Texas Instruments: Reduce emissions and improve immunity of isolated CAN systems
“As more and more systems operate at different voltages, from elevators to electric vehicles, and even maritime systems, isolated CAN transceivers have become an indispensable part.
Recently, Texas Instruments Neel Seshan published a blog post titled “Reducing Emissions of Isolated CAN Systems and Improving Noise Immunity.” The following is the full text of the article:
As more and more systems operate at different voltages, from elevators to electric vehicles, and even maritime systems, isolated CAN transceivers have become an indispensable part.
These transceivers combine the priority and arbitration functions of the CAN (controller area network) standard into one, and provide the advantages of isolation (breaking ground loops, pressure drop resistance, common mode transient immunity, etc.), which helps To maintain reliable communication between the two voltage domains in the system.
As with non-isolated CAN systems, the main problem of using isolated CAN systems is the electromagnetic compatibility (EMC) performance of isolated CAN transceivers. EMC performance is measured by two parameters:
1. Emissions generated by the device
2. Immunity caused by interference in the system
Emission is the unnecessary release of electromagnetic energy. Under ideal circumstances, low emission ensures that the subsystem can operate reliably without affecting the performance of adjacent subsystems.
Depending on the market (industrial or automotive) and application, the system must meet different emission standards. Although launch testing is performed at the system level, designers usually select components that meet component-level requirements. This helps to ensure that a single device does not exceed its own limit. In addition, system design and circuit board layout also play an important role in the overall launch performance of the system. Among many emission tests, the Zwickau standard is a rigorous test for automotive applications, focusing on the emission performance of CAN transceivers.
The EMC test board has three isolated CAN transceivers connected to the same bus. The emission measurement is carried out at the test point, and one of the transceivers transmits a square wave signal with a 50% duty cycle and 250 kHz.
Placing a common mode choke (CMC) between the transceiver and the bus will filter out some of the emissions. In automotive and industrial applications, common mode chokes are commonly used.
The transmission data performed by the ISO1042 of the EMC test board at the traditional CAN data rate is shown in the figure below.
ISO1042 (transmission performed at 500 kbps)
Some certification bodies require data collection when using CMC, which also helps keep emissions low. Under the same conditions, ISO1042 is superior to competing equipment in terms of launch.
Immunity refers to the ability of a device to operate correctly in the presence of interference. In order to prove the noise immunity of isolated CAN devices, we performed a direct power injection (DPI) test on the same circuit shown in Figure 1, but with a different coupling network. Scan the frequency of the noise injected on the bus and check the difference between the transmit and receive modes through a mask test. The noise signal injected by the test includes continuous wave (CW) noise signal and amplitude modulation (AM) noise signal. The AM signal is 80% 1-kHz signal. Changes that exceed a certain voltage limit (±0.9 V) in the vertical direction or a time limit (±0.2μs) in the horizontal direction are regarded as failures.
We performed the test under two different conditions:
1. 36 dBm injected noise without common mode choke
2. 39 dBm noise signal with common mode choke
The following two figures show the ISO1042 figure of traditional CAN under two conditions. In both cases, the isolated CAN performance is higher than the limit line, indicating that it has passed the DPI test. Passing these immunity tests can ensure reliable communication and reduce system errors and failures.
ISO1042 DPI test without common mode choke
ISO1042 DPI test with common mode choke
It is expected that the integrated isolated CAN device can meet the same emission and immunity specifications as the non-isolated CAN device. Considering the low emission and high immunity of small packages, ISO1042 and ISO1042-Q1 meet the stringent requirements of industrial and automotive applications.
ISO1042 demo video
Please watch the ISO1042 demo video “Interoperability of isolated FD nodes.”