Voltage Ripple Control of Flying Capacitor Three-Level Inverter With Variable Switching Frequency PSPWM

Generating multiple voltage levels requires a number of capacitors in a flying capacitor (FC) inverter, which is of crucial importance to the cost and power density of the whole system. Often the flying capacitance requirement is a tradeoff between accepted voltage ripple peak-to-peak value and the switching frequency of the inverter. Based on the popular phase-shifted pulsewidth modulation (PSPWM), the peak-to-peak value of the FC voltage ripple is first derived as a function of operating conditions in the time-domain. Since it is not an optimized way to simply adjust the flying capacitance for making the voltage ripple meet the requirement under constant switching frequency of the inverter, a variable switching frequency PSPWM (VSF-PSPWM) for FC voltage ripple control is presented in this article. From the point of view of FCs, the proposed VSF-PSPWM can satisfy the same requirement of the voltage ripple peak-to-peak value with a lower switching frequency and hence reduce the switching losses, comparing with the normal constant switching frequency PSPWM (CSF-PSPWM). Due to the real-time variation of switching frequency, the proposed VSF-PSPWM demonstrates the advantage in electromagnetic interference peak reduction. The simulation and experimental results are presented to evaluate the performance of the proposed method against the traditional CSF-PSPWM.

I. Introduction

In Recent years, multilevel inverter has caused great concern and has been developing extensively for medium- and high-voltage applications [1]–[4], such as electric propulsion, traction drive, and renewable energy sources. In the process of multilevel inverter development, the most popular topologies can be divided into three groups: the neutral point clamped (NPC) inverter [5]; the flying capacitor (FC) inverter [6]; the cascaded H-bridge inverter [7]. Among them, the FC inverter has some merits including of only one dc power supply, with merits of modularity, flexible control and lack of unbalance in the split dc-link capacitors. On the other hand, the FC inverter has an obvious demerit in that it needs additional flying capacitors to generate multiple voltage levels, where the volume, weight, and cost are of crucial importance. Hence, there is an important concern in design and selection of flying capacitors, with one basic idea to achieve smaller capacitance.