A low cost high efficiency inverter for photovoltaic AC module application

The unfolding-type flyback inverter operating in Discontinuous Conduction Mode (DCM) has been previously investigated as a low cost solution for a photovoltaic AC module application. The aim of the present work is to improve the efficiency of the scheme by adopting Continuous Conduction Mode (CCM) operation. The paper presents the control modeling and analysis of the flyback-CCM inverter and discusses its control challenges. These include 1) the presence of a right-half plane (RHP) zero in the direct output current control transfer function, 2) the inevitable DCM operation when the instantaneous AC voltage is low, and 3) issues associated with the large variation in the power level. An indirect average current control scheme to avoid the problem of the RHP zero has been proposed and its effectiveness verified experimentally. The results show an improvement of 7.4% in the weighted California efficiency over the benchmark flyback-DCM scheme for a 200W PV panel. The total harmonics distortion and output power factor satisfy IEC61727. Practical implementation problems faced in the sensing of current are also addressed.