The single-phase five-level transformer-less inverters have been gradually applied in photovoltaic (PV) grid-connected systems due to low voltage stress and excellent waveform quality. However, they also suffer from serious leakage current as conventional three-level inverters. In order to reduce the leakage current, a single-phase five-level transformer-less inverter is proposed in this paper. The proposed inverter guarantees the common-mode (CM) voltage is clamped to a half of the dc-link voltage when outputting ${\pm V_{dc}/2}$ and zero voltage levels. Consequently, the high-frequent CM voltage is avoided and the leakage current is suppressed. The voltage stresses of eight switches and two flying capacitors are only Vdc/4, thus the system investment cost and total losses are reduced. The conduction and switching losses are analyzed detailly and a comparison with some existing topologies is also made. Finally, the simulations and experiment are carried out based on a 1 kW prototype. The results show that the proposed single-phase five-level inverter can obtain both excellent differential-mode (DM) and CM performance.

Single-phase full-bridge transformerless topologies, such as the H5, H6, or the highly efficient and reliable inverter concept (HERIC) topologies, are commonly used for leakage current suppression for photovoltaic (PV) applications. The main derivation methodology of full-bridge topologies has been used based on both a DC-based decoupling model and an AC-based decoupling model. However, this methodology is not suited to the search for all possible topologies, and cannot verify whether they are inclusive. Part I of this paper will propose a new topology derivation methodology based on unipolar sinusoidal pulse width modulation (USPWM) to search all possible full-bridge topologies for leakage current suppression. First of all, a unified circuit model is proposed, instead of the DC- and AC-based models. Secondly, a mathematic method called the MN principle is then proposed to search for all possible topologies, and a derivation procedure is provided. It was verified that all existing topologies could be found using the proposed method; furthermore, seven new topologies were derived. The proposed topology derivation methodology is extended to search topologies under Double-Frequency USPWM (DFUSPWM). Twenty topologies under USPWM and four topologies under DFUSPWM have been derived.

This paper proposes a topology derivation methodology to achieve small leakage current or zero leakage current for photovoltaic application. The core of the proposed method is a unified topology model and MN principle to show how to derive all possible topologies based on unipolar sinusoidal pulse width modulation (USPWM) and double-frequency USPWM (DFUSPWM). Part II of this paper discusses the topology synthetization method to achieve zero leakage current. Two types of neutral point clamped (NPC) topologies based on USPWM and DFUSPWM are elaborated. Two possible connections for the NPC cell are introduced, and detailed NPC topology derivation procedures are also provided. All existing NPC topologies are derived, and twenty-two new NPC topologies are found based on the new topology derivation methodology for a single-phase, full-bridge, transformerless inverter.