Flyback Micro-inverter Research Articles

Optimal Design of a Flyback Microinverter Operating under Discontinuous-Boundary Conduction Mode (DBCM)

The flyback converter has been widely used in Photovoltaic microinverters, operating either in Discontinuous, Boundary, or Continuous Conduction Mode (DCM, BCM, CCM). The recently proposed hybrid DBCM operation inherits the merits of both DCM and BCM. In this work, the necessary analytical equations describing the converter operation for any given condition under DBCM are derived, and are needed due to the hybrid nature of the modulation strategy during each sinusoidal wave. Based on this analysis, a design optimization sequence used to maximize the weighted efficiency of the inverter under DBCM is then applied. The design procedure is based on a power loss analysis for each converter component and focuses on the appropriate selection of the converter parameters. To achieve this, accurate, fully parameterized loss models of the converter components are implemented. The power loss analysis is then validated by applying the optimization methodology to build an experimental prototype operating in DBCM.

LCL filter design for grid-connected single-phase flyback microinverter: a step by step guide

<span lang="EN-US">Recently, <em>LCL</em> has become amongst the most attractive filter used for grid-connected flyback inverters. Nonetheless, the switching of power devices in the inverter configuration creates harmonics that affect the end application behavior and might shorten its lifetime. Furthermore, the resonance frequencies produced by the <em>LCL</em> network contribute to the system instability. This paper proposes a step-by-step guide to designing an <em>LCL</em> filter by considering several key aspects such as the resonance frequency and maximum current ripple. A single-phase grid-connected flyback microinverter with an <em>LCL</em> filter was designed then constructed in the MATLAB/Simulink environment. Several different parameter variations and damping solutions were used to analyze the performance of the circuit. The simulation result shows a promising total harmonic distortion (THD) value below 5% and harmonic suppression up to 14%.</span>

An Efficient Grid-tied Flyback Micro-inverter with DCM Control Strategy

In two-stage micro-inverter photovoltaic (PV) applications, DC/DC converter is used to obtain the highest DC power from the PV module. In this type of inverter, the rising of voltage from the PV module to a grid voltage level is limited to a certain value. Moreover, the absence of the isolation between the input and output makes it is less efficient. For these reasons, an efficient single-stage grid-tied flyback PV micro-inverter with discontinuous conduction mode (DCM) control strategy is proposed to feed an alternating current (AC) to the main grid with a lower value of the total harmonic distortion (THD). The control strategy is based on a sine sinusoidal pulse width modulation (SPWM) technique to control the main switch of flyback inverter. Also, a simple perturb and observe (P&O) maximum power point tracking (MPPT) technique has been presented to obtain the MPP point from the PV module for any environmental conditions. The proposed control was verified using PSIM software and simulation results is obtained. The proposed control is tested under different weather conditions for solar irradiance and temperature, as a result, a pure sin wave current has been injected into the grid with a lower harmonics value. Finally, the small size, low cost and high reliability of single stage flyback micro-inverter is presented without the need for DC/DC converter.

An Analog Control Strategy With Multiplier-Less Power Calculation Circuit for Flyback Microinverter

This letter proposed an analog control strategy for a flyback microinverter, which can calculate the output power by multiplier-less power calculation circuit, while the maximum power point tracking is realized by controlling the maximum duty cycle of the main switch. The proposed analog control method is quite simple with low cost. It is applied with a 100-W single-phase microinverter in a discontinuous conduction mode, and the simulation and experimental results demonstrate its feasibility.

Performance optimisation of the grid‐connected flyback inverter under improved hybrid conduction mode

In the unfolding-type interleaved two-switch flyback inverter (ILTFI) operating in discontinuous conduction mode (DCM), the hybrid control strategy combining the one-phase DCM and the two-phase DCM has a significant impact on improving the efficiency under all load conditions. In the situation where the marginal power of this control strategy is increased, the converter may enter CCM. Consequently, the converter cannot track the reference current and the output current quality is notably decreased. To tackle this issue, a novel hybrid control method is proposed which varies the switching frequency based on the output power. The proposed approach is immune to the loss analysis and power losses of the components. Hence, it is independent of components selection and their characteristics. In this case, the smooth transition between the one-phase and the two-phase operation modes is guaranteed without affecting the output power quality and the stability of the converter in DCM. The control complexity of the proposed scheme is low and the converter can be easily controlled in DCM. The performance of the flyback microinverter with the proposed hybrid control scheme is verified by the simulation and the experimental results together with the loss analysis.

