In accordance with the requirements of the railway industry EN50155, the electromagnetic compatibility of the power supply products used in the high-speed rail gating system shall meet the EN 50121-3-2 certification requirements, and the surge must pass through the differential mode (line-line) 1KV / - ground) 2KV. A high-iron gating system, to enhance the equipment anti-interference ability to reduce the risk of failure, the use of isolated power supply. However, the surge test was carried out at the time of the certification test. It was found that the differential surge of the system could meet the requirements. However, when the common mode surge test was carried out, the whole device was seriously curved and caused many IC damage. To this end, the following design of the equipment circuit analysis and rectification.

　Cause Analysis:
　First of all, in this case the use of isolated power supply, in general, common-mode surge test should not appear arc problem. So we need to analyze the circuit structure of the customer's entire equipment. Figure 1 for the power part of the block diagram, the front use of the Jinyang sun power supply filter FC-CX3D for EMC protection, the main control panel power supply using URF1D05QB-50W power supply, while the power supply in parallel with URB1D15YMD-6W for the motor drive Control board power supply, IB0505S-1W is mainly used for communication isolation chip power supply, the general structure is as follows.

Figure 1 high-iron gating equipment main circuit structure

      It can be seen from the figure, SP485 communication module of the EMC protection circuit PE through the two power supply isolation and power supply input EMC filter filter PE together. This connection is a normal application, but it is the problem for the surge.

Figure 2 Surge failure analysis

      According to the above connection, it can be seen from Fig. 2 that CY is the Y capacitor of the original secondary side of the isolated power supply, and Cj is the distributed capacitance equivalent to the original secondary side of the isolated power supply. Since the SP485 EMC protection circuit PE and the filter PE exists in the connection relationship, when the common mode surge L-PE or N-PE test, which is equivalent to the two isolated power supply of the original Y capacitor in series partial pressure. If the L-PE 2KV common mode surge test, if the two capacitors are equal, then the respective partial pressure 1KV, that is, Figure 2, - - between the two points there is a pressure difference of 1KV. At this time in the system wiring, if the spacing between the two points can not meet the 1KV isolation distance, the common-mode surge test, there will be isolated input and isolation output between the arc problem.

　Experimental Verification:
　After analyzing the circuit structure and determining how the signal output is connected to the PE, how to determine the distance between the isolation input and the isolation output does not meet the withstand voltage requirements Here are two ways:
　First, through the PCB layout to find Figure 2, two isolated power supply isolation input and isolation between the alignment of the line spacing (generally hit the arc), find the minimum distance and disconnect here. If it is found after the break does not exist the phenomenon of the arc, you can basically determine the isolation input and isolation between the alignment of the line can not meet the requirements caused by pressure.
　Second, the input source of the isolated power supply can be connected with the output ground to become non-isolated, because they are equipotential, that is, there will not be the phenomenon of firing arc.
　Through the above two methods, can determine whether it is due to isolation between the power input and output alignment problem between the lead to lead the arc.

　Rectification process:
　Through the analysis to determine the isolation between the power input and output alignment between the lack of common mode surge caused by high pressure differential at both ends. This will be the ignition of the line to disconnect, there will no longer appear ignition. At the same time, if there is the same problem elsewhere, after the front of the fire is disconnected, the common mode path is not enough to move to the next spacing, so these isolation spaces need to be disconnected and meet the common mode voltage spacing requirements.

　Sum up:
　1. Analysis method: If the above-mentioned system structure is caused by the common mode surge, the isolated input and isolation output isolation distance is not enough to lead to the phenomenon of ignition, you can take two ways to verify: First, the fire Of the spacing off or growth; Second, the isolation will become non-isolated, the formation of equipotential.
　2. Power Distribution: If the system is designed, if the isolated power supply or isolation IC is used and the output of the isolated power supply or the output of the isolation IC is connected directly to the system chassis by direct or indirect connection, if the system has a common mode surge In this case, it is necessary to pay attention to the tolerance distance of the common-side common-mode voltage in the layout isolation input and isolation output. Do as follows: 1KV - 2mm; 2KV - 3mm; 4KV - 4mm and above.
　3. Solution: First, increase the isolation input and isolation output spacing to meet the common mode voltage requirements; Second, the isolated power supply or isolation IC output and the housing of the PE connection disconnect and ensure that other signals and Chassis spacing; Third, in the power input line and PE between the increase in varistor common mode voltage suppression, but this time should pay attention to the input and PE frequency withstand voltage requirements.