For a product that has been well designed and has correct shielding and grounding measures, there will still be conducted interference emission or conducted interference into the product. When the conducted emission (CE) is unqualified, the radiated emission (RE) of the device may also be unqualified due to the antenna effect. In order to meet the CE and CS (conducted susceptibility) limit values specified by the EMC standard, it is a good way to use an EMI filter.
Usually some form of filtering is used to reduce the emission of power lines and signal lines. The attenuation of the filter determines the source and load impedance. That is, if the filter does not match the source and load impedance, the minimum transmission signal (EMI) power will be generated. Also consider whether the electromagnetic interference is common mode or differential mode. Common mode refers to the ground-referenced noise voltage on two conductors, and differential mode refers to the voltage of one conductor relative to the other. In general, both types of electromagnetic interference need to be attenuated.
In electromagnetic shielding technology, any wire that directly penetrates the shield will cause the shield to fail. In practice, many cases (cabinets) with shielding problems have failed electromagnetic compatibility tests because of conductors directly passing through the shielding. This is one of the typical problems that designers who lack electromagnetic compatibility experience are confused. One of the effective ways to solve this problem is to use a filter at the port of the cable to filter out unnecessary frequency components on the cable, which can not only reduce the electromagnetic radiation generated by the cable, but also prevent the environmental noise induced on the cable from being transmitted. Inside the device. In a nutshell, the function of the filter is to allow only the signal frequencies necessary for the work to pass, and to have a great attenuation effect on the signal frequencies that are not necessary for the work, so as to minimize the chance of interference. From the perspective of electromagnetic compatibility, the power cord is also a conductor that passes through the chassis, and its impact on the electromagnetic compatibility of the equipment is the same as that of the signal line. Therefore, a filter must be installed on the power line. Especially in recent years, switching power supplies have been widely used. In addition to small size, high efficiency, and wide voltage regulation range, the characteristics of switching power supplies are also a major feature of strong electromagnetic interference emission. If a filter is not installed on the power line, it is impossible to meet the electromagnetic requirements. Compatible requirements.
The filter installed on the power line is called the power line interference filter, and the filter installed on the signal line is called the signal line interference filter. The reason for this division is that in addition to having sufficient suppression of electromagnetic interference, the two have some special considerations respectively: the signal filter should consider that the filter cannot have a serious impact on the working signal and cannot cause a signal. Distortion. In addition to ensuring that the power filter meets the filtering requirements, it should also be noted that when the load current is large, the inductance in the circuit cannot be saturated (causing the performance of the filter to decrease).
According to the relative relationship between the frequency of the interference signal to be filtered and the operating frequency, the interference filter has low-pass filters, high-pass filters, band-pass filters, band-stop filters, and other types.
※Low pass filter
The low-pass filter is the most commonly used one. It is mainly used in occasions where the frequency of the interference signal is higher than the frequency of the working signal. For example, in digital equipment, the pulse signal has rich high-order harmonics. These high-order harmonics Not necessary for circuit operation, but they are a strong source of interference. Therefore, in digital circuits, low-pass filters are commonly used to filter out unnecessary high-order harmonics in the pulse signal, and only retain the lowest frequency that can maintain the normal operation of the circuit.
The power line filter is a low-pass filter, which only allows 50Hz current to pass, and has a great attenuation to other high-frequency interference signals. The commonly used low-pass filter is a combination of inductors and capacitors. The capacitors are connected in parallel between the signal line to be filtered and the signal ground (to filter out differential mode interference current) or between the signal line and the chassis ground or the ground ( Filter out common mode interference current) The inductor is connected in series with the signal line to be filtered. According to the circuit structure, there are single capacitor type (C type), single inductor type, L type, reverse Г type, T type, and π type.
※High pass filter
The high-pass filter is used in occasions where the interference frequency is lower than the signal frequency, such as filtering the interference caused by power harmonics on some sensitive signal lines close to the power line.
※Band pass filter
Band-pass filters are used where the signal frequency only occupies a narrow bandwidth. For example, a band-pass filter should be installed on the antenna port of a communication receiver to allow only communication signals to pass.
※Band stop filter
The band stop filter is used where the interference frequency bandwidth is narrow and the signal frequency is wide. For example, a band stop filter with a band stop frequency equal to the transmitting frequency of the radio station should be installed at a cable port very close to a high-power radio station.
When selecting a power line filter, several indicators should be considered. The first is the rated voltage/rated current; the second is the insertion loss, the leakage current (the DC power filter does not consider the size of the leakage current), the structure size; the last is the withstand voltage test.
Since the inside of the filter is generally potted, the environmental characteristics are not a major issue. However, the potting materials used and the temperature characteristics of the filter capacitor have a certain impact on the environmental characteristics of the power filter.
The volume of the filter is mainly determined by the inductance in the filter circuit. The larger the volume of the inductance coil, the larger the volume of the filter.
In order to reduce the grounding resistance, the filter should be installed on a conductive metal surface or connected to the grounding point through a braided grounding strap to avoid a large low impedance caused by a slender grounding wire.