Increasing demands for power supplies have contributed to the population of high-frequency ac (HFAC) power distribution system (PDS), and in order to increase the power capacity, multilevel inverters (MLIs) frequently serving as the high-frequency (HF) source-stage have obtained a prominent development. Existing MLIs commonly use more than one voltage source or a great number of power devices to enlarge the level numbers, and HF modulation (HFM) methods are usually adopted to decrease the total harmonic distortion (THD). All of these have increased the complexity and decreased the efficiency for the conversion from dc to HF ac. In this paper, a nine-level inverter employing only one input source and fewer components is proposed for HFAC PDS. It makes full use of the conversion of series and parallel connections of one voltage source and two capacitors to realize nine output levels, thus lower THD can be obtained without HFM methods. The voltage stress on power devices is relatively relieved, which has broadened its range of applications as well. Moreover, the proposed nine-level inverter is equipped with the inherent self-voltage balancing ability, thus the modulation algorithm gets simplified. The circuit structure, modulation method, capacitor calculation, loss analysis, and performance comparisons are presented in this paper, and all the superior performances of the proposed nine-level inverter are verified by simulation and experimental prototypes with rated output power of 200 W. The accordance of theoretical analysis, simulation, and experimental results confirms the feasibility of proposed nine-level inverter.

In conventional hybrid multilevel inverters (MLIs), the voltage balance controls of floating capacitors are usually realized by redundant switching states using high-frequency modulation. The switching frequency, as well as the switching loss, is hence increased, and the topologies are limited to low-frequency output occasions as a result. In switched-capacitor MLIs (SCMLIs), the capacitors are self-balanced, since they are charged to fixed voltage levels by the input source directly or indirectly. By integrating switched-capacitor techniques into conventional hybrid MLIs, a pair of symmetric/asymmetric hybrid MLIs is proposed in this paper with boost ability. Complicated balance controls are avoided for capacitors voltage, thus the modulation gets simplified. Compared with SCMLIs, the backend H-bridges that withstand the accumulated voltage stress are removed. The peak inverse voltages of devices are decreased, thus only low-voltage components are needed. Meanwhile, the component numbers are reduced greatly. All these advantages contribute to high efficiency and reduced costs. Consequently, the proposed hybrid MLIs are especially appropriate for synthesizing a staircase output directly from a input source of low-voltage. Analyses and their operational principles are firstly presented for the proposed topologies. Comparisons are then carried out to illustrate their superiority. To evaluate the performance, simulation and experimental prototypes are implemented, whose results confirm the feasibility of the proposed hybrid MLIs.

A novel single-phase nine-level switched-capacitor inverter (9LSCI) is presented with quadruple boost ability and reduced components. The proposed topology with single dc source employs only eight switches to realize nine-level output, self-voltage balance of capacitors, quadruple boost and inductive-load ability, thus, the effective cost is cut down compared to other switched-capacitor multilevel inverters (SCMLIs). Different from other SCMLIs, the proposed 9LSCI has no need for back-end H-bridge, of which four switches need to withstand the peak voltage of output. Hence, total standing voltage can be reduced. The operation principles containing the self-voltage balance of capacitors are described in detail. The quantitative comparisons, modified cost function, as well as the loss evaluations are examined in depth. Finally, multicarrier phase disposition pulsewidth modulation method is adopted and a laboratory prototype is implemented with the rated output of 220 V–500 W. The experimental results also verify the feasibility of the proposed topology.

This paper proposed a single-source multilevel inverter (MLI) based on the novel K-type unit (KTU). The 13-level topology with two KTUs and 1.5 voltage gain is firstly introduced and its operation modes at different output levels are described in the paper. The two capacitors in series connection of each KTU can get self-voltage balance due to their symmetric operation in a cycle, reducing the complexity of control compared with conventional MLIs. The analysis of self-balance and capacitance calculation are given in detail. Afterward, the single-source generalized structure equipped with more KTUs is presented for increasing output levels. The output levels are significantly increased with additional KTUs, and the voltage gain rises as well. Moreover, through the comparative study against other MLI topologies proposed in recent years, the advantages of the proposed KTU topology are indicated in the aspects for reduced components, self-balance, voltage stress, and overall cost. Finally, the 13-level simulation and 1kVA experimental prototype with fundamental frequency modulation are implemented to verify the feasibility and transient performance of the proposed topology.

In this paper, switched Z-source/quasi-Z-source dc-dc converters (SZSC/SQZSCs) are proposed for the photovoltaic (PV) grid-connected power system, where the high step-up dc-dc converters are required to boost the low voltage to high voltage. The boost factor is increased by adding another one switch and diode to the output terminals of traditional Z-source/quasi-Z-source dc-dc converters. Not only does the output capacitor function as the filter capacitor; it is also connected in series into the inductors' charging loops when both switches are turned on. Compared with existing Z-source based structures, higher boost factor is realized through a small duty cycle (smaller than 0.25). On the one hand, the instability caused by the saturation of the inductors can be avoided. On the other hand, a larger range can be reserved for the modulation index of the backend H-bridge when they are used for the dc-ac conversion. Moreover, much fewer passive components are employed when compared with the recently proposed hybrid 3-Z-network topologies that have the same voltage gain, which can enhance the power density and decrease the cost. The performances of the proposed converters, including their operational principles in continuous and discontinuous current modes, voltage and current parameters of components, and impacts of parasitic parameters, are analyzed. The simulation and experimental results are given to verify the aforementioned characteristics and theoretical analysis.