## 1 Introduction  
The increasing demand for the use of electrical energy, taking place in the recent years, has led to serious concerns about sustainability and the related environmental issues. Consequently, a great attention is being given to the production of electrical energy using renewable energy resources such as photovoltaic (PV) generators and wind turbines due to their abundance and almost zero environmental impacts [1]. Therefore, distributed renewable generation of electricity both in small-scale and large-scale power systems is gaining much interest in the recent years. In particular, PV energy has been wide spread during the last ten years due to the reduction in the cost of PV modules, increase of their efficiency and easy maintenance [2, 3]. Although the basic PV system principles and elements remain the same, three main types of of PV systems can be distinguished: stand-alone, grid-connected, and hybrid. Grid-connected PV systems account for more than 99% of the PV-installed capacity compared to stand-alone systems [4]. Grid connection of PV systems requires a highly efficient inverter which is considered the main element in PV installations. Grid-connected PV systems can be built by using either two-stage or single-stage approaches. In the two-stage approach an intermediate DC-DC power stage is inserted between the PV module and the DC-AC grid-tied inverter [5; 6; 7]. This intermediate stage is responsible for decoupling the PV system operating point from the PV inverter grid control by enabling a suitable average voltage at the inverter DC side. In two-stage grid-connected PV systems, each stage has its independent task and the control design is relatively simple for each stage. The single-stage grid-connected PV system consists of a PV generator, a DC-link capacitor and DC-AC inverter used both to extract the maximum power from the PV generator and to inject a sinusoidal current into the grid with near unity power factor [8; 9]. In conventional PV systems, a single inverter, called a central inverter, is connected with several PV panels and if a partial shading takes place, the efficiency of the entire PV field is degraded. This main disadvantage of central inverters is overcome by using microinverters that are suitable for small-scale residential uses where each PV module is connected to an inverter separately forming an AC module with its own Maximum Power Point Tracking MPPT control. Therefore, the total harvested power increases under a partial shadowing condition since the power of each PV panel is controlled independently and separately from the others. Moreover, single-stage microinverters are connected to the AC side directly, hence eliminating the DC cable losses and therefore improving further the system efficiency. Voltage step-down DC-AC inverters based on H-bridge topology are conventionally used to connect a PV system to the grid generating a sinusoidal output voltage for supplying an AC load. These kinds of inverters are conventionally preferred because they have inherent control-to-output linearity in large-signal sense. Nevertheless, in these kinds of inverters, the maximum output voltage at the AC side must be always smaller than the DC voltage. Therefore, in single-stage cases based on voltage source H-bridge topology, many strings of PV panels are connected in series to increase the output voltage [10]. The efficiency of the two-stage system is negatively affected by using increased component count and switching devices. Moreover, the size and the cost are higher compared to single-stage grid-connected PV systems. The low reliability is another shortcoming of the two-stage approach. To reduce the overall cost and complexity, single-stage structures can be used. Single-stage grid-connected DC-AC conversion systems with boosting voltage capability has recently attracted the attention of many researchers. Single-stage structures of microinverters not only perform DC-AC conversion but also perform voltage boosting.