EMC starts: Huawei Switch disassembly

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1. Static reactance Disturbance

Ideally, our system is a hollow and airtight metal box, according to the theory of electromagnetic field, the outside of any static power supply can not affect the inside of the box circuit operation. However, such a system is of little practical significance. An actual system must include an external interface (i.e., it is impossible to seal). An actual (metal-cased) system should be able to put the electrostatic discharge applied to the ground quickly enough (before it accumulates enough electrostatic charge to affect the system's work). This brings the following requirements (only for metal enclosure devices):

1) Good grounding terminal

2) Any bare metal that may be affected by static electricity should have a good connection with the grounding terminal

3) Any voids on the device housing should be small enough to allow static electricity to "traverse" the inside of the device through the metal void

2. Chassis Panorama

This chassis is a top cover with a large upper cover and chassis (very large!). ) Area of metal contact, unfortunately forgot to take pictures. The impression of the upper cover covers the left, top, right, and rear four faces of the chassis as a whole.

3. Power input and AC/DC

The ground line of the power input is directly attached to the chassis shell and not to AC/DC, that is to say, the chassis of this large metal as a relatively perfect electrostatic discharge circuit (for the surge and other tests should also be beneficial). The blue device above the AC/DC power input socket should be a varistor?

4. AC/DC output

The AC/DC output is directly attached to the motherboard (PCB power input and work).

5. Motherboard-to-chassis connection

As you can see, the screws used to secure the motherboard are large areas of exposed copper foil. To disassemble the motherboard, the lower chassis housing is:

Each mounting hole is raised to a larger area. Correspondingly, from the bottom of the chassis, the mounting hole is sunken.

Each fixed PCB screw is connected to the chassis shell on a PCB (workplace or plug-in, see below). The PCB works on the ground and connector through the chassis shell.

6. Connection between motherboard connector and chassis, motherboard

In two different cases.

6.1 Welding the connector on the motherboard is well lapped with the chassis housing

The chassis housing in the figure has a corner at the connector hole and is well connected with the plug reed. The plug-in enclosure is then connected directly to the PCB's working place.

6.2 Connector housing is not in good contact with the chassis

At this time the connector housing on the PCB using a separate ground plane (a plurality of contact with the chassis shell of the connector shell Common Ground plane), and on the one hand on the PCB and the working ground between the capacitor, on the other hand through the above-mentioned screw and large area copper foil connected to the chassis shell. The connector on the PCB is shared with the working ground through the chassis. The two are not directly connected on the PCB (there is a capacitive connection between the two). In this way, when the electrostatic discharge of such plug-in, static electricity will first be released through the chassis shell, and will not run to the PCB work on the floor.

7. Other Miscellaneous

7.1 Light Touch switch

The grounding pin (tactile switch via panel operation) is connected to the connector, so the tactile switch does not work in the case of an uneasy fit into the chassis.

7.2 Network Interface Protection

In addition to the usual local side TVs, the transformer side is connected to the plug-in area via a varistor (unknown value) (eventually connected to the working place via the chassis).

The network LED is placed on the PCB, through the long guide beam, without using the RJ45 with LED, to avoid static electricity through the LED signal line interference to the system operation.

EMC starts: Huawei Switch disassembly