The Design Of CAN Bus Interface Circuit And The Study Of Its Conducted Immunity

CAN bus technology was primarily used in the field of automotive industry. With the development of science and technology, this technology has been widely used in the field of industrial control area. In practical application, due to the complex electromagnetic environment, CAN bus is vulnerable to interference from the conducted current induced by external electromagnetic field.Both the design of a CAN interface circuit and the method for inhibiting the conducted interference of RF field are studied in this paper. Firstly, a CAN interface circuit is made, having the ability to inhibit the conducted RF interference. Based on the results of the conducted RF immunity test data, the mechanism of the conducted RF interference and the effect of the common mode choke on suppressing such interference are analyzed. The possible failure reason of the common mode choke at high frequency is also analyzed. Secondly, the relationship between the RF field on the shield layer of the CAN bus and the induced current at the core line is studied. The minimum frequency of induced current which results in the failure of CAN bus communication is determined. The practical method, which can effectively suppress the conducted RF interference in the CAN interface circuit, is obtained finally.The main work of this project is as follows:1, Both a CAN interface circuit and a communication monitoring circuit are designed and realized. The corresponding test software is also programmed using C language.2, Under conditions of different EMI elements, different test levels, and different grounding methods of shield layer, conducted RF immunity tests of the CAN interface circuit are performed according to the national standard GB/T17626.6-2008.3, According to the theoretical and experimental analysis, the available frequency band., of the common mode choke for inhibition of conducted RF interference is obtained. An equivalent circuit model of common mode choke at high frequency is established. Then the relationship between its inductance value and frequency is obtained; The common mode choke model is simulated by CST software, and then the result of its magnetic field distribution is obtained; An equivalent circuit model of shielded twisted pair is also established. Thus, the relationship between RF field on the shield layer and the induced current at the core line is obtained. Based on the second equivalent model and measurement results, the minimum amplitude of the induced current, which can result in the failure of the CAN bus communication, is obtained.The above research work indicates that only with the comprehensive application of common mode choke, shielded twisted pair of which the shield layer is grounded at both end, and the optical isolator, the CAN interface circuit can pass the conducted RF immunity test of level3.