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Grid-connected converters employ LCL-filters, instead of simple inductors, because they allow lower inductances while reducing cost and size. Active damping, without dissipative elements, is preferred to passive damping for solving the associated stability problems. However, large variations in the grid inductance may compromise system stability, and this problem is more severe for parallel converters. This situation, typical of rural areas with solar and wind resources, calls for robust LCL-filter design. This paper proposes a design procedure with remarkable results under severe grid inductance variation. The procedure considers active damping using lead-lag network and capacitor current feedback. Passive damping is also discussed. The design flow, with little iteration and no complex algorithms, selects the proper ratios between the switching and resonance frequency, the grid and converter inductance, and the filter capacitance and total inductance. An estimation for the grid current total harmonic distortion (THD) is also proposed. Simulation and experiments validate the proposals. [ABSTRACT FROM PUBLISHER]

A curved switching surface (SS) for inverters control with superior characteristics is geometrically derived in this paper. In order to avoid inaccuracies introduced by simplification or assumptions, the analysis is performed entirely using a versatile geometrical method in the normalized domain. Unlike previous approaches, both the output voltage and the capacitor current are considered as varying references to establish a more accurate control law with enhanced performance. The proposed normalization technique provides remarkable insight into the behavior of system, the leading to a pure geometrical treatment that is general and applicable to any possible inverter. As a result of a rigorous analysis, a control law for inverters defined as natural SS is proposed and thoroughly characterized. In addition to the enhanced dynamic response, fixed-frequency operation is one of the key features of the proposed control scheme. In order to formally demonstrate fixed-frequency operation, a transformation from the natural SS to its pulsewidth modulation (PWM) equivalent is performed, revealing duality between boundary control using curved SS and traditional PWM. This is a significant advancement toward the unification and understanding of traditional modulation against modulation produced by curved SSs. Finally, an additional novel concept is explored: operation in mixed monopolar and bipolar mode using the natural SS. This new mixed operating mode overcomes physical limitations of the inverter structure in monopolar mode around the region of zero voltage cross (both the problem identification and solution are investigated). Experimental results of a 1.5-kVA inverter operating at fixed moderate frequency are presented to validate the natural SS performance, illustrate the benefits of the normalization technique, and demonstrate the monopolar and mixed operating mode. [ABSTRACT FROM AUTHOR]

A general procedure for the selection of a curved switching surface (SS) to control buck-type converters is presented in this letter. The analysis is based on the normalized representation of ideal SSs for different loading conditions. The normalization process leads to a unique representation of the SSs for any possible buck converter. A set of graphics in three dimensions is introduced to give a spatial sense of the behavior of the converter and its control requirements during transients. As a result of the investigation, a switching surface referred to in this letter as the natural unloaded SS is selected, providing excellent transient behavior and no overshoot during startup. For any buck converter with typical parameters, this control scheme produces, in one switching action, a minimum of 99 % of the desired output voltage. The general concept of using second-order SS is also geometrically analyzed in this letter to clarify its characteristic features and disadvantages. Experimental results for a typical buck converter are presented to illustrate the transient behavior of the converter during startup and sudden load changes. The results confirm the virtues of the control scheme. [ABSTRACT FROM AUTHOR]

Three-phase pulsewidth modulated converters are usually controlled to achieve sinusoidal currents in the grid side and tight voltage regulation in the dc-link capacitor. The implementation of linear controllers based on small-signal models is a well-established solution for these converters. However, the dynamic performance can be improved only to a limited extent and it deteriorates under different operating conditions which may result in the uncontrollability of the dc-bus voltage during transients. In this work, a state-plane model and control strategy are introduced to improve the dc-link dynamics for three-phase converters. The proposed method provides fast, reliable, and consistent transient responses under the entire operating range, including rectifier and inverter modes. First, a normalized model that describes the natural dynamic behavior of the converter is derived and represented in the state-plane. In this way, the dynamic evolution of the operating point is described in a graphical domain as circular trajectories with well-defined characteristics. The insight provided by these natural trajectories is used to develop a control strategy to improve the dc-link dynamics by selecting a unique combination of circular paths to solve the transients. As a result, the operating point follows well-defined trajectories to achieve fast and consistent dc-link voltage responses under a wide operating range. The effectiveness and feasibility of the proposed control strategy are validated by several simulation and experimental results. [ABSTRACT FROM AUTHOR]

