A brief discussion on how data line chokes can improve electromagnetic compatibility and make cars safer

In recent years, the proportion of electronic devices in cars has grown significantly, resulting in more functions to increase the safety, reliability and convenience of the car. Corresponding to this are the increasing requirements for bus systems: to ensure reliable communication between the most diverse control units, especially for safety-critical applications controlled via bus systems such as Controller Area Network (CAN) or FlexRay systems, which must meet the highest electromagnetic compatibility specifications. Common mode chokes ( CMC ) in the data lines can enhance the protection against faults caused by electromagnetic compatibility problems.

The complexity of control functions in modern motor vehicles lies in three aspects: faster data rates, deterministic latency performance and fault tolerance. Cost-oriented functions or multimedia functions are connected by LIN (Local Interconnect Network) or MOST (Media Oriented Systems Transport) buses, while CAN or FlexRay bus systems are used for safety-critical applications such as engine control, ABS systems and airbags. CAN and FlexRay use twisted pairs, have deterministic latency and can achieve fast data rates (CAN bus: 1Mbit/s; FlexRay bus: 10Mbit/s). The physical layer and data transmission protocols of both bus systems have been optimized to ensure high reliability. However, given the increasing complexity of modern vehicles, these measures alone cannot completely prevent malfunctions caused by electromagnetic compatibility problems. Controller Area Network ( CAN

) bus chokes produced by EPCOS prevent electromagnetic compatibility ( EMC ) problems in automotive networks and thus increase safety. CAN bus electromagnetic compatibility and electromagnetic interference Automotive bus systems must meet high-level electromagnetic compatibility requirements: immunity to transients, electrostatic discharge (ESD) and electromagnetic interference (EMI ) , while also not interfering with other electronic components, that is, interference radiation must be minimized. However, with the increasing proportion of electronic devices in vehicles, it is not possible to test electromagnetic compatibility in advance under all conditions, which means that there is a risk of malfunctions or even damage to the control device. To solve these problems, it is usually necessary to distinguish between differential and common mode interference. Differential mode interference coincides with the data signal, while common mode interference is to ground and is caused by imbalance and parasitic effects. To minimize common mode interference, special attention must be paid to the routing of the bus signal lines, terminal filters, connectors and the circuit board itself. Parasitic capacitance and inductance of feedthroughs or connectors, as well as the layout of bus signal leads on the circuit board, can cause asymmetries and generate common-mode interference. A common method for determining RF-affected electromagnetic compatibility is the Te direct RF power injection (DPI) method: a signal from a signal generator (up to 36 dBm) is coupled into the bus leads and the signal output is observed. If a fault occurs, the signal level of the access signal is recorded. This process is repeated step by step for each corresponding frequency in the relevant range. The interference radiation of a specific bus structure is determined by a test receiver, which is used to measure the common-mode voltage at the bus and all inputs and outputs. The test structure shown in Figure 1 is used for electromagnetic interference measurement and determination of the effect of bus chokes.

The DPI method was used to measure RF immunity and interference emission on a test board with and without EPCOS chokes. The corresponding results for the CAN bus can be seen in Figures 2 and 3. Obviously, the use of the B82789 common-mode choke greatly improves RF immunity. At the same time, the use of chokes in the data line also significantly reduces interference emission.

Data line chokes can significantly increase the reliability of CAN bus systems. Although higher inductance chokes result in better electromagnetic compatibility due to higher attenuation, factors such as stray inductance, signal integrity, ground drift and lead robustness must be considered when selecting the most suitable choke.

EPCOS now offers various types with different package sizes and versions to meet specific automotive requirements. The B82789 series chokes are designed for powerful automotive bus systems such as CAN or FlexRay. When applied to the data lines, they can suppress coupled interference and prevent interference from the data bus. The operating voltage of this series is 42VAC or 80VDC, the rated inductance of the chokes is between 11 and 100μH, and the rated current is between 150 and 300 mA.

FlexRay electromagnetic compatibility and electromagnetic interference

FlexRay is a serial, deterministic delay and fault-tolerant bus system. FlexRay, defined by the FlexRay Consortium, is designed to meet the higher requirements of future automotive networks, especially fast data rates, real-time responsiveness and fault tolerance. The current focus is on faster data rates. The

physical layer application guide clearly states the general requirements for chokes in the data lines of FlexRay networks, such as lead resistance (less than 1Ω), inductance (greater than 50μH) and stray inductance (less than 1μH). The determination of electromagnetic radiation and electromagnetic interference is the same as for the CAN bus. However, due to the higher transmission rate of 10Mbit/s, the signal integrity needs to be checked more carefully. The corresponding evaluation results of DPI determination for test boards with chokes of various inductances and without chokes are shown in Figure 4. Figure 5 shows the results of electromagnetic interference measurements using another test board, also for chokes with different inductances. It is obvious that the use of data line chokes improves electromagnetic compatibility and reduces interference radiation. The effect is greater when the inductance is higher. However, higher inductance values ​​have a certain negative impact on signal integrity.

The eye diagram is suitable for studying the signal integrity of FlexRay. If the curve of the data signal is outside the shaded range, data transmission is guaranteed. These requirements can be achieved by using the B82789C0104 choke with an inductance of 100 μH (see Figure 6). In view of the results of these EMC measurements and the fact that the influence on the data signal is negligible (i.e. the signal integrity is guaranteed), the EPCOS B82789C0104 N002 (bifilar winding) choke was selected as the reference model for the FlexRay physical layer qualification test.

Model library facilitates product development

EPCOS now offers appropriate simulation models to improve the efficiency of designs with new chokes. To this end, the electrical characteristics of the choke over the entire relevant frequency range have been defined and included in the corresponding models. Since the terminal capacitance and inductance as well as the resistance of the connection depend on the user design, they are not included in the model. Customers can evaluate the function of the choke without having to carry out time-consuming overall system design. EPCOS now offers simple models for initial design evaluation, which enable fast and accurate simulations. Advanced models provide more accurate simulation results, but require longer simulation times.