Application of EMI filter in flyback switching power supply

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Application of EMI filter in flyback switching power supply [Copy link]

In recent years, switching power supplies have developed rapidly due to their high efficiency, small size and good output stability. They have been widely used in various fields such as industry, national defense, and home appliances. However, due to the high frequency, high di/dt and high dv/dt during the operation of switching power supplies, the electromagnetic interference problem is very prominent. How to reduce the EMI of the product and make it pass the EMC standard demonstration test such as FCC or IEC1000 has become a very concerned issue for global switching power supply designers and electromagnetic compatibility (EMC) designers. EMI disturbance is usually difficult to describe accurately, and the design of the filter is usually through repeated iterations, calculations and production in order to gradually approach the design requirements. Starting from the principle of EMI filtering, this paper analyzes its common mode and differential mode noise models respectively, and gradually gives the method of implementing EMI filters in flyback switching power supplies.
1. The construction principle of electromagnetic interference filter
When the switching power supply is working, electromagnetic interference can be divided into two categories. One is the external interference introduced by the power supply line. At the same time, the switching power supply itself
is also an interference source, which interferes with the power grid, the switching power supply itself and other equipment through the coupling channel. Therefore, electromagnetic interference belongs to a two-way interference signal. Electronic equipment is both the object of noise interference and a source of interference. Electromagnetic interference is usually analyzed by common mode and differential mode interference.
Common mode interference is the interference between the current carrier and the earth: the interference is equal in magnitude and consistent in direction. It exists between any one side of the power supply relative to the earth, or between the neutral line and the earth. It is mainly generated by | d v / dt |, and | di / dt | also generates a certain common mode disturbance. Differential mode interference is the interference between current carriers: the interference is equal in magnitude and opposite in direction. It exists between the power supply phase line and the neutral line and between the phase lines. EMI filters should meet the requirements of electromagnetic compatibility (EMC) and must be bidirectional RF filters. On the one hand, they should filter out external electromagnetic interference introduced from the AC power line, and on the other hand, they should prevent the equipment from emitting noise interference to the outside, so as not to affect the normal operation of
other . EMI filters should suppress both series mode and common mode interference.
EMI filters are low-pass devices composed of L and C. In order to obtain maximum attenuation in the stop band, the impedance of the filter input and output ends must be opposite to the impedance of the noise source connected to them, that is, for low-impedance noise sources, the filter must be high impedance (large series inductance); for high-impedance noise sources, the filter must be low impedance (large parallel capacitance). For EMI filters, these principles apply to common mode and differential mode. EMI filters are usually placed at the front end of the switching power supply connected to the power grid. They are low-pass filters composed of series reactors and parallel capacitors. The equivalent impedance of the noise source is Zsource and the equivalent impedance of the power grid is Zsink. The filter indicators (fstop and Hstop) can be achieved by a first-order, second-order or third-order low-pass filter. The calculation of the filter transfer function is usually approximated at high frequencies, that is, for an n-order filter, all ωk-related terms are ignored (when k＜n), and only the ωn-related terms are taken.