Electronic Enthusiast Network Report (Text/Li Ningyuan) In the automotive communication network, CAN BUS refers to the vehicle multi-channel communication system, and its purpose is to simplify the vehicle wiring harness and speed up the communication speed. CAN BUS will allow multiple devices (ECUs) to be connected to one communication line and exchange data with each other, and can be divided into CAN and CAN FD systems according to different protocols and communication speeds. The communication speed of CAN is 500Kbps, and CAN FD can reach up to 8Mbps.

Both systems communicate using shielded twisted-pair wires, all with differential voltages. In terms of data length, CAN can only vary from 1 to 8Byte, while the data length of CAN FD can vary from 1 to 64Byte. As mentioned above, CAN BUS will allow multiple devices to be connected to one communication line and exchange data with each other. Generally, there are three topologies: P2P connection, bus connection and star connection (Ethernet and LVDS only support P2P connection). In an actual vehicle, usually the above three connection methods are mixed to form an in-vehicle network.

What are the troubles caused by the upgrade of vehicle CAN to CAN FD

With the upgrade of CAN to CAN FD protocol, some problems began to emerge. The first problem is that high speed will cause resonance phenomenon, leading to an increase in the possibility of misjudgment; the second problem is that the electromagnetic radiation frequency band shifts to high frequency, making it more difficult to deal with EMC.


(Influence of resonance phenomenon, TDK)

It can be seen that under 8Mbps CAN FD, the resonance phenomenon is the most serious, which will affect the judgment of dominant and recessive areas. The rate of 500Kbps can ensure sufficient response time after the resonance phenomenon ends, but if the time is shortened by 1 bit, the ECU error rate will increase significantly. There are two main reasons for the resonance phenomenon, one is the reflection caused by the branch point of the CAN harness (topology design), and the other is the leakage inductance and parasitic capacitance from the ECU to the harness.

To solve the resonance phenomenon, one is to consider the design, and the other is to reduce the leakage inductance and parasitic capacitance as much as possible from the common mode filter.

How Common Mode Filters Solve Resonance Problems

The mode conversion characteristic of the common mode filter applied by CAN FD, that is, the corresponding Ssd21 parameter is extremely important. The industry’s highest level of mode conversion characteristics is achieved through the unique structural design including the winding process, which minimizes leakage inductance, parasitic capacitance, and mode conversion characteristics, and is optimized for CAN-FD applications that are more susceptible to resonance than before.

In order to achieve a good Stray C corresponding to the requirements of CAN FD, the common mode filter should preferably cover a wide temperature range (-40°C to 150°C). The reliability of the common-mode filter is also a prerequisite to ensure good resonance. Generally, metal terminals and laser welding are used to improve the reliability of the device. Compared with the more traditional riveting methods on the market, the reliability level has been improved by several levels.


(Comparison of radiated EMI data with or without adding a common mode filter, TDK)

In the case of not adding a common mode filter, the suppression effect on data spikes is obviously much worse than that after adding it. In the case of such a good radiation suppression, will the differential signal be affected, suppressing data transmission? The answer is no. After adding the common mode filter, the differential signal waveform will not deteriorate compared with before, so there is no need to worry that the suppression effect will spread to the signal transmission.

In addition, if the transmission rate is fast, in order to avoid data loss or distortion, the common-mode filter itself will meet the corresponding insertion loss/return loss requirements. More detailed solutions depend on the design of the PCB.

How to deal with CAN BUS ESD

In the previous article discussing ESD, we said that ESD suppressors and TVS diodes are effective means to solve ESD protection and surge protection. In vehicle communication networks, varistors and ceramic chip capacitors are protective devices with excellent protection functions.

Varistors include silicon carbide varistors and zinc oxide varistors. Zinc oxide is more common, and many manufacturers use zinc oxide as the basic material. High ESD tolerance is one of the most important characteristics for protecting devices. After multiple voltage tests, different varistors will have different voltage changes, and excellent high ESD tolerance varistors will have little change in the test.


(Varistor ESD Resistance Test, TDK)

How to choose a suitable varistor is not complicated. First, ensure that the rated voltage is slightly lower than the breakdown voltage of the varistor, and then the electrostatic capacity. Generally speaking, when the varistor voltage is the same, the larger the electrostatic capacity, the better the electrostatic absorption capacity and durability. But what cannot be ignored is that the corresponding communication speed needs to be taken into account when selecting, and the large electrostatic capacity cannot be blindly selected so as to affect the communication speed.

Another question that attracts more attention is whether the varistor will respond slower than the TVS diode in the CAN BUS. In fact, there is no need to worry about this problem. The performance of the two types of devices on the ESD clamping voltage is similar, and the suppression effect of the two different protection devices will appear before 1ns. The difference from varistors is that TVS in one direction will absorb the differential voltage together because of its low breakdown voltage, resulting in communication errors. Varistors do not affect the differential voltage.

summary

In the electronic equipment of various control systems of automobiles, in order to realize high-speed communication with a larger amount of data, the communication speed of the CAN communication system has begun to further increase. In the Internet era where communication speeds are accelerated, high-performance electronic components give the system sufficient reliability at high communication speeds.

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