An improved control method to reduce harmonic level for a single phase grid-connected flyback micro-inverter of a small scale solar PV

One of an attractive solution for PV ac-module is flyback-type micro inverter since it has a number of advantages include simple control, stable current injection and potentially low-priced technique. But, it has high voltage ripple in the DC-link that may cause high harmonic distortion. Hence, a smooth DC voltage is required in order to maintain the total harmonic distortion within standard level. DC link voltage can be smoothened by using capacitor. However, the size of this capacitor is large and hence affects the weight and size of the inverter package. This paper proposes a control method to reduce harmonic level for flyback-type micro-inverter. MPPT (maximum power point tracker) technique used in this study is perturb & observe because it is the simplest online MPPT method with high efficiency. Several experiments are carried out to examine the effectiveness of the proposed method. The results show that the proposed method can reduce the harmonic level significantly and can be used as an alternative solution.

Photovoltaic Flyback Microinverter With Tertiary Winding Current Sensing

This paper presents a new low cost, non-invasive, and isolated current sensing technique for the grid-tied photovoltaic (PV) flyback microinverter. This is accomplished by using the flyback transformer itself as a current sensor, achieved by introducing a tertiary winding to the flyback transformer. The mathematical integration of the tertiary winding's open circuit voltage through a ground-clamped-integrator results in the sensing of the magnetizing current. Since the magnetizing current is a combination of both primary and secondary current, control of both grid current and maximum power point tracking (MPPT) is implemented by sensing only the magnetizing current. This allows the PV, primary, secondary, and the grid current loops to be free of any invasive current sensors. Moreover, controlling the magnetizing current provides an alternative solution to an inherent problem with continuous conduction mode (CCM), the control complexity. Linear ramping and de-ramping of the magnetizing current allows for a set–reset hysteresis control to be implemented, resulting in CCM control simplicity that is akin to the boundary and discontinuous conduction mode. A grid-tied microinverter prototype is presented for verification, achieving the following experimental result: 1.9% grid current THD, 0.9988 power factor, above 99% static MPPT efficiency and dynamic efficiency of 98.50%.

Photovoltaic flyback micro-inverter with power decoupling technique

This paper proposes a low-cost single-phase micro-inverter for grid-connected photovoltaic (PV) system. The lifetime of the conventional flyback micro-inverter is shortened, because lifetime of a large electrolyte capacitor is shortened. For this reason, the need for a large electrolyte capacitor is avoided by proposing power decoupling (PD) circuit. Dual advantages are achieved by proposed circuit, first high-power decoupling with small capacitances and other to protect the main MOSFET from spike voltage stress during turn off time without needing for additional a snubber circuit. Consequently, PD circuit is already used as a snubber circuit to absorb the leakage energy in the transformer which may destroy the switch and thus the voltage spike on the main MOSFET decreased. In addition, operating principle, modes, and control scheme of the proposed micro-inverter are discussed. As the simulation results, the input power ripple of the single-phase power fluctuation is under than 4%, unity power factor (P.F) and the total harmonic distortion (THD) of the proposed inverter output current is less than 5%. PSIM tool box is provided to simulate the proposed system and the simulation results are adequate.

A Hybrid Boost–Flyback/Flyback Microinverter for Photovoltaic Applications

For photovoltaic applications, the flyback microinverter with pseudo-dc-link is popular as a simple topology but brings large transformer turns ratio and thus large leakage inductance, which would deteriorate the converter efficiency. To solve this issue, based on the nonisolated pseudo-dc-link structure, this paper proposed a hybrid boost–flyback/flyback (BF/F) microinverter. This new topology is operated at the BF mode for the most segment of a half grid cycle and the F mode for the rest. During the BF mode, high voltage gain with low voltage stress is easily available in minimized transformer turns ratio. Besides, the leakage energy is recycled and the turn- off voltage spike of the main switch is clamped, as a result of a passive snubber inherently contained in this mode. Given that the BF mode is lack of step-down function, the F mode is developed to regulate the output voltage even for values lower than the input voltage. The operation and characteristic of the hybrid BF/F microinverter in boundary conduction mode are analyzed in detail, and the mathematical expression of reference current is derived theoretically to guarantee high power quality. Finally, a 240 W prototype was implemented to validate the theoretical analysis and the benefits of the proposed topology.

Modeling and Comparisons of Aggregated Flyback Microinverters in Aspect of Harmonic Resonances With the Grid

In this paper, performances on two kinds of aggregated flyback microinverters are compared in aspect of harmonic resonances, which are controlled by current feedback control and peak current control. Based on the Norton models of flyback microinverters obtained by a small-signal modeling approach, potential harmonic resonances with the grid are analyzed. Following that, an impedance-based stability criterion is proposed, where the resonance frequency can be calculated and predicted based on the derived Norton models and the system admittance matrix of aggregated microinverters. After comprehensive analysis and comparisons, it is found that the peak-current-control-based flyback microinverters has better performance on damping the harmonic resonances than the current feedback controlled one. Simulations and experiments are carried out for verifying the effectiveness of the presented models and the correctness of the analysis results.

