The Generation Mechanism and Elimination Strategy of Narrow- and Error-Pulse for Cascaded H-Bridge NL-PWM Modulation

There is a wide application prospect of Nearest level pulse width modulation (NL-PWM) in cascaded H-bridge (CHB) converter, however, it will produce narrow- and error-pulse in the process of implementation, which has a negative impact on CHB efficiency, output voltage quality and safe operation of power devices. In this paper, the NL-PWM strategy is improved based on round function to solve the narrow pulse of traditional NL-PWM, and a step wave delay update strategy is proposed to solve the error-pulse of NL-PWM. This paper analyzes the basic principle of improved NL-PWM and its ability to avoid narrow pulse of power device, shows the harmonic characteristics of improved NL-PWM, analyzes the generation mechanism of error pulse, and calculates the error of output voltage caused by the proposed step wave delay update strategy. The correctness of theoretical analysis and the effectiveness of the proposed method are verified by simulation and experiment. The research shows that the improved NL-PWM can effectively prevent narrow pulse while maintaining good harmonic characteristics, and the proposed step wave delay update strategy can avoid error pulse and has little impact on the output of NL-PWM.