Single-stage voltage step-up inverters, such as the Dual Boost Inverter (DBI), have a large operating range imposed by the high step-up voltage ratio, which together with the converter of non-linearities, makes them a challenge to control. This is particularly the case for grid-connected applications, where several cascaded and independent control loops are necessary for each converter of the DBI. This paper presents a global current control method based on a combination of a linear proportional resonant controller and a non-linear sliding mode controller that simplifies the controller design and implementation. The proposed control method is validated using a grid-connected laboratory prototype. Experimental results show the correct performance of the controller and compliance with power quality standards.

Photovoltaic (PV) microinverters have grown rapidly in the small-scale PV market, where typical two-stage converters are used to connect one PV module to the single-phase AC grid. This configuration achieves better performance in terms of energy yield compared with other PV configurations. However, the conversion efficiency of a two-stage system is the main drawback, especially when a high-voltage gain effort is required. In this context, single-stage microinverter topologies have been recently proposed since only one power conversion stage is required to extract the maximum power of the PV module and inject the AC power to the grid. This single-stage configuration allows considerable improvement of the overall efficiency of microinverters by reducing the number of elements in the system. However, the main challenge of these topologies is their control, since all variables of the converter are composed by the AC waveform with DC-bias. In this paper, four control strategies are analyzed for the mainstream single-stage topology, which is the dual-boost inverter (DBI). Classical linear control and three non-linear strategies, namely finite control set–model predictive control, flatness-based control, and sliding mode control, are detailed. The main contribution of this work is a complete comparison of the control strategies, to give insights into the most suitable control strategy for the DBI in PV microinverter application.

The Differential Boost Inverter (DBI) is a single-stage step-up topology composed of two bidirectional boost converters whose outputs are connected in differential mode to generate an ac output voltage. The DBI poses significant modeling challenges, since all variables operate with both dc and ac components, requiring a highly complex dynamical model to accurately capture its behavior. Also, the operating range of this converter is restricted both in the modulation indexes, which is limited in the interval between 0 and 1, and in the capacitor voltage which must always be above the input voltage. In addition, a large harmonic content in the converter variables when working at certain operating points can increase the Total Harmonic Distortion (THD), degrading the performance. This work presents a model that considers both the dc and fundamental ac components of the converter variables. Based on this model, the allowable operating ranges for the modulation indexes and capacitor voltages are defined. Furthermore, the model is extended to determine the harmonic content of the capacitor voltage enabling the identification of the operating ranges in which a low THD is achieved. The proposed analysis introduces a flexible dc component in the modulation, without assuming a fixed dc bias, which broadens the valid operating region and enhances the potential for THD minimization. Finally, a DBI prototype has been built to validate the model and analysis, with experimental results confirming the mathematical relationships at representative operating points.