EFT Problems of the Industrial Grade Switch


1.Problem Description for the Product EMC

A certain product is an industrial switch product with 24V DC input. The external communication interface has a 100-mega optical fiber interface and a 100-mega Ethernet interface, which uses a dual 24V DC input and only has one connection in the experiment. The product requires 4KV, 5K/100KHz interference frequency in EFT test. The criterion is A. During the test process, data need to be transmitted, and no packet loss can occur in the middle. The phenomenon happening is the loss of the prototype in the test process.

Problem Analysis

The EFT test level of the power supply port of industrial grade switch is relatively high. At present, the power supply interface is three-stage common mode filter, and then it enters the single board through DC power module. Although it is a three-stage filter circuit, it may not be enough for such a harsh level. The structure of the prototype is very compact. In the internal structure, the power board is placed above the CPU board. It is likely to affect the CPU board and switch board through radiation interference. The whole prototype is made up of six metal plates. The overlap between metal panels is not good enough, which results in the loss of packets caused by the unobstructed release of interference on the digital ground of the motherboard.

Rectification measures

1. External filter is installed at the power input port.
Result: The number of lost packets has been greatly reduced and the overall situation has been greatly improved. In the latter optimization, the external filter can be removed and only the Y capacitor can be added on the power board.
2. The prototype has two grounding wires connected to the earth, one is the 5PIN input ground, while the other is the grounding stud of the prototype connected to the earth. And one of the grounding wires is not connected in the experiment.
Results: The number of lost packages increased significantly. It shows that the ground loop of the prototype has a great influence on the test results.
3. When the prototype is dismantled, it is found that the PGND on the power board passes through the stud in the upper right corner to the housing.
In rectification of the site, the screw hole is insulated from the stud on the basis of the external filter.
Result: no package lost.

The final rectification measures are to add the Y capacitance on the power supply and to the use power supply board insulate to be close to the screw of the CPU board.
1. For the power port EFT interference processing, it is mainly in the power filter processing. On the basis of the current three-stage filter, only the Y capacitor can be added.
2. For the crust of PCB board, we must make a good plan to avoid the disturbing circuit path affecting the digital signal communication.

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