“In automotive and industrial systems interconnected with the Controller Area Network (CAN) communication bus, the number of controllers used is increasing. For designers, this means that they must consider the electrical noise environment in a wide frequency range-from high-frequency radiated electromagnetic interference (EMI) to common mode conducted interference, as well as the connection and disconnection of various loads such as motors and relays And the voltage spike caused by the start and stop of the alternator/generator. Although CAN buses are suitable for harsh electrical environments, they are prone to various failure modes without proper protection.
In automotive and industrial systems interconnected with the Controller Area Network (CAN) communication bus, the number of controllers used is increasing. For designers, this means that they must consider the electrical noise environment in a wide frequency range-from high-frequency radiated electromagnetic interference (EMI) to common mode conducted interference, as well as the connection and disconnection of various loads such as motors and relays And the voltage spike caused by the start and stop of the alternator/generator. Although CAN buses are suitable for harsh electrical environments, they are prone to various failure modes without proper protection.
This article explains the potential causes of CAN failures and introduces common isolation techniques. Then the article introduces solutions from vendors such as Texas Instruments, RECOM Power, NXP Semiconductors, and Analog Devices (designers can use these solutions to protect CAN devices), as well as guidance on how to effectively implement these solutions (including the use of evaluation boards) . The provided solutions include discrete implementations (that is, based on a single CAN transceiver) and integrated solutions based on single-chip and dual-chip isolated CAN bus designs.
The cause of the failure and the need for isolation
CAN bus failure can be caused by many reasons: the ground potential difference between the subsystems; common noise sources such as common mode energy and radiated energy; and high-voltage noise and spikes on the power distribution bus. To ensure the robust operation of CAN bus interconnect devices in automotive and industrial systems, two types of isolation are required:
Isolated from the power bus
Isolation of the communication bus connecting the various subsystems
Compared with integrated solutions, solutions with separate power and signal paths are usually cheaper and more efficient. These solutions also enable designers to independently optimize the isolation levels of the two paths. The designer is free to choose the type of isolation technology most suitable for the specific application. Options include magnetic isolation, optical isolation and capacitive isolation. A detailed discussion of the various isolation options is beyond the scope of this article, but for a better understanding, please refer to “How to Choose the Right Electrical Isolation Technology for IoT Sensors.”
There is also a difference between basic electrical insulation (using materials that do not allow current to flow) and reinforced insulation. The required insulation level depends on the voltage level involved and whether there is a connection from the accessible part to the ground. Basic insulation provides primary protection against electric shock. A system with a voltage greater than 60 V DC or 30 V AC is considered a dangerous system and requires at least one level of protection. The system does not necessarily have fail-safe measures, but any failure will be limited to the system. Reinforced or double insulation can provide secondary protection. This can ensure the safety of users in the event of a failure. If the system is connected to the mains voltage, it is required to have reinforced insulation.
Design trade-offs between isolation solutions
The isolation options in the CAN bus system include discrete solutions in which power and signals are separately isolated, and fully integrated power and signal isolation solutions. The integrated solution can also include related protection features, such as high electrostatic discharge (ESD) robustness and radio frequency (RF) immunity, so that it can be used in automotive and industrial applications without the need for additional protection devices, such as transients. State voltage suppressor diode.
There is a performance trade-off between size and efficiency between these different solution options. In terms of solution size, the single-chip solution is the smallest, with a typical package of about 330 square millimeters (mm2). Two-chip solutions are larger, usually about 875 mm2. Due to the size of the external DC-DC converter and required supporting components, the discrete solution is much larger, with a typical size of approximately 1,600 to 2,000 mm2.
In addition, there is a trade-off in efficiency. The larger the solution, the higher the efficiency. However, because the power levels involved are often quite low (3 V to 5 V at up to 15 milliamps (mA)), thermal shock may not be important in the design. The efficiency of single-chip and dual-chip solutions range from 50% to 60%, and the efficiency of discrete isolation solutions using external DC-DC converters can be as high as 75% to 80%.
Discrete isolation solution for CAN transceivers
Isolated CAN transceivers are relatively simple devices, such as the ISO1042DWR isolated CAN transceiver from Texas Instruments, which has 70 V bus failure protection and flexible data rates (Figure 1). ISO1042DWR devices provide basic or reinforced isolation options. The basic ISO1042 transceiver is designed for industrial applications.
ISO1042 supports a data rate of up to 5 Mbps in CAN flexible data rate (FD) mode, and the data transmission speed is much faster than traditional CAN. The device uses a silicon dioxide (SiO2) insulating sheet with a withstand voltage of 5000 Vrms. ISO1042 enables designers to select the best bus protection device according to the specific needs of individual applications. When used with an isolated power supply, the device can prevent noise currents in the data bus or other circuits from entering the local ground to avoid interference or damage to sensitive circuits.
