ABSTRACT – Increasing the voltage of energy sources, such as photovoltaic (PV), fuel cells, and battery storage units, requires a voltage-boosting technique. This work presents a new single-phase 7-level switched-capacitor inverter with a reduced number of switches and a voltage boost gain of 3. The proposed topology with a PV source can achieve 7-level output, and self voltage balance of capacitors using only 8 switches. The proposed inverter does not use a back-end H-bridge with four switches that must withstand the peak load voltage, and the voltage stress of all switches does not exceed half of the peak load voltage. The simulation is carried out in MATLAB/Simulink software.
INTRODUCTION
Multilevel inverters (MLIs) have been known as one of the most popular solutions for power conversion of renewable energy sources, such as photovoltaic (PV) cells, wind turbines, and fuel cells. The main advantages of MLIs are higher voltage operating capability, lower dv/dt, low voltage stress on switches and better harmonic performance. In general, there are three main types of multilevel inverter topologies: neutralpoint-clamped multilevel inverter (NPC-MLI), flying capacitor multilevel inverter (FC-MLI) and cascade H-bridge multilevel inverter (CHB-MLI). As the number of output levels increases, the number of clamping diodes in NPC-MLI, flying capacitors in FC-MLI, and isolated DC sources in CHB-MLI also increase significantly. Furthermore, NPC and FC topologies require auxiliary circuits and complicated control algorithms to maintain the voltage balance of the capacitors. In addition, a front-end DC–DC boost converter or a load-end transformer is required to achieve a higher AC voltage. All of these make the inverter system complex, bulky, and expensive. As a new approach to overcome the aforementioned drawbacks, a switched-capacitor (SC)-based multilevel inverter (SCMLI) has been extensively investigated. The SCMLI can effectively increase the number of output levels with switched capacitors and boost the input voltage without any bulky transformer or inductor. The SC based T-type topology was presented with nine-level output voltage, which can maintain the voltage balance of capacitors without auxiliary balance circuits. However, two symmetrical DC sources are required to regulate the voltage of the clamping capacitors. A back-end H-bridge (HB) is used for generating negative voltage levels. However, four switches on the HB must withstand the peak output voltage, making it unsuitable for high-voltage applications. On the other hand, a single-DC source inverter without HB was presented. This topology has lower voltage stress on individual switches; however, as the number of voltage levels increases, the number of capacitors and power switches also increases. Two types of SC-based nine-level inverters with a single DC source were presented respectively. Both can achieve the voltage balance of capacitors without auxiliary circuits and control. Nonetheless, their voltage boost gain is only 2. The topology with a voltage boost gain of 4 and the least number of switches and capacitors was proposed. However, two of the total switches have to block the peak load voltage. A single-source inverter with a voltage boost gain of 4 was presented. The voltage stress of its switches does not exceed twice the input DC voltage, but the switch count is high.