Comprehensive comparison of THD and common mode leakage current of bipolar, unipolar and hybrid modulation schemes for single phase grid connected full bridge inverters

This paper presents a comprehensive analysis and comparison among the three commonly used PWM schemes for single phase full bridge inverters namely, bipolar, unipolar and hybrid modulation schemes. The inductor current ripple and THD are analyzed using time domain analysis methods and the performance of the three modulation schemes are compared in detail for a range of modulation indices. An interesting result is that in order to achieve the same THD, compared with unipolar PWM, the average switching frequency (considering all the switches) of hybrid PWM is same but the average switching frequency of bipolar PWM needs to be around 3.6 times, when the modulation ratio is between 0.6 and 1. This results in the corresponding increase in the device switching losses. The common mode voltage between the negative terminals of the input and the grid, and the resulting leakage current are also analyzed and compared among the three schemes. Finally, a SiC MOSFETs based 1 kW full bridge inverter using the three modulation schemes has been built to validate the theoretical analysis. The inductor current ripple, THD and common mode leakage current analysis are verified through experiment.