Interfering filtering in electromagnetic compatibility design, most electronic product designers generally limit the interference filter: "Electronic products must be used by power cord to conduct emission tests and power lines, must be used on power cord Interfering filter. " For other functions of interference filters, it is very much, which leads to experiments with other test items, such as radiation emission, radiation immunity, and signal lines. In fact, electromagnetic interference filters are very important for the smooth and most electromagnetic compatible tests and the function of ensuring the product. When the following interference issues appear, it is often due to the imperfect filtering measures.
1. Equipment's chassis or cabinet shield is very perfect, but still produce radiation launch of exceeding standard;
2. Separate devices do not have any electromagnetic interference (radiation emission and immunity are completely qualified), but when the necessary external cable is connected, there is an interference problem;
3. When the electric speed pulse is injected on the signal cable, it is faulty;
4. Cannot test by radiation immunity
5. Cannot be tested by the conductivity of the cable bundle
6. Do not pass electrostatic discharge test;
7. Interference between the wires in the cable or between the cables, causing the device to achieve a predetermined function. The following is a brief introduction to how to solve the above problems described above with a filter.
1) Although the chassis or cabinet is shielded, radiation emission exceeds the standard, or cannot be tested by radiation immunity
This is due to the role of the outer trail on the chassis or cabinet. One of the antennas is a tutaneous, that is, if an antenna has high radiation efficiency, its receiving efficiency is also very high. Therefore, the outer trailer of the device can produce strong radiation, and can also effectively receive spatial electromagnetic waves, into the device, and form interference to the circuit. For some reason, an interference current is formed on the outer trailer, which are transmitted from the chassis and radiate electromagnetic waves as radiation antenna. The method of solving this problem is to install a filter at the port of the cable to remove the interference current filter.
2) Independent devices do not have any electromagnetic interference (radiation emission and immunity are completely qualified), but when the necessary external cable is connected, there is an interference problem;
This issue is the same as the essence of the first type of problem, that is, the outer trail is equivalent to the antenna. When there is no cable, it is equivalent to no radiation antenna and receiving antenna, so it is easy to test the radiation emission and the immunity test, but these cables are used as radiation antennas and receive antennas, resulting in the radiation of the equipment, and external space The sensitivity of interference is improved. The solution is to install the filter at the port of the cable, filtering the electromagnetic energy received from the space before they reach the electronic circuit, on the other hand, preventing the interference energy in the electronic circuit from entering these conductors and radiates by means of conductor radiation. .
3) When the electric speed pulse is injected on the signal cable, it is faulty;
We know that the frequency of electricity pulse is very high, these interferences through capacitive coupling clamps, form interfering currents on the cable, direct flow through circuits on one aspect, forming interference on circuitry, and generating radiation on the other hand Circuit formation interference. The solution is to use a shielded cable and an installed filter.
4) Do not pass the conductive sensitivity test on the cable bundle
Electrical fast transient pulse group anthambient test is to verify anti-interference capabilities of transient disturbance caused by lightning, ground fault or switching electro-sensitive load. This test is a pulse group test coupled from a rapid transient pulse that is coupled to the power supply line, the control line, and the signal line, which is naturally also solved by filtering at the cable port.
5) Can't pass electrostatic discharge test;
The effect of electrostatic discharge on the device circuit is largely due to the high-frequency electromagnetic field around the electrostatic discharge current, and these electromagnetic fields are very high, so it is easy to receive by the wire, and the circuit forms an interference, and a device is doing electrostatic discharge test. When discharge is discharged on the movable panel, the circuit has failed. After inspection, it is found that the panel is a bundle cable, and the electromagnetic field generated by the electrostatic discharge current on the panel should have noise current in the cable bundle to form interference. After installing the filter at the port of the cable, the problem is solved.
With the universal application of the switching power supply, the installation filter is already necessary for the power cord entrance. Since the switching power supply works in high power pulse state, it produces strong electromagnetic radiation, which induces conductive emissions on the line. If the filter is not used, it is not possible to meet the electromagnetic compatibility test.
In the design, the interference filter is often divided into two categories of power cord interference filters and signal line interference filters. From the circuit, the two types of filters are the same, all of which are low-pass filters, which are mainly because the two have a much more suppression of electromagnetic interference, there are some respectively. Special considerations. The signal filter also considers that the filter does not have a serious impact on the work signal, and cannot cause the distortion of the signal. In addition to ensuring the safety requirements, the power filter must be noted that when the load current is large, the inductance in the circuit cannot be saturated (resulting in a decrease in filter performance).
EMC design verification essential test instrument: http://www.ocetest.com
(Conduct test, radiation test, EMS test, antenna, near field probe)
The article is taken from the network, there is Shenzhen national test editorial finish
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