When choosing a filter, the three most important parameters are rated current, rated voltage and cut-off frequency.
(1) The rated current refers to the working current flowing through the filter. When the operating current of the power line filter exceeds the rated current, it will not only cause the filter to overheat, but also cause the low-frequency filtering performance of the filter to decrease. This is because the inductance in the filter will saturate in the case of a large current, which will reduce the actual inductance. Therefore, it is very necessary to leave a certain margin when determining the rated working current of the filter. Generally, the rated current value of the filter should be 1.5 times the actual current value.
(2) Determination of rated voltage. If the wires to be filtered will not be impacted by high voltages such as electrostatic discharge, electromagnetic pulse, surge, etc., the working voltage of the circuit is the rated working voltage of the filter. If this is not the case, for example, the cables dragged outside the equipment will be impacted by high voltage, and this situation needs to be fully considered and sufficient margin is left.
(3) Determination of cut-off frequency. The selection of the cutoff frequency must ensure that the passband of the filter covers the bandwidth of the functional signal, ensure the normal operation of the device, and filter out unnecessary high-frequency interference to the greatest extent possible. For analog signals, the cutoff frequency is well defined, as long as the cutoff frequency is greater than the bandwidth of the signal. For digital pulse signals, the cutoff frequency can be set to 1/lr, where tr is the rise/fall time of the pulse. If it is a periodic pulse signal, 15 times the pulse repetition frequency can also be taken as the cut-off frequency.