This paper presents analysis, design, and implementation of an isolated grid-connected inverter for photovoltaic (PV) applications based on interleaved flyback converter topology operating in discontinuous current mode In today's PV inverter technology, the simple and the low-cost advantage of the flyback topology is promoted only at very low power as microinverter Therefore, the primary objective of this study is to design the flyback converter at high power and demonstrate its practicality with good performance as a central-type PV inverter For this purpose, an inverter system rated at 2 kW is developed by interleaving of only three flyback cells with added benefit of reduced size of passive filtering elements A simulation model is developed in the piecewise linear electrical circuit simulator Then, the design is verified and optimized for the best performance based on the simulation results Finally, a prototype at rated power is built and evaluated under the realistic conditions The efficiency of the inverter, the total harmonic distortion of the grid current, and the power factor are measured as 9016%, 442%, and 0998, respectively Consequently, it is demonstrated that the performance of the proposed system is comparable to the commercial isolated PV inverters in the market, but it may have some cost advantage

An Interleaved High-Power Flyback Inverter for Photovoltaic Applications

This paper presents an approach to efficiency optimization in digitally controlled flyback dc-dc converters over wide ranges of operating conditions. Efficiency is characterized and optimized based on power loss modeling and multivariable nonlinear constrained optimization over power-stage and controller parameters. A valley switching technique is adopted to reduce MOSFET turn-on switching loss in discontinuous conduction mode. An optimization procedure is formulated to minimize power loss weighted over a range of operating points, under a cost constraint. A lookup table-based digital controller is applied to achieve on-line efficiency optimization by programming switching frequencies and operating modes based on the efficiency optimization processes. The proposed on-line efficiency optimization approach is verified by experimental results on a low cost 65 W flyback dc-dc prototype.

Efficiency Optimization in Digitally Controlled Flyback DC–DC Converters Over Wide Ranges of Operating Conditions

Droop control by means of virtual resistance (VR) control loops can be applied to paralleled dc–dc converters for achieving autonomous equal power sharing. However, equal power sharing does not guarantee an efficient operation of the whole system. In order to achieve higher efficiency and lower energy losses, this paper proposes a tertiary control level including an optimization method for achieving efficient operation. As the efficiency of each converter changes with the output power, VR values are set as decision variables for modifying the power sharing ratio among converters. A genetic algorithm is used in searching for a global efficiency optimum. In addition, a secondary control level is added to regulate the output voltage drooped by the VRs. However, system dynamics is affected when shifting up/down the VR references. Therefore, a secondary control for system damping is proposed and applied for maintaining system stability. Hardware-in-the-loop simulations are conducted to validate the effectiveness of this method. The results show that the system efficiency is improved by using tertiary optimization control and the desired transient response is ensured with system damping secondary control.

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Tertiary and Secondary Control Levels for Efficiency Optimization and System Damping in Droop Controlled DC–DC Converters

This paper presents a new concept of module-integrated converters called PV balancers for photovoltaic applications. The proposed concept enables independent maximum power point tracking for each module, and dramatically decreases the requirements in terms of electrical rating, size, and manufacturing cost for power converters. The power rating of a PV balancer is less than 20% of its counterparts, and the manufacturing cost is thus significantly reduced. In this paper, two architectures of PV balancers are proposed, analyzed, realized, and verified through simulation and experimental results. It is anticipated that the proposed approach will be a low-cost solution for future photovoltaic power systems.

PV Balancers: Concept, Architectures, and Realization

In this paper, a weighted-efficiency enhancement control method for an interleaved flyback inverter using a synchronous rectifier is proposed based on photovoltaic ac modules. In this control method, the operation of the synchronous rectifier is classified according to the voltage spike across the main switch. When the voltage spike across main switch is lower than the rating voltage of main switch, the operation of the active clamp circuit is interrupted to reduce the switching loss of the auxiliary switch. At this time, the synchronous rectifier is operated for the zero-voltage switching of main switch. When the voltage spike across main switch is higher than the rating voltage of main switch, the active circuit is activated to reduce the voltage spike. The synchronous rectifier is operated to reduce the conduction loss of the secondary output diode. Thus, the switching loss of the main switch can be reduced in the low power region, and the weighted efficiency can be improved. A theoretical analysis and the design principles of the proposed method are provided. The method is validated through simulation and experimental results.

Weighted-Efficiency Enhancement Control for a Photovoltaic AC Module Interleaved Flyback Inverter Using a Synchronous Rectifier

This paper presents efficiency characterization and optimization in conventional flyback DC-DC converters over wide ranges of operating conditions based on power loss modeling and multi-variable non-linear constrained optimization over design parameters and controller parameters. A valley switching technique is applied to reduce MOSFET turn-on switching loss in discontinuous conduction mode (DCM). An optimization procedure is formulated to minimize power loss weighted over a range of operating points, under a cost constraint. Experimental results with a 65 W flyback prototype demonstrate that combined design-time optimization, and controller optimization can lead to significant efficiency improvements.

Efficiency characterization and optimization in flyback DC-DC converters

This paper presents a look-up table based digital control approach to on-line efficiency optimization over wide ranges of operating conditions. The proposed digital controller allows programmable modes of operation based on off-line efficiency characterization and optimization. Applied to a flyback dc-dc converter, the controller adopts valley switching in discontinuous conduction mode (DCM), which significantly reduces the MOSFET turn-on switching loss. Furthermore, solutions to the problem related to undesirable frequency hopping, commonly observed in other valley-switching control schemes, are presented. Dynamic operation of the controller demonstrates smooth mode transitions and consistent transient responses over various operating conditions. Experimental results show that efficiency of a low-cost 65-W flyback prototype exceeds 92% at all operating points over approximately 3:1 range of input voltages and 10:1 range of loads.

On-line efficiency optimization in flyback dc-dc converters over wide ranges of operating conditions

This paper presents simplified sensing and analog-to-digital (A/D) conversion techniques, targeting low-cost, low-pin-count controller implementation for digitally controlled flyback DC-DC converters with on-line efficiency optimization. Precise output voltage regulation is accomplished by sensing an auxiliary winding voltage on the primary side at the end of the diode conduction interval, thus eliminating the need for secondary-side circuitry and isolation in the feedback path. Furthermore, simple single-comparator analog-to-digital (A/D) conversion techniques are adopted for input current and input voltage sensing, allowing implementation of on-line efficiency optimization over wide ranges of operating points. Experimental results are shown for a digitally controlled 65-W flyback prototype.

Sang Hee Kang

Papers

Efficiency Optimization in Digitally Controlled Flyback DC–DC Converters Over Wide Ranges of Operating Conditions

Efficiency characterization and optimization in flyback DC-DC converters

On-line efficiency optimization in flyback dc-dc converters over wide ranges of operating conditions

Simplified sensing and A/D conversion for digitally controlled flyback DC-DC converters with on-line efficiency optimization