Study On Digital Active Common-mode EMI Filter For PWM Inverter

With the development of power electronics, PWM inverters are getting more widely used, such as PWM drives, UPSes etc. However, high frequency switching of power semiconductor devices like IGBTs and MOSFETs bring high dv/dt and di/dt, which produce large electromagnetic interference (EMI). In real electrical and electronic equipments, the common-mode electromagnetic interference (CM EMI) usually constitutes a major part of the total conducted EMI, especially beyond MHz level. Because the high frequency leakage current returns through the earth ground to form a CM circuit loop, the area of the CM interference loop is relatively larger compared with that of the differential- mode interference, this cause serious radiated EMI due to the significant"antenna effect"of the CM current loop, so the CM EMI is a major cause of radiated EMI. Thus, the reduction of CM EMI should be the main object of switching converter EMI attenuation.This paper proposed a novel digital controlled active common-mode EMI filter for PWM inverters, it can achieve CM EMI digital attenuation by using the advanced digital single processor and intelligent optimization algorithm. This filter is composed of three parts: the first is the CM current detection and processing; the second is digital close-loop controlled system; the third is parallel connection compensation circuit.In this paper, the author first proposed two kinds of parallel connection compensation circuit based on the different DC input voltage grades. One is called the direct-parallel connection compensation circuit, and the other is called indirect-parallel connection compensation circuit. In order to resolve the phase synchronization problem between the compensation CM current and the self CM current of the inverter, this paper proposed a CM noise evaluation method based on current energy and intelligent optimized searching close-loop controlled algorithm based on simulated annealing algorithm. Along with the variation of the phase deflection between the compensation CM current and the self CM current of the inverter, the energy of the CM current after compensating will appear several smaller values and a minimum value. Therefore, in order to conduct the best adjustments of the time of the rising and falling edge of the compensation circuit PWM driving waveform, whether the close-loop searching algorithm can find the minimum energy value of the CM current after compensating or not become the key to reflect the capability of the optimal searching algorithm. Simulated annealing algorithm can avoid the smaller value during the searching process, and can find the lowest value finally, the algorithm has good robust capability, and easy to implement. Experiments have been done on the platform of single phase half bridge inverter with a digital controlled active CM EMI filter based on the newest DSP TMS320F2812 of TI Company. The experiment results show that the CM interference has been attenuated efficiently when the inverter works, verified the correctness and validity of the proposed digital controlled active CM EMI filter.