your
battery management systems (BMS) Neel Seshan, Product Marketing Engineer, Isolation, Interface
Group
Copyright © 2018, Texas Instruments Incorporated
SLLA411 – July 2018
Submit Documentation Feedback
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