How filters work

    The filter is composed of inductance and capacitance of the low-pass filter circuit, it allows the useful signal of the current through, the interference signal with high frequency has a large attenuation. Since there are two kinds of interference signals: differential mode and common mode, the filter should attenuate these two kinds of interference. There are three basic principles:

　　A) introduce the high-frequency interference current of live wire and zero wire to the ground wire (common mode) or the high-frequency interference current of live wire to the zero wire (differential mode) with the characteristics of capacitor passing high frequency to separate low frequency;

　　B) high frequency interference current is reflected back to the interference source by using the impedance characteristics of the inductance coil;

　　C) using the characteristic that interference suppression ferrite can be used to convert the interference signal in a certain frequency band into heat, the appropriate interference suppression ferrite magnetic ring and magnetic bead can be directly placed on the cable requiring filtering for the frequency band of a certain interference signal

　　5 Importance of high frequency insertion loss of power filter

　　Although the electromagnetic compatibility standards only limit transmission to 30MHz (old standard to 50MHz, new standard to 10MHz), the suppression of transmission must not ignore the effect of high frequency. Because, the high frequency conduction current on the power line can cause radiation, making the radiation emission of the equipment exceed the limit. In addition, the test waveform in the transient pulse sensitivity test often contains very high frequency components, and if these high frequency interferences are not filtered out, the sensitivity test of the equipment will fail.

　　There are two main reasons for the poor high-frequency characteristics of the power line filter, one is the spatial coupling caused by internal parasitic parameters, the other is the imperfect filter. Therefore, the way to improve the high frequency characteristics is also from these two aspects.

　　Internal structure: the connection line of the filter should be arranged in one direction according to the circuit structure. When space allows, a certain distance should be maintained between the inductor and the capacitor. If necessary, some isolation plates can be set up to reduce the spatial coupling.

　　Inductance: control parasitic capacitance of inductance according to the method described above. Use multiple inductors in series if necessary.

　　Differential filter capacitor: the lead of the capacitor should be as short as possible. Understand what this requirement means: keep the line between the capacitor and the wire (live and zero) that needs to be filtered as short as possible. If the filter is installed on the circuit board, the circuit board wiring will be equivalent to the capacitor lead. At this point, be careful to ensure that the timing of the capacitor lead is the shortest.

　　Common mode capacitance: capacitor leads should be as short as possible. This requirement is understood and noted in the same way as differential modulus capacitance. However, the common-mode high-frequency filtering characteristic of the filter is mainly guaranteed by the common-mode capacitance, and the frequency of common-mode interference is generally higher, so the high-frequency characteristic of the common-mode filtering capacitor is more important. The effect of high frequency filtering can be improved by using three - terminal capacitance. But pay attention to the correct use of three-terminal capacitors. That is, keep the ground as short as possible, and the length of the other two wires has little effect. If necessary, a penetrating capacitor can be used. At this time, the performance of the filter itself can be maintained above 1GHz.

　　Special note: do not forget to check the common mode transmitting emission of the power cord at a frequency when the radiation emission of the equipment does not meet the standard requirements. The radiation emission is probably caused by the common mode transmitting current.

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