What are the applications of panel shielding?
1. The shielding of the cabinet (or shielding box):
In practice, the electromagnetic shielding body is not a totally enclosed shielding body, that is, it is not continuously and uniformly electrically. In the actual design of the chassis and shielding box structure, there are usually the introduction and extraction of power lines and control lines. There are operation keys on the panel, openings for the display, and ventilation holes on the rear panel. The chassis is not electrically continuous, and the electrically discontinuous chassis will reduce its shielding effectiveness. Here are some basic practices in chassis design:
(1) Structural materials:
① Most of the materials suitable for the bottom plate and chassis are good conductors, such as copper and aluminum, which can shield the electric field. The main shielding mechanism is reflection rather than absorption;
② Ferromagnetic materials are required to shield the magnetic field , Such as high permeability alloys and iron. The main shielding mechanism is absorption rather than reflection;
③ In a strong electromagnetic field environment, the material is required to shield the two components of the electric field and the magnetic field, so a structurally sound ferromagnetic material is required. The shielding efficiency is directly affected by the thickness of the material and the quality of the bonding and grounding methods;
④ For plastic shells, the shielding layer is sprayed on the inner wall of the plastic shell, or metal fibers are mixed during vapor molding.
(2) Gap:
① As far as possible, each seam and discontinuity between the bottom plate and the cabinet should be overlapped as well as possible. The worst electrical connection plays a decisive role in the shielding effectiveness of the housing;
② Ensure the metal-to-metal contact at the joint to prevent leakage and radiation of electromagnetic energy.
(3) Penetration and opening:
① Pay attention to the degree to which the overall shielding efficiency is reduced by the cable passing through the cabinet. When a typical unfiltered wire passes through the shield, the shielding efficiency is reduced by more than 30dB.
② When the power cord enters the cabinet, all should pass through the filter box. The input end of the filter is best able to pass out of the shielding casing; if the filter structure is not suitable to pass out of the casing, a compartment should be set up for the filter where the power cord enters the casing.
③ When the signal line and control line enter or exit the cabinet, they must pass through an appropriate filter. Multi-core connectors (sockets) with filter pins are suitable for this occasion.
(4) Overlap:
① Use the same metal overlap as much as possible. Make sure that the DC resistance of the overlap is not greater than 2.5 milliohms;
② When attaching different metals, pay attention to the relative positions of various metals in the electrochemical sequence table. The potential difference should be as small as possible, and have appropriate anti-corrosion measures. When metal laps with far different levels are placed, a metal gasket with an intermediate level needs to be placed between the two metal surfaces;
③ Repair the lap surface to obtain the largest contact area;
④ Clean all mating surfaces before lap. To prevent oxidation, the mating surfaces should be lapped immediately after the protective layer is removed;
⑤ For permanent lap joints, all joint surfaces should be connected by welding or brazing as much as possible. RF bonding should be given priority to permanent bonding.
2. Plug-in shielding
When the working frequency of the plug-in box is high and it is easy to interfere with the work of other plug-in boxes, or when the work is very sensitive and susceptible to interference from other plug-in boxes, it is necessary to consider the shielding performance of the plug-in box. The shielding of the plug-in box is more complicated. The following methods are generally used:
(1) The panel is made of metal, generally U-shaped panels are used;
(2) In order to ensure good lap between the panels, it is best to use metal reeds;
(3) It is better to use metal reeds or conductive linings between the panel and the frame The pads are guaranteed to be bonded;
(4) The bonding of the frame must be well guaranteed;
(5) The upper and lower of the box generally have ventilation and heat dissipation requirements, and porous sheets can be used;
(6) To discharge the static electricity on the panel in time, the Fortunately, there is a metal pin, which has two functions of electrostatic discharge and positioning; there must be a positioning hole and a grounding spring in the corresponding pin in the chassis.
3. Cable shielding: Since electromagnetic waves are present in every corner, a slight negligence may cause serious losses. Therefore, the shielding of various objects must be carefully planned. Among the various objects, the first is the shielding of the cable. The braided metal mesh is most commonly used for the shielding of the cable. The factors that affect its shielding effectiveness include: shielding material and thickness, the termination method of the shielding layer and the joints used, the online Standing wave ratio, the length and direction of the cable itself; in addition, in fact, the flexibility of the cable after adding the shielding layer should also be paid attention to. In addition, the shielding layer of the cable should be insulated, because improper grounding will generate noise; at the same time, except for coaxial cables, the shielding layer should not be regarded as the signal return line. In addition, the shielding layer of the cable used to transmit high-frequency signals should be grounded at both ends. In addition to the shielding layer of the cable, the joint should also be considered; at the same time, the shielding of the joint should be properly grounded, and the shielding effectiveness should not be lower than the cable shielding.
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