Model-Based Current Sharing Approach for DCM Interleaved Flyback Micro-Inverter

Current sharing control is a challenge for a discontinuous conduction mode (DCM) micro-inverter based on interleaved flyback topology. To solve this problem, this study proposes a novel and systemic model-based approach. Firstly, an accurate fourth-order model is presented for the interleaved flyback circuit, which takes the two flybacks’ parameter mismatch and coupling into account. Secondly, based on the presented model, a continuous time sliding mode current controller is proposed to tackle the output imbalance caused by parameter mismatch, coupling and disturbance. The proposed controller is derived from the Lyapunov function without switching conditions. Finally, the effectiveness of the proposed model and control method is validated by simulation tests using MATLAB/SIMULINK. Simulation results show that the proposed approach improves the current sharing for the interleaved flyback micro-inverter when compared to the conventional current sharing approach.

Investigation of microinverter based on the two-switch DC-DC flyback converter topology

This paper presents an investigation of a novel scheme of the single stage flyback microinverter based on two-switch [1]. The purpose of this topology of microinverter is to transform the DC power generated by the DC voltage input to a standard electric grid AC voltage. The single stage two-switch DC-DC flyback microinverter for the case CL output filter ensures the isolation between the DC power sources and the AC sources by a flyback transformer; has high efficiency (more than 96%) if the parameters of dual two-transistor stage: M1 to M4 are chosen correctly and Total Harmonic Distortion THD less than 5 %. However, the design of this microinverter is characterized by a low number of components.

Impedance‐based analysis of grid harmonic interactions between aggregated flyback micro‐inverters and the grid

Grid harmonic interactions due to aggregated flyback micro-inverters are investigated in this study. An impedance-based model based on the Norton model of flyback micro-inverters, which are with quasi-resonant peak-current control, is obtained by adopting the small-signal modelling approach. As a supplement to the existing output impedance modelling of string inverters with linear controllers, further study on micro-inverters under non-linear control is addressed in this study. Based on the derived impedance model, the admittance matrix of aggregated micro-inverters connected to the grid is formulated and the impedance-based analysis of the system stability is also presented. Consequently, by modelling the output admittance of the quasi-resonant peak-current controlled flyback micro-inverters, harmonic interactions with a distorted grid can be effectively forecasted. Results obtained from modelling, analysis and verifications have shown that the proposed method is a simple and valid way of dealing with the harmonic quasi-resonance problems.

A Generalized and Flexible Control Scheme for Photovoltaic Grid-Tie Microinverters

In this paper, design and implementation of a flyback photovoltaic (PV) microinverter based on the direct digital synthesis (DDS) technique has been described for both the standalone and the grid-connected operation. The DDS technique adopted provides flexibility in the implementation of various control schemes of the PV microinverter on a simple low-cost digital signal processing type of microprocessor (dsPIC). As compared with the conventional look-up-table method used for generating sinusoidal output voltage waveforms by digital signal processing, a much higher resolution can be obtained in the voltage phase angle and magnitude owing to the adaptive nature of the look-up table implemented within the DDS architecture. The DDS technique is used in the implementation of all control schemes of a PV microinverter, such as maximum power point tracking (MPPT), phase-locked-loop (PLL), anti-islanding, and low-voltage ride-though (LVRT), with an integrated software run on a simple microcontroller. A dedicated computer simulation model is developed, where the PV panel model, the PLL in DQ reference frame, the MPPT algorithm, and the anti-islanding and LVRT features are all taken into account. The experimental results obtained on a 120-W PV flyback microinverter have verified the validity of the proposed technique for both the steady-state and the transient-state operation. The DDS technique is thus found to be quite convenient for application to module integrated converters.