These isolated CAN transceivers have a variety of safety-related certifications; these are important safety standards and certifications for any device that provides enhanced and/or basic isolation options):
・ 7071-VPK VIOTM and 1500-VPK VIORM (enhanced and basic options) conforming to DIN VDE V 0884-11:2017-01
・ 5000-VRMS isolation capability up to 1 minute according to UL 1577 standard
・ IEC 60950-1, IEC 60601-1 and EN 61010-1 certification
・ CQC, TUV and CSA certification
For designers considering ISO1042, there are two evaluation board options. The ISO1042DWEVM evaluation module provided by Texas Instruments enables engineers to evaluate the high-performance, reinforced isolation CAN ISO1042 in a 16-pin wide-body SOIC package (package code DW). EVM is a two-chip solution with enough test points and jumper options to evaluate the device with a minimum of external components.
RECOM Power provides R-REF03-CAN1 evaluation board for ISO1042. The R-REF03-CAN1 evaluation board demonstrates the ISO1042 isolated CAN transceiver powered by the R1SX-3.305/H isolated DC-DC converter. To power the reference board, only a 3.3 V external power supply is required. With this reference board, designers can quickly develop and analyze isolation systems.
Texas Instruments’ ISO1042 is optimized for industrial CAN applications, while NXP’s TJA1052i high-speed CAN transceiver is specifically for electric vehicles (EV) and hybrid electric vehicles (HEV), which require electrical isolation barriers between high and low voltage components (Figure 2 ).
The TJA1052i is designed for lithium-ion (Li-ion) battery management, regenerative braking, and 48 V to 12 V level shifting. The device can also be used to isolate high-voltage on-demand pumps and motors in belt elimination projects. The TJA1052i certified by AEC-Q100 implements the CAN physical layer (PHY) defined in ISO 11898-2:2016 and SAE J2284-1 to SAE J2284-5. Three isolation levels are currently available: 1 kV, 2.5 kV and 5 kV. Like ISO1042, TJA1052i also requires an external isolated power supply.
Integrated power and signal isolation solutions
Although discrete DC-DC converter implementations are generally more efficient than integrated solutions, integrated solutions have the following advantages:
Small circuit board area
Authentication is easier
Analog Devices’ ADM3055E/ADM3057E are 5 kV rms and 3 kV rms isolated CAN transceivers with integrated isolated DC-DC converters (Figure 3).
These devices are powered by a single 5 V power supply and provide a completely isolated solution for CAN and CAN FD. By continuously adjusting the switching frequency, the radiated emissions from the high-frequency switching of the DC-DC converter are limited to the EN 55022 Class B limit. Safety and regulatory certifications (pending approval at the time of writing) for 5 kV rms isolation voltage, 10 kV surge testing, and 8.3 mm creepage distance and clearance ensure that the ADM3055E meets the application’s reinforced isolation requirements. The ADM3057E is packaged in a 20-lead wide body SOIC package with an isolation voltage of 3 kV rms and a creepage distance of 7.8 mm.
In order to support the design and development work based on ADM3055E/ADM3057E, Analog Devices provides the EVAL-ADM3055EEBZ evaluation board. The ADM3055E and ADM3057E integrate the logic side on-off keying (OOK) signal isolation channel and the isoPower DC-DC converter of Analog Devices. When transmitting on a double-layer printed circuit (pc) board using surface mount ferrite beads, It can provide a regulated isolated power supply far below the EN55022 Class B limit.
Texas Instruments provides a different CAN communication power and signal isolation method based on a two-chip solution, which uses a dual-channel isolation data and power device ISOW7841, and the CAN transceiver TCAN1042H (Figure 4).
Integrating the transformer inside the ISOW7841 chip saves space not only in the x and y dimensions, but also in the z (height) dimension. To evaluate ISOW7841, the ISOW7841EVM evaluation module can be used. When using two chips, the ISOW7841 device can be placed on one side of the circuit board, and the CAN device can be placed on the other side, thereby reducing the overall space of the circuit board.
This two-chip solution eliminates the need for any additional components in the design to generate isolated power. Compared to a solution that uses a discrete transformer to generate the required isolated power, the size of the isolation solution is less than a quarter. A related reference design uses a single power supply input from 3 V to 5.5 V, and the digital signal refers to the input power level on the side of the circuit board. Then, ISOW7841 uses an integrated DC-DC converter to generate isolated power for powering the CAN transceiver on the other side of the circuit board. The signal on the power side of the circuit board is isolated and connected to the CAN transceiver, which converts the single-ended digital signal into a differential CAN format.
In order to protect the CAN bus from general noise sources such as ground potential difference between subsystems, common mode energy, and radiated energy, as well as potential faults caused by high-voltage noise and spikes on the power distribution bus, power and signal isolation is essential.
As mentioned above, the isolation options for CAN bus systems include discrete solutions in which power and signals are isolated separately, as well as fully integrated power and signal isolation solutions. The latter can also include related protection functions, so that it can be used in automotive and industrial applications It can be used without additional protection devices, such as suppressor diodes.