How to use filters to solve the problem of electromagnetic interference?

Most electronic product designers' understanding of interference filters is generally limited to: "Electronic products must use interference filters on the power line to pass the power line conducted emission test and power line immunity test." However, little is known about other functions of interference filters, which leads to the failure of other test items, such as radiated emission, radiated immunity, and conduction sensitivity on the signal line, after the product design is completed.

In fact, the electromagnetic interference filter is very important for smoothing most of the electromagnetic compatibility test and ensuring the function of the product. When the following interference problems occur, it is often due to imperfect filtering measures.

1 The equipment's chassis or cabinet shielding is very perfect, but it still produces excessive radiated emissions.

2 The independent equipment does not have any problems of electromagnetic interference (radiated emission and immunity are fully qualified), but when the necessary external cables are connected, interference problems occur.

3 A fault occurs when electrical fast pulses are injected on the signal cable.

4 Cannot pass the radiation immunity test.

5 Cannot pass the conduction sensitivity test on the cable harness.

6 Cannot pass the electrostatic discharge test.

7 The conductors in the cable or the cables interfere with each other, causing the device to fail to achieve its intended function.

The following is a brief introduction on how to use filters to solve the above problems.

1 Although the chassis or cabinet is well shielded, the radiated emissions exceed the standard or fail the radiation immunity test.

This is because the external drag cable on the chassis or cabinet acts as an antenna. One characteristic of the antenna is reciprocity, that is to say: if an antenna has a high radiation efficiency, then its reception efficiency is also very high. Therefore, the external drag cable of the equipment can not only generate strong radiation, but also effectively receive the space electromagnetic wave and transmit it into the equipment, causing interference to the circuit. For some reason, interference currents are formed on the external drag cable, and these currents are conducted out of the chassis, and the cable is used as a radiation antenna to radiate electromagnetic waves. The way to solve this problem is to install a filter at the port of the cable to filter out the interference current.

2 The independent equipment does not have any problems of electromagnetic interference (radiated emission and immunity are fully qualified), but when connecting the necessary external cables, interference problems occur.

This problem is essentially the same as the first type of problem, that is, the external drag cable is equivalent to an antenna. When there is no cable, it is equivalent to no radiating antenna and receiving antenna, so it is easy to pass the radiated emission and immunity test, but when the cable is dragged, these cables act as radiating antenna and receiving antenna, resulting in increased radiation of the device and the impact on the outside The sensitivity of interference is increased. The solution is to install a filter at the port of the cable to filter out the electromagnetic energy received by these conductors from the space before they reach the electronic circuit. On the other hand, to prevent the interference energy in the electronic circuit from entering these conductors and radiating through the conductor .

3 When electrical fast pulses are injected on the signal cable, a fault occurs

We know that the frequency of electrical fast pulses is very high. These interferences are coupled into the cable through capacitive coupling clamps, forming interference currents on the cable. These currents flow directly into the circuit on the one hand, which causes interference to the circuit, and on the other hand generates radiation. The circuit forms interference. The solution is to use shielded cables and add filters.

4 Cannot pass the conduction sensitivity test on the cable bundle

The purpose of the electrical fast transient pulse group immunity test is to verify the immunity of transient disturbances caused by lightning, ground faults, or switching inductive loads. This test is a pulse group test composed of many fast transient pulses coupled to the power line, control line, and signal line. Naturally, it can also be solved by filtering at the cable port.

5 Cannot pass the electrostatic discharge test

The effect of electrostatic discharge on the circuit of the equipment is largely due to the high-frequency electromagnetic field around the electrostatic discharge current. These electromagnetic fields are very easy to be received by the wire due to the high frequency and interfere with the circuit. When a device is doing an electrostatic discharge test It was found that the circuit malfunctioned when discharging on the movable panel. After inspection, it is found that there is a bundle of cables behind the panel. The electromagnetic field generated by the electrostatic discharge current on the panel induces a noise current on the cable bundle, forming interference. After installing the filter at the port of the cable, the problem is solved.

With the widespread application of switching power supplies, it is already a necessary measure to install a filter at the entrance of the power cord. Because the switching power supply works in a high-power pulse state, it will generate strong electromagnetic radiation, which is induced to form conductive emissions on the line. If no filter is used, it is impossible to pass the EMC test.

In the design, interference filters are often divided into power line interference filters and signal line interference filters. Speaking from the circuit, the two types of filters are the same, and they are both low-pass filters. The reason for this division is that, in addition to both having the greatest possible suppression of electromagnetic interference, there are also some Special considerations. The signal filter must also consider that the filter must not have a serious impact on the working signal and cannot cause signal distortion. In addition to ensuring that the power filter meets the safety requirements, it should also be noted that when the load current is large, the inductance in the circuit cannot be saturated (causing the filter performance to deteriorate).

If you want to know more, our website has product specifications for filters, you can go to ALLICDATA ELECTRONICS LIMITED to get more information