Incorporation of Harmonic Injection in an Interleaved Flyback Inverter for the Implementation of an Active Anti-Islanding Technique

Over recent years, the output power of micro-inverters [module-incorporated inverter (MIC)] is progressively pushed to higher levels, following the current photovoltaic (PV) modules market trends. The interleaved Flyback inverter—under discontinuous conduction mode of operation (DCM)—is an appealing solution for PV applications which are based on MIC technology. This topology provides increased power level for distributed PV generation exploitation with simple control configuration, high efficiency, and reduced filter size. In this paper, the harmonic injection capability is incorporated into the interleaved Flyback inverter for the implementation of an active anti-islanding scheme. A suitable technical solution is proposed that bypasses the unfolding H-bridge, without affecting the active power generation of the inverter and without requiring any hardware modification. In addition, a mathematical model based on the instantaneous power balance theory is presented, being a powerful analysis tool for inverters operating under complete magnetic discharge such as the Flyback inverter (either conventional or interleaved) under DCM or boundary conduction mode of operation. The proposed model predicts accurately the steady-state operational behavior of the Flyback inverter under current harmonic injection, either in grid-tied or islanding operation. The proposed technical implementation method and mathematical model are verified through simulation and experimental results. Finally, an anti-islanding set-up based on the proposed harmonic injection technique is presented and implemented into the Flyback micro-inverter. The proposed set-up is verified through experimental results on various quality factor values, as well as for weak grid conditions.

Highly efficient flyback microinverter for grid-connected rooftop PV system

This paper proposes a high efficiency DC-DC flyback converter with a resonant full-bridge inverter to use in PV systems. The flyback converter is composed of a resonant active-clamp circuit that limits the voltage stress and provides zero voltage switching (ZVS) turn-on and turn-off of the power switches. Therefore, the switching losses of the high frequency primary switches are negligible. A resonant full-bridge inverter with ZVS of the high frequency switches is adopted to make the overall efficiency high. Moreover, using a film capacitor in the DC link for the power decoupling, the lifespan of the microinverter is increased. A 250W prototype of the proposed microinverter has been implemented and the performance is analyzed with different loading condition. The feasibility analysis of the proposed topology for the PV modules in different weather condition confirms the superiority of the proposed microinverter for rooftop PV system compared with the existing topologies.

Modeling and Simulation of High-Efficiency Interleaved Flyback Micro-inverter For Photovoltaic Applications

PV generation is one of the fast growing renewable generation system, it helps to meet the ever-growing power demand. Among the various PV generation techniques available, the micro inverter system which finds its application in roof top systems is rated as one of the best option for maximum energy harvest from each solar panel. High step up dc-dc boost converter with Interleaved Fly back boost converter topology and Maximum power point tracking method (MPPT) can used to vary the duty cycle so as to improve the efficiency and extract maximum available power from each panel in both healthy and partial shading condition. With this background, this paper discusses the advantage of micro-inverter system over the existing string-inverter technology and tries to validate the advantage of fly back boost converter technology with hardware results.

Hybrid Discontinuous/Boundary Conduction Mode of Flyback Microinverter for AC–PV Modules

The current-source flyback inverter has been proven an excellent solution for ac–PV modules. The two principal modes of operation of this converter are the discontinuous conduction mode (DCM) and the boundary conduction mode (BCM). Although BCM offers higher power density, it suffers at lower power levels, because of the significantly increased switching frequency that consequently leads to higher switching losses and lower overall converter efficiency. In this paper, a hybrid operating mode is presented, that combines both DCM and BCM during a utility grid period, so as to improve the converter performance as well as the efficiency and the power density on a wider power range. The mathematical analysis that describes the control specifics especially for a smooth transition between the two modes of operation is presented and demonstrated through computer simulations and experimental results on a laboratory prototype.

Single-Stage Grid-Connected Flyback Microinverter Operating in DCM for Photovoltaic AC Modules

this paper, a single-stage grid-connected flyback microinverter is proposed. The proposed flyback microinverter has some advantages such as high voltage gain, high efficiency, low cost, small size, simple control and high power factor. The proposed system is used to connect the PV panel to the grid with achieving maximum Power Point Tracking (MPPT) control. The converter operates in DCM to inject a sinusoidal current into the grid with unity power factor. A complete system has been simulated using PSIM program. The simulation results are obtained to validate the system. Keywords-Stage, grid-connected, Microinverter, Flyback, DCM, MPPT.

Control Strategy of Flyback Microinverter With Hybrid Mode for PV AC Modules

This paper presents a control strategy of a flyback microinverter with hybrid operation mode for photovoltaic ac modules. The proposed control strategy consists of two components: the proportional-resonant (PR) controller with the harmonic compensator (HC) and the hybrid nominal duty ratio. Compared to the conventional control strategy using the proportional-integral controller, the PR controller with HC provides a higher system gain at the fundamental and harmonic frequencies of the grid without using a high proportional gain in both operation modes. Then, it enhances the tracking speed and disturbance rejection performances satisfying the desired stability. Moreover, by applying the hybrid nominal duty ratio yielded from the proposed operation mode selection, the disturbance rejection is achieved more effectively, and the control burden is reduced. Finally, the simulation and experimental results were shown to verify the tracking speed and disturbance rejection performances of the proposed control strategy.