Over the last decade, energy demand from the power grid has increased significantly due to the increasing number of users and the emergence of high-power industries. This has led to a significant increase in global emissions with conventional energy generation. Therefore, the penetration of renewable energy resources into the power grid has increased significantly. Photovoltaic systems have become the most popular resources as their protentional is enormous, thus, the worldwide installed PV capacity has increased to more than 635 gigawatts (GW), covering approximately 2% of the global electricity demand. Power electronics are an essential part of photovoltaic generation; the drive for efficient power electronic converters is gaining more and more momentum. Presently, multilevel inverters (MLI) have become more attractive to researchers compared to two-level inverters due to their abilities to provide lower electromagnetic interference, higher efficiency, and larger DC link voltages. This paper reviews multilevel inverters based on their classifications, development, and challenges with practical recommendations in utilizing them in renewable energy systems. Moreover, PV systems with various maximum power point tracking (MPPT) methods have been extensively considered in this paper as well. The importance and the development of a modified multilevel inverter are also highlighted in this review. In general, this paper focuses on utilizing multilevel inverters for PV systems to motivate and guide society to focus on inventing an efficient and economical multilevel inverter that has the combined capabilities of these converters reported in the literature.

With the rapid development of modern energy applications such as renewable energy, PV systems, electric vehicles, and smart grids, DC-DC converters have become the key component to meet strict industrial demands. More advanced converters are effective in minimizing switching losses and providing an efficient energy conversion; nonetheless, the main challenge is to provide a single converter that has all the required features to deliver efficient energy for different types of modern energy systems and energy storage system integrations. This paper reviews multilevel, bidirectional, and resonant converters with respect to their constructions, classifications, merits, demerits, combined topologies, applications, and challenges; practical recommendations were also made to deliver clear ideas of the recent challenges and limited capabilities of these three converters to guide society on improving and providing a new, efficient, and economic converter that meets the strict demands of modern energy system integrations. The needs of other industrial applications, as well as the number of used elements for size and weight reduction, were also considered to achieve a power circuit that can effectively address the identified limitations. In brief, integrated bidirectional resonant DC-DC converters and multilevel inverters are expected to be well suited and highly demanded in various applications in the near future. Due to their highlighted merits, more studies are necessary for achieving a perfect level of reducing losses and components.

Studies on resonant power converters (RPCs) have received much attention due to the rapid growth of their potential in modern power and renewable energy applications such as photovoltaic, electric vehicles, wind, and fuel cells. Consequently, a significant number of studies focusing on RPC topologies for renewable energy applications is available. Generally, these studies have addressed several aspects, such as the development of their soft switching feature, smooth waveforms, high power density, and high efficiency. With this in mind, the present paper aims to review the development of the RPCs, challenges in their development, and comparison between common topologies as highlighted in literature. In addition, the use of RPCs in various applications of renewable energy is highlighted, focusing on some of the recently utilized topologies based on their constructions and achievements. There are still several issues and challenges in research that need to be considered for future improvement of the RPC performance in renewable energy applications. The improvement may require modifications on the circuit design or control strategies.

This paper presents proposal of a five-level LLC resonant DC–DC converter design procedure for battery chargers. The five-level inverter side of the proposed converter is connected to a transform-less LLC resonant tank to ensure operating at high frequency and achieve soft switching. The proposed converter has less weight, size, and cost. It is also much simpler in terms of implementation, and has smooth energy conversion to the load. The proposed converter is designed to work within the range close to the resonant frequency, to ensure higher power density and efficiency. Thus, the range of operating frequency is set to be (91 kHz < fsw < 110 kHz), while the LLC parameters is designed to achieve resonant frequency fr = 100 kHz. Therefore, it is designed to achieve zero voltage switching (ZVS) for all switches, which enhances the efficiency as well. The theoretical analysis outcomes were confirmed by simulation studies conducted using MATLAB/SIMULINK. An experimental model was also developed and validated with 100 VDC input voltage, which delivered output power of 100 W, 48 V, with efficiency around 96.9%. Selected findings are presented to confirm the effectiveness of the suggested converter.

The design and analysis of a five-level cascaded LLC resonant tank converter was presented in this study. The proposed converter steps-down the voltage, and the construction of the designed converter and the tank selected parameters forces the input impedance of the resonant tank to behave as a capacitor. Besides, the lack of any transformer in this design is beneficial in terms of cost reduction and the number of elements. The performance of the proposed converter was evaluated in a MATLAB-SIMULINK platform and based on the outcome of testing three different switching frequencies and measuring the ZCS for all switches, there was an agreement between the simulation and theoretical findings. Hence, the results conformed the ability of the proposed converter to provide a wide range of load operation (0.8 ≤ Fn ≥ 1.6) with input voltage of almost 400V and output voltage of 48V.