Analysis And Active Suppression Of Common-Mode EMI In Single Phase Inverter

With the development of power electronics, PWM inverters are getting more widely used, such as PWM drives, UPS 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) from several 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) analysis of typical characteristics of common mode (CM) EMI and prediction of CM EMI spectrum of single-phase full-bridge inverter; 2) reduction of CM EMI through automatic tuning of gating signals; and 3) evaluation of CM noise level based on the energy of CM current.The source and propagation path of CM EMI in single-phase inverters are investigated. The conclusion is that CM EMI is due to the dv dt charging and discharging the parasitic capacitors of the power devices. After examining the existing circuit model of CM current, some shortage of the model has been found. A new parallel circuit model is proposed. Time- and frequency-domain analysis are conducted to obtain CM current waveforms under trapezoidal excitement and CM noise spectra in 150kHz-30MHz frequency range. The CM current is of decaying oscillatory waveform. The simulated CM EMI waveforms and spectra are compared with the experimental results to verify the validity of the proposed model.The impacts of modulation strategy on CM emissions are discussed. Theoretically, for single-phase full-bridge inverters, if bipolar SPWM is used, the two phase-legs provide perfect CM compensation for each other and the CM current can be cancelled out automatically. Due to inevitable deviation in signal transmission characteristics of devices, transmission delay of each gating signal can be different, so the CM current cannot be cancelled out perfectly. To address this, a simple solution is to fine-tune the gating signals before they are transmitted. The tuning process consists of evaluation of CM noise level, which is based on the energy of CM current, and an automatic search algorithm. The relationships between the CM current energy and the differences of the transmission delays on different situation are given in simulation. The experimental results on a TMS320F2812 based test setup are also given to verify the validity of the method. The THD% of the output voltage is also very low, which indicates that this method can be easily incorporated with the inverter control algorithm.