Analysis And Suppression Of CM EMI In Power Electronic Equipment

Electromagnetic interference is produced due to transient voltage and currentcaused by high speed switching of power devices to nearly devices. Common-modeEMI (CM EMI) usually constitutes a major part in total EMI, which produce fromseveral kilo hertz to tens of million hertz and the electromagnetic compatibility(EMC)standard is not met. The research work of this paper includes three aspects:1) EMCdesign of power electronic equipment;2) analysis of typical characteristics of CM EMIand prediction of CM EMI spectrum of single-phase full-bridge converter;3)suppression of CM EMI based on tuning of gating signals and active parallelcompensatory.Firstly, taking single-phase full-bridge converters as the typical example, theconclusion is that CM EMI is due to the dv/dt charging and discharging the parasiticcapacitor of the power devices. On the basis of CM EMI propagation path whenswitches operate, a equivalent model for simulation of CM EMI in the full-bridgeconverter is proposed, which concludes CM EMI source based on the series of noisesource and CM impedance. The CM EMI in full-bridge converter with unipolar SPWMmodulate technique is analyzed in theory. The simulation is performed in the150kHz～30MHz frequency band, and the simulated CM current spectra are compared with theactual experiments to verify the validity of the proposed model.Secondly, the CM EMI in full-bridge converter with bipolar SPWM modulatetechnique is analyzed. Theoretically, for single-phase full-bridge converters, if bipolarSPWM is used, the two phase-legs provide perfect CM compensation for each other andthe CM current can be cancelled our automatically. Due to inevitable deviation in signaltransmission characteristics of devices, transmission delay of each gating signal can bedifferent, so the CM current cannot be cancelled out perfectly.Thirdly, aiming at the conditions with two complementary common mode noisesources like single-phase full-bridge converter, a method based on self-correction ofdriven pulses are used to suppress CM EMI in this paper. The suppression methodconcludes two parts:1) the evaluation algorithm based on the energy of CM current;2)the adjustment strategy based on intelligent searching. The relationship between the CMcurrent energy and the differences of the transmission delays on different situation aregiven to verify the validity of method. Finally, experiments confirm that the spectra of CM current obviously decreasedafter adjusting driven pulses, in order to determine the transmission delay lagging timeof switch pairs with the CM current minimization, which means the effectiveness of theproposed suppression method.