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How to improve the high frequency performance of the filter?
Aug 17, 2018

How to improve the high frequency performance of the filter?

There are two main reasons for the high frequency characteristics of the power line filter. One is the spatial coupling caused by internal parasitic parameters, and the other is the imperfection of the filter components. Therefore, the method of improving the high frequency characteristics is also started from these two aspects.

Internal structure: The connection of the filter should be arranged in one direction according to the circuit structure. Under the condition of space, the inductance and the capacitance are kept at a certain distance. If necessary, some isolation plates can be set to reduce the spatial coupling.

Inductance: Control the parasitic capacitance of the inductor as described previously. If necessary, use multiple inductors in series.

Differential mode filter capacitor: The lead of the capacitor should be as short as possible. To understand the meaning of this requirement: the connection between the capacitor and the wire (hot and neutral) that needs to be filtered is as short as possible. If the filter is mounted on a board, the traces on the board will be equivalent to the leads of the capacitor. At this time, pay attention to ensure that the capacitor lead of the timing is the shortest.

Common mode capacitor: The lead of the capacitor should be as short as possible. The understanding and precautions for this requirement are the same as for the differential mode capacitor. However, the common mode high frequency filtering characteristics of the filter are mainly guaranteed by the common mode capacitor, and the frequency of the common mode interference is generally high, so the high frequency characteristic of the common mode filter capacitor is more important. The use of a three-terminal capacitor can significantly improve the high frequency filtering effect. But pay attention to the correct use of the three-terminal capacitor. That is, the grounding wire should be as short as possible, and the length of the other two wires has little effect on the effect. A feedthrough capacitor can be used if necessary, and the performance of the filter itself can be maintained above 1 GHz.

Special note: When the radiation emission of the device does not meet the standard requirements at a certain frequency, do not forget to check the common mode conducted emission of the power line at this frequency. The radiation emission is likely caused by this common mode emission current.

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