This paper presents a new microinverter topology that is intended for single-phase grid-connected PV systems. The proposed microinverter topology is based on a flyback converter, where an extra switch is added to separate the decoupling capacitor from the PV Module, which allows for a high voltage and voltage ripples across its terminals. This results in reducing the power decoupling required capacitance. In this manner, long life-time low power density film capacitors can be used instead of life-time limited high power density electrolytic capacitors, resulting in remarkable increase of microinverter's lifespan. The main advantages of the proposed topology are summarized as: 1) eliminating the double-frequency power ripple using a small film capacitor; 2) using long lifetime film capacitors, which will improve the reliability of the inverter; and 3) requiring no additional circuitry to manage the transformer leakage energy. A 100-W microinverter prototype was built to verify the proposed topology. Experimental results show that the proposed topology and its control scheme can realize the power decoupling, while maintaining very good conversion efficiency numbers.

A Single-Stage Microinverter Without Using Eletrolytic Capacitors

The unfolding-type flyback inverter operating in discontinuous conduction mode (DCM) is popular as a low-cost solution for a photovoltaic (PV) ac module application. This paper aims to improve the efficiency by using a scheme based on continuous conduction mode (CCM) for this application. Design issues, both for the power scheme and the control scheme, are identified and trade-offs investigated. An open-loop control of the secondary current, based on feedback control of the primary current, is proposed in order to bypass the difficulties posed by the moving right half plane zero in the duty cycle to secondary current transfer function. The results presented show an improvement of 8% in California efficiency compared to the benchmark DCM scheme for a 200-W PV module application. The output power quality at rated power level is capable of meeting IEC61727 requirements. The stability of the flyback inverter in CCM has been verified at selected working conditions.

A Low Cost Flyback CCM Inverter for AC Module Application

Modeling and simulation of photovoltaic (PV) power systems have become increasingly important with wide acceptance and integration of solar energy in modern electric grids. The transcendental nonlinear equations describing the PV generator, which are coupled with the detailed switching models of the power electronic converters, generally result in slow and inefficient simulations, especially when long-term analyses are required. This paper focuses on simple and efficient modeling approaches that are suitable for long-term and large PV system analyses. This study provides a simplified PV-cell model and its parameterization, guaranteeing that the I-V characteristic curves pass through the typical points given in manufacturers' datasheets. Furthermore, several power interface models are provided for fast simulation purpose. A classical two-stage power processing system with intermediate dc link used as a string inverter, as well as a single-stage conversion unit used in distributed module-dedicated PV applications, are taken as application examples. The generalized modeling approach is thoroughly evaluated by comparing the simulation results with the experimental data of a practical 2.4-kW grid-tied PV solar unit. The proposed methodology is shown to have advantages over conventional modeling approaches to simulate long-term grid-tied operation.

Efficient Approaches for Modeling and Simulating Photovoltaic Power Systems

This paper presents a novel zero-voltage switching (ZVS) approach to a grid-connected single-stage flyback inverter. The soft-switching of the primary switch is achieved by allowing negative current from the grid side through bidirectional switches placed on the secondary side of the transformer. Basically, the negative current discharges the metal-oxide-semiconductor field-effect transistor's output capacitor, thereby allowing turn on of the primary switch under zero voltage. To optimize the amount of reactive current required to achieve ZVS, a variable-frequency control scheme is implemented over the line cycle. In addition, the bidirectional switches on the secondary side of the transformer have ZVS during the turn- on times. Therefore, the switching losses of the bidirectional switches are negligible. A 250-W prototype has been implemented to validate the proposed scheme. Experimental results confirm the feasibility and superior performance of the converter compared with the conventional flyback inverter.

Analysis and Implementation of a Single-Stage Flyback PV Microinverter With Soft Switching

Boundary conduction mode (BCM) and discontinuous conduction mode (DCM) control strategies are widely used for the flyback microinverter. The BCM and DCM control strategies are investigated for the interleaved flyback microinverter concentrating on the loss analysis under different load conditions. These two control strategies have different impact on the loss distribution and thus the efficiency of the flyback microinverter. For the interleaved flyback microinverter, the dominant losses with heavy load include the conduction loss of the power MOSFETs and diodes, and the loss of the transformer; while the dominant losses with light load include the gate driving loss, the turn-off loss of the power MOSFETs and the transformer core loss. Based on the loss analysis, a new hybrid control strategy combing the two-phase DCM and one-phase DCM control is proposed to improve the efficiency in wide load range by reducing the dominant losses depending on the load current. The optimal design method based on the boundary condition of the hybrid control is also presented. The experimental results verify the benefits of the proposed control.

An Optimal Control Method for Photovoltaic Grid-Tied-Interleaved Flyback Microinverters to Achieve High Efficiency in Wide Load Range

This paper presents an investigation into the use of a flyback based grid-inverter operating in continuous conduction mode (CCM) for a photovoltaic AC module application. Detailed analysis of operation, including evaluation of achievable power factor and current stresses, is presented. A design approach for the flyback-CCM inverter is presented and discussed. The performance of the proposed inverter is compared with a benchmark flyback-DCM (discontinuous conduction mode) scheme. Simulation results demonstrate the advantages and limitations of the proposed scheme. The impact of the bifurcation and subharmonics observed at low power levels are discussed. A mitigation method is proposed for this problem and its operation verified.

A flyback-CCM inverter scheme 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.

Yanlin Li

Papers

A Low Cost Flyback CCM Inverter for AC Module Application

A flyback-CCM inverter scheme for photovoltaic AC module application

A low cost high efficiency inverter for photovoltaic AC module application