The principle and function of power supply filter

The power filter, also known as "power EMI filter" or "EMI power filter", is a passive bidirectional network, an electrical device that effectively filters the frequency point of a specific frequency in the power supply or the frequency outside the frequency point. When we choose a power filter, we should mainly consider three indicators; first, voltage and current, second, insertion loss, and finally, structural size. Since the filter is generally potted inside, environmental characteristics are not the main problem. However, the temperature characteristics of all potting materials and filter capacitors have a certain impact on the environmental characteristics of the power filter.

The power filter is a passive bidirectional network with the power supply at one end and the load at the other end.

The principle of the power supply filter is an impedance mismatch network: the greater the impedance mismatch between the input and output sides of the power supply filter and the power supply and load sides, the more effective it is in attenuating electromagnetic interference.

Many people believe that the role of power line filters is to enable equipment to meet the requirements of electromagnetic compatibility standards for conducted emissions and conducted sensitivity. Power line filters are also important in suppressing strong radiated interference generated by equipment. Strictly speaking, the role of power line filters is to prevent electromagnetic interference generated by the equipment itself from entering the power line, and to prevent interference on the power line from entering the equipment. The power line filter is a low-pass filter that allows DC or 50Hz operating current to pass through, but does not allow higher frequency electromagnetic interference current to pass through. The power line filter is bidirectional, which can prevent interference on the power grid from entering the equipment (Figure F-2),

Figure F-2 Filter out the adverse effects of power grid interference on the equipment, so that the equipment meets the requirements of conducted sensitivity; and prevent the electromagnetic interference in the equipment from passing through. The power line is transmitted to the power grid, so that the equipment meets the requirements of conducted emission (Figure F-3). Equipment that can generate strong interference and equipment that is sensitive to external interference must use power line filters. Equipment that can generate strong interference includes: equipment containing pulse circuits (microprocessors), equipment using switching power supplies, equipment using thyristors, variable frequency speed regulation equipment, equipment containing motors, etc. Sensitive circuits include: equipment using microprocessors, small signal analog circuits, etc.

Figure F-3 Filtering equipment conducted emissions