Basic knowledge of EMC theory filter design

1. Basic concept of filter circuit

The filter circuit is a frequency selective network composed of inductance, capacitance, resistance, ferrite bead and common mode coil. Low pass filter is widely used in EMC suppression technology. In order to reduce the external radiation of power and signal cables, the interface circuit and power circuit must be designed with filter.

The efficiency of the filter circuit depends on the impedance characteristics of both sides of the filter circuit. In the low impedance circuit, a simple inductive filter circuit can get a 40dB attenuation, while in the high impedance circuit, it has little effect; in the high impedance circuit, a simple capacitance filter circuit can get a good filtering effect, and it has little effect in the low impedance circuit. In the filter circuit design, the capacitance is close to the high impedance circuit design, and the inductance is close to the low impedance circuit design.

The insertion loss of the capacitor increases with the increase of the frequency, until the frequency reaches the self resonance frequency, because there is a series connection between the inductance of the conductor and the electrode of the capacitor and the capacitor on the circuit, the insertion loss begins to decrease.

2. Power EMI filter

Power EMI filter is a passive two-way network, one end of which is connected with power supply and the other end is connected with load. In the higher frequency band of the concerned attenuation band, the power EMI filter can be regarded as "impedance mismatch network".

The results of network analysis show that the larger the impedance mismatch between the two sides of the filter impedance, the more effective the attenuation of EMI energy. Because the common mode impedance of the power line side is generally low, the impedance of the power side of the filter is generally high. In order to get better filtering effect, high input resistance should be provided for the power supply side with low impedance

For the load side with high input impedance, the filter with low output impedance shall be equipped.

The common power filter has a good filtering effect on the interference signal below tens of megabytes. In the higher frequency range, because of the inductance effect of capacitance, its filtering performance will decline. For high frequency interference, the feedthrough filter should be used. Because of its structural characteristics, the filter has good filtering characteristics, and its effective frequency band can be extended to GHz, so it is widely used in wireless products.

The most important problem in the use of filters is grounding. Only a well grounded filter can play its filtering role, otherwise it is of no value. Pay attention to the following problems when using the filter:

1) The filter is placed at the entrance of the power supply;

2) The feedthrough filter shall be placed on the metal wall of the case (cabinet);

3) The filter is directly connected with the cabinet, and the protective paint cannot be applied under the filter;

4) The input and output leads of the filter cannot be parallel or crossed.