Analysis and application of near-field c

Analysis and application of near-field coupling characteristics of EMI filter  

Source: Magnetic Components and Power Supply, May 2015, author: Chen Qingbin, Chen Wei, Page view: 26168  

EMI filter is an important means of EMI suppression at present. With the gradual increase of power density, the distance between devices becomes smaller and smaller.  EMI filter's near-field magnetic coupling is serious, so there are few researches and no method to improve EMI filter's low-band differential mode filtering performance by utilizing EMI filter's near-field coupling.  This paper presents a new method to improve the performance of EMI filter near the resonant point by series resonance of capacitor and inductor.  

1 the introduction  

EMI filter is the last barrier to suppress conducted electromagnetic interference at present. The filtering function of EMI filter is necessary for most switching power converters to pass the EMI standard. EMI filter is an important remedial measure, especially when EMI design is not carried out at the beginning of product design.  In view of the importance of EMI filter, many experts and scholars have studied the distribution parameters of EMI filter and the coupling relationship and interaction between components [1-6].  

In the switching power supply, EMI filter is compact because of the device placement. FIG. 1 shows the general structure of EMI filter, and the relatively large size between devices is affected by near-field coupling effect [7].  

At present, the industry mainly focuses on the cancellation of EMI filter elements' distribution parameters to improve the high frequency performance of EMI filter [1-6]. However, there are few studies on the near-field coupling between EMI filter elements, and there are no methods or studies that consider the use of EMI filter's distribution parameters to improve the low frequency performance of EMI filter.  Because the low frequency performance of EMI filter directly determines the volume of EMI filter, improving the low frequency performance of EMI filter is an effective means to solve the problem of EMI filter miniaturization.  Therefore, EMI characteristics of magnetic elements in the SWITCHING power supply and the near-field coupling characteristics of the filter are the research focus and one of the effective technical means of EMI filter miniaturization.  

Near field coupling includes magnetic field coupling and electric field coupling.  In EMI filters, magnetic field coupling is mainly used [7-8].  Magnetic field coupling refers to the process in which the side generating the magnetic field couplings the signal to other devices through the magnetic field.  The device that produces the interfering magnetic field is the interfering body, and the device that can be coupled to the interfering magnetic field is the interfered body.  In EMI filter, magnetic component and capacitor are the main interferers and interferes.  Magnetic coupling mainly occurs between magnetic components and between magnetic components and capacitors.  

In this paper, the near-field coupling effect and decoupling method of EMI filter are analyzed in detail, and a new method to improve the performance of EMI filter in low band differential mode is proposed.  Combined with the decoupling equivalent circuit of EMI filter, a method is proposed to improve the performance of EMI filter in low frequency band by utilizing the near-field coupling characteristics of EMI filter.  The experimental results verify the correctness and feasibility of the theoretical analysis.