Disassemble Huawei switches and learn Basic EMC operations

1. Electrostatic immunity

Ideally , Our system is a hollow and airtight metal box , According to the theory of electromagnetic field , No external static power source can affect the circuit operation inside the box . However, such a system has no practical significance . An actual system must include external interfaces （ It is impossible to seal ）. A practical （ Metal shell ） The system should be able to （ Before accumulating electrostatic charges sufficient to affect the operation of the system ） Discharge the static electricity applied on it to the ground quickly enough . This brings the following requirements （ Only for metal clad equipment ）：

    1) Good grounding terminal

    2) Any bare 、 All metals that may be affected by static electricity should be well connected with the grounding terminal

    3) Any gap in the equipment shell should be small enough , So that static electricity cannot pass through the metal gap “ through ” To the inside of the equipment

2. Chassis overview

This case has an upper cover , The upper cover is larger than the chassis （ It's big ！） Area of metal contact , Unfortunately, I forgot to take pictures . In the impression, the upper cover covers the left side of the chassis as a whole 、 On 、 Right 、 The last four sides .

3. Power input and AC/DC

The ground wire of the power input is directly connected to the chassis shell , And didn't receive AC/DC On , That is to say , Take this large piece of metal as a perfect electrostatic discharge circuit （ It should also be good for tests such as surge ）.AC/DC The blue device above the power input socket should be a varistor ？

4. AC/DC Output

AC/DC The output is directly connected to the motherboard （PCB Power input and working place ）.

5. Connection between motherboard and chassis

You can see , There is a large area of exposed copper foil under the screw used to fix the motherboard . Disassemble the motherboard , The chassis shell below is like this ：

Each mounting hole is bulged with a large area . Corresponding , From the bottom of the chassis , The mounting hole is sunken .

Each fixed PCB The screws will PCB Somewhere on the （ Working place or connector place , See below ） Connect to the chassis shell .PCB The working ground and connector ground on the are grounded together through the chassis shell .

6. Motherboard connector and chassis 、 Connection between mainboards

There are two situations .

6.1 The connector welded on the motherboard is well overlapped with the chassis shell

In the figure, the chassis shell has a corner at the connector hole , It is well connected with the spring of the connector . At this time, connect the connector housing directly to PCB Where I work .

6.2 The contact between the connector shell and the chassis is not good enough

At this time, the connector shell is PCB Use a separate ground plane on （ Several connector housings that have poor contact with the chassis shell share the ground plane ）, Then on the one hand PCB Jumper capacitance between upper and working ground , On the other hand, it is connected to the chassis shell through the screws and large-area copper foil mentioned above .PCB The ground and working ground of the connector on the chassis are grounded together . stay PCB The two are not directly connected （ There is a capacitive connection between the two ）. such , When electrostatic discharge is carried out on such connectors , Static electricity will first be discharged through the chassis shell , Instead of running to PCB On the working floor .

7. Miscellaneous

7.1 Tap the switch

（ Touch switch operated by panel ） The grounding foot of the is connected to the connector ground , Therefore, without installing into the chassis , Touching the switch will not work .

7.2 Network interface protection

In addition to the commonly used local side TVS Outside , The ground on the transformer side passes through the varistor （ Unknown resistance value ） Connect to the connector ground （ Finally, it is connected to the workplace through the chassis ）.

The network indicator is placed in PCB On , Lead out through the long light guide column , Instead of using belts LED Of RJ45 seat , Avoid static electricity LED Signal line interferes with system operation .

From the blog Garden to bid farewell to the era 《EMC start ： Huawei switch dismantling 》