According to the characteristics, frequency range, voltage, impedance and other parameters of the interference source, as well as the requirements of the load characteristics, a filter should be appropriately selected, generally considering:
Firstly, it is required that the electromagnetic interference filter can meet the attenuation characteristics and insertion loss requirements of the load requirements within the corresponding operating frequency range. If the attenuation amount of a filter cannot meet the requirements, a multi-stage connection can be used to achieve higher attenuation than a single stage. Different filters can be cascaded to achieve good attenuation characteristics and insertion loss in a wide frequency band.
Secondly, it is necessary to meet the requirements of the working frequency of the load circuit and the frequency to be suppressed. If the frequency to be suppressed is very close to the frequency of the useful signal, a filter with very steep frequency characteristics is required to filter out the suppressed interference frequency and only allow it to pass through the useful frequency signal.
Thirdly, at the required frequency, the impedance of the EMI filter RFI filter must match the impedance of the interference source and load it is connected to. If the load is high impedance, the output impedance of the power filter should be low impedance; If the impedance of the power supply or interference source is low, the output impedance of the filter should be high; If the power source impedance or interference source impedance is unknown or varies over a large range, it is difficult to obtain stable filtering characteristics. In order to obtain good and stable filtering characteristics of the filter, a fixed resistor can be connected simultaneously at the input and output terminals of the filter.
Fourthly, the filter must have a certain withstand voltage capability. The power filter should be selected based on the rated voltage of the power supply and interference source, so that it has a sufficiently high rated voltage to ensure reliable operation under all expected working conditions and can withstand the impact of input instantaneous high voltage.
Fifthly, the power filter is allowed to pass through the rated current that should be consistent with the continuous operation in the circuit. A high constant current will increase the volume and weight of EMI filters; A low constant current will also reduce the reliability of EMI filters.
Sixth, the power filter should have sufficient mechanical strength, simple structure, light weight, small size, convenient installation, and safety and reliability.