Analysis And Suppression Of Common-mode EMI In Power Electronic Equipment

Electromagnetic interference is produced due to high dv /dt and high di /dt caused by high speed switching of power devices to nearby devices. Common-mode EMI (CM EMI) usually constitutes a major part in total EMI. The EMI is not only influences the load's usual work and withdraws the load's work life but also has detrimental effects on inverter itself. In order to analysis of the common-mode EMI features, accurately prediction Common-mode current, the effective suppression of common mode EMI, The three main researches are listed as following:Firstly, Taking Buck converters as the typical example, this paper carried out an in-depth analysis of the common-mode electromagnetic interference in order to get the relationship between the CM EMI and key operation parameters. A CM equivalent circuit of the converter is firstly proposed, and the relationship between the equivalent of CM noise source and operation parameters like input voltage, switching frequency are analyzed. To verify the theoretical analysis, simulations are carried out on an IGBT Buck converter to test the actual influences of input voltage, switching frequency, duty ratio on the CM EMI. The results indicate that CM EMI is not affected by the duty ratio and it is proportional to the input voltage and switching frequency.Secondly, common-mode conducted EMI in single-phase full-bridge converter is analyzed in this paper. According to Buck circuit and its equivalent circuit model similarity, the CM equivalent circuit of single-phase full-bridge converter is proposed, which is a novel equivalent circuit of common mode EMI based on noise source in parallel. Compared the conducted common mode EMI in full-bridge circuit with bipolar SPWM and unipolar SPWM modulate technique in theory. It is conclusion that the conducted common mode current with bipolar SPWM modulation technique is theoretically zero because the two legs provide perfect CM current compensation for each other. But the CM current of the single-phase full-bridge converter with bipolar SPWM modulation technique is not zero in the real case. Different transmission delays of two drive signals in bipolar SPWM modulation technique always produce large CM current. Experimental results validate fine-tunning driving signals before they are transmitted can suppression the common mode EMI in full-bridge converter efficiently.Thirdly, aim at the conditions with two complementary common mode noise sources like single-phase full-bridge converter, a method of optimizing drive pulses through pre-shifting the pulse edges are used to suppress CM EMI in this paper. Experimental results show that this method can effective suppress CM EMI without any additional hardware. Aim at the conditions without two complementary common mode noise sources like single-phase half-bridge converter, an active compensatory common mode EMI suppression technique with an external compensating circuit is adopted. Two kind of push-pull compensating circuit where the DC biased voltage source is provided by DC bus voltage of converter or external electric supply is discussed. The calculating process of output capacitor and coupling capacitor in active compensating circuit are discussed by using the common mode current equivalent mode based on noise source in parallel. The driven circuit of compensating circuit is also designed in this paper.