Example of noise suppression using common mode choke coils for CAN/Ethernet in the automotive market

1. Background

In recent years, the electrification of the automobile market has become increasingly obvious. In the future, with the popularization of new energy vehicles, additional functions such as ADAS will become more and more abundant, the number of electronic devices installed in each car will also increase accordingly, and the types of electronic devices will become more and more diversified.

In the process of automobile electrification, in order to realize the communication between electronic devices in the car, the installation of in-vehicle LAN has become indispensable. Since the in-vehicle LAN can transmit a large amount of information, it is indispensable for high-speed communication, and due to the demand for high-reliability communication quality, a large number of unique in-vehicle interfaces are used. Among the in-vehicle LANs, CAN (Controller Area Network) is particularly widely used. The transmission speed of CAN can reach up to 1Mbps, and it is connected to the bus in a differential manner. Multiple nodes can be connected to the CAN bus, realizing a one-to-many communication network structure. Due to the high-reliability control method of CAN, it can be widely used in a series of applications from car navigation systems to engine control systems.

In recent years, with the popularization of in-vehicle camera applications, more information such as video signals needs to be transmitted at high speed, and in-vehicle Ethernet has also come into being. In-vehicle Ethernet is based on traditional industrial Ethernet technology and also uses the 100Base-TX physical layer protocol. It is an interface that can perform differential transmission at a rate of 100Mbps. One of the in-vehicle Ethernet technologies is BroadR-Reach promoted by the standards organization "OPEN Alliance SIG". BroadR-Reach uses a pair of UTP (unshielded twisted pair), which can achieve bidirectional transmission of 100Mbps. It is expected to be applied to in-vehicle cameras and popularized around this.

2. Noise issues in vehicle-mounted equipment

If a communication failure occurs in the in-vehicle LAN, it will directly affect the equipment and the safety of human life. Therefore, there are almost strict requirements for the reliability of communication quality, especially for factors that affect the communication quality, that is, the so-called electromagnetic noise. The electromagnetic noise problem must not only prevent noise from affecting the wireless communication used in the car, but also ensure a certain degree of anti-interference ability for the noise generated by the surroundings to ensure the quality of communication. The communication method of CAN and in-vehicle Ethernet uses differential transmission, so the problem of electromagnetic noise can be reduced to a certain extent. Differential transmission is to transmit signals by applying current in opposite directions on a pair of transmission paths. The magnetic flux generated on the transmission line is offset because of the opposite directions. Therefore, differential transmission is difficult to generate electromagnetic noise. This is its characteristic.

However, regarding the noise mixed in from the surrounding environment, if the direction of this noise is in the same direction on the transmission line, it is difficult for differential signal transmission to interfere, thus causing communication failure. Therefore, even the differential transmission method cannot completely solve the noise problem, because noise will still be generated for some reasons, which can be roughly listed as the following situations, ① output signal imbalance, ② line imbalance, ③ and noise from some other circuits in the device, etc. If the signal generated by the communication IC in ① does not become an ideal differential signal, it will generate signal components transmitted in the same direction, and these components will be released as noise; if in ② due to the imbalance of line length and impedance, the same direction noise will also be generated in the reverse signal. In addition, if the noise generated by other circuits inside the device in ③ is transmitted to the inside of the substrate, the radiation in the device will also mix with noise.

In view of the above factors, even if the in-vehicle LAN uses differential transmission, noise will be generated for various reasons, so corresponding noise countermeasure products must be used. The quality of noise countermeasure products used in in-vehicle LAN needs to reach automotive grade and have the ability to withstand more demanding environments.

3. Noise Countermeasure Products

Common mode chokes are commonly used as noise countermeasure products for differential interfaces such as CAN and in-vehicle Ethernet. The basic structure of a common mode choke is shown in Figure 1: There are two windings in a core. When the currents in opposite directions flow through the two windings, the magnetic flux generated in the core will cancel each other out, while when the currents in the same direction flow through, the magnetic fluxes will be generated in the same direction, so the impedance is higher. In this way, the common mode choke can pass the differential signals in opposite directions but suppress the common mode noise transmitted in the same direction.

As for the noise countermeasure products in CAN, Murata has commercialized the corresponding common mode choke DLW43SH series. The appearance and main features of this series of products are shown in Figure 2. The DLW43SH series is a common mode choke used for noise countermeasures in vehicle-mounted LANs such as CAN and FlexRay. It is a surface mount product with a size of 4.5×3.2×2.6mm. The characteristic value is a common mode inductor with a maximum value of 100μH. In addition, the cutoff frequency of DLW43SH is about 1000MHz, which can correspond to the high frequency requirements of high-speed differential signals. The operating temperature range is -40~+125℃, so it is suitable for a wide range of vehicle-mounted applications.

As a miniaturized product of the DLW43SH series, the DLW32SH series is currently under development. The appearance and main features of this series are shown in Figure 3. The DLW32SH series has a small size of 3.2×2.5×2.3mm and can achieve the same performance as the DLW43SH series. The characteristic of its structure is that a metal bracket is used in the terminal part of the product to alleviate the influence of thermal expansion and contraction caused by temperature changes, and the applicable temperature range reaches -55~+150℃.

In addition, Murata is currently developing the DLW43MH series as a common mode choke for automotive Ethernet. The appearance and main features of this product are shown in Figure 4. The size of the DLW43MH series is the same as the DLW43SH series, 4.5×3.2×2.6mm, and the common mode inductance value has been increased to 200μH, so it can play an effective noise countermeasure effect in a wider frequency band. Since a lot of effort has been put into the winding and the common mode to differential mode noise has been improved, the noise improvement effect will be even better.

4. Confirmation of the effectiveness of noise countermeasures

In order to confirm the noise suppression effect of the DLW43SH series and DLW32SH series in CAN, we used the CAN evaluation board to measure the noise. The noise measurement was carried out in Murata's anechoic chamber, and the peak detection measurement was carried out in the test environment of CISPR25 specified in the automotive electronic equipment standard. The evaluation board inputs a 250kHz pulse signal to the CAN using the TXD terminal of the transceiver, and at the same time, the output CAN signal is transmitted to the cable to detect the noise. The common mode chokes used are DLW43SH510XK2 and DLW32SH510VK2. The measurement results are shown in Figure 5. As shown in Figure 5, high-frequency noise with a signal frequency of 250kHz is generated, but because a common mode choke is used, the noise is suppressed to a maximum of 20dB.

Next, we will talk about the results of the noise countermeasure evaluation for in-vehicle Ethernet. The so-called measurement is to use a communication evaluation board corresponding to in-vehicle Ethernet to measure the noise radiated during communication between the boards. The test environment is the same CISPR25 standard test environment as CAN as mentioned above. The common mode choke coil used is DLW43MH201XK2. The test results are shown in Figure 6. As can be seen from Figure 6, if the common mode choke coil is not used, noise will be generated at 66MHz, and high-frequency noise will also be generated. If the DLW43MH201XK2 is used, the noise peak part is relatively reduced, indicating that the noise is controlled in a wider frequency band.

5 Conclusion

Murata's noise countermeasure products for the automotive market introduced the common mode choke coil DLW43SH series, DLW32SH series, and DLW43MH series products using CAN and automotive Ethernet as examples, and demonstrated noise countermeasure examples. Murata will continue to enrich its product lineup for the automotive market in the future, contributing to the progress of electrification and solving noise problems in the automotive market.