Active EMI Filtering Technology for Power Electronic Systems


Student thesis: Doctoral Thesis

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Award date11 Aug 2022


In a power electronic system, semiconductor devices work in switched mode to pursue high efficiency and flexible yet stable output. The fast switching transition generates electromagnetic interference (EMI), comprising differential-mode (DM) and common-mode (CM) emissions. Passive EMI filters are widely used to attenuate EMI. However, the bulky volume of passive filters becomes a bottleneck in increasing the power density of the entire system. Active EMI filters are thereby proposed to use active components to replace part of the passive components for volume reduction. This thesis is devoted to developing the active EMI filtering technology for power electronic systems in three aspects: DM filter, input CM filter, and output CM filter.

The first work has advanced the power semiconductor filter (PSF), which is an active DM filter. The concept of the PSF is based on utilizing a series pass device (SPD) operated in linear mode to regulate the input current of switching converters. A fixed-frequency dynamic ramp modulator is proposed to avoid input current oscillation and reduce power dissipation of the SPD. An optimized fast-current regulation circuit is designed to achieve high DM EMI attenuation. A method to predict the EMI suppression performance of the PSF is proposed. Furthermore, the ground loop inductance in the fast-current regulation circuit is shown to significantly impair the filtering performance, and PCB layout guidelines are given to mitigate this adverse effect. A 100W buck-boost PFC prototype with the PSF satisfies EMC standard EN55015 class B in the whole range of 150kHz to 30MHz.

The second work has proposed an advanced input active CM filter (ACF). It has the merits of wide bandwidth, high attenuation, and general multistage structure. The working bandwidth of the ACF ranges from 150kHz to 30MHz. By cascading several ACF sections, a multistage structure that exhibits higher filtering attenuation can be designed. The number of cascaded sections is optimized by considering the required filtering attenuation. The performance of single-stage and multistage ACFs is evaluated on two commercial power supplies with rated power of 90W and 1000W, respectively. Experimental results show that the physical volume of the ACF is significantly smaller than that of the passive filter, and the power dissipation of the ACF is similar to its passive counterpart.

The third work has proposed a new active filtering architecture for PWM inverter-fed motor drive systems. The stepwise CM voltage at the output of inverters causes EMI and damage to motor bearings. A new output ACF is proposed to compensate for the steep rising/falling edges of CM voltage for EMI reduction and bearing protection. An input ACF is also necessary to attenuate the CM noise injected into the grid. Hence, a holistic assessment of this new active filtering architecture, comprising input and output ACFs, is demonstrated. Moreover, ferrite materials and the winding configuration of the CM transformer used in the output ACF are investigated. The relationship between the input and output ACFs is also discussed. This work aims to provide new perspectives and implementation guidelines for the ACFs in motor drive systems.