Three-phase pulsewidth-modulated converters require closed-loop controllers to meet the performance requirements and to achieve reliable responses over the entire operating range. Conventionally, by using the synchronous reference frame (SRF) representation, different linear and nonlinear control strategies have been proposed to improve the converter's dynamic performance. However, the achieved improvements cannot be objectively assessed due to the absence of a performance limit standard reference of the converter's dynamics. In this article, the theoretical minimum time solutions for three-phase converters in the SRF are derived and analyzed, providing an objective reference point for the system's dynamic performance evaluation. The normalized dynamic model of the converter in the SRF is derived and combined with the time-optimal control principle to find the theoretical minimum transient time solutions. The state-plane representation of these minimum time responses enables a straightforward and comprehensive interpretation of the converter's dynamic behavior, and it simplifies the derivation of closed-form expressions that characterize the minimum time transient parameters. The introduced analysis, minimum time dynamic responses, derived transient's expressions, and dynamic performance evaluation are validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Wide-bandgap and silicon power semiconductor devices require advanced pulsewidth gate-driver capability to successfully convert power for high-performance operation. In this article, a simplified forward isolated resonant converter structure with an integrated planar transformer is proposed, a cost-effective solution which eliminates the need of an output filter inductor while rearranging the clamp circuit for reduced component ratings and voltage stress. The first design option uses a simple back-to-back Zener diode voltage clamp circuit for low-power dc–dc isolated power supply applications. The second design employs a lossless voltage-clamped converter with auxiliary energy-recovering winding with an application example for an isolated gate-driver power supply. Through analysis of the converter, the switching frequency has been selected in relationship to the transformer inductance and the equivalent circuit capacitance for the benefit of soft-switching power device transitions. The proposed resonant converter structure features an integrated planar transformer designed with a specific layer disposition to ensure low interwinding capacitance. The converter offers isolated supplies for a gate-driver power devices pair, such as half-bridge SiC devices. The simulation and experimental results are obtained from a gate-driver application platform to demonstrate the validity of the proposed isolated dc–dc converter design and integrated planar transformer with a total conversion efficiency of 87.7%. [ABSTRACT FROM AUTHOR]

Utility power lines voltage levels vary in different geographical locations, ranging from 208 to 480 Vrms. Power factor correction (PFC) converters are typically designed for a particular input voltage and do not accept a wide range of line voltages. This results in multiple product designs and revisions and increased production costs to accommodate the various ac voltage levels used around the world. A PFC converter capable of coping with a wide input voltage range, from 90 Vrms up to 530 Vrms, would significantly decrease costs and streamline development. In this article, an extended input voltage range PFC converter is proposed, which provides a high and almost flat efficiency curve throughout the entire operating voltage range with an 800 VDC bus voltage. The proposed converter has a flexible bridgeless structure with simple control, low-current ripples, low common mode noise, and startup inrush current handling capabilities. Theoretical analysis and waveforms are discussed and experimental results for a 2-kW prototype are given, thereby validating the expected behavior. The operation of the proposed extended input voltage range PFC converter is also compared to the traditional totem-pole PFC showing more than 1% efficiency improvements at full load and a higher power factor in the proposed design. [ABSTRACT FROM AUTHOR]

Fringing losses play a detrimental role in high-frequency transformers. The tendency toward higher power density and miniaturization of power converters leads to higher switching frequency and enforces the use of low-profile components such as gapped planar transformers. Due to the air gap, lower magnetizing inductance and better regulation of LLC (Lm, L2, Cs) can be achieved, but fringing fluxes can also be induced, resulting in extra magnetic losses and more hotspots. These losses are highly dependent on the frequency and the core material, so it is critical to model them for gapped planar transformers in LLC resonant converters, which operate at high frequencies and use ferrite material. In this article, fringing losses for gapped ferrite transformers in LLC converters are thoroughly modeled in order to provide a precise prediction regardless of the materials and core geometries. The proposed method provides an accurate and compact formula for predicting the fringing losses of planar transformers. This formula is obtained based on the finite-element method so as to consider and evaluate different design parameters. An LLC resonant converter with different planar transformers is implemented to show the compatibility of the proposed model. Experimental results show the higher accuracy of the proposed model compared to traditional ones and confirm that the proposed loss model can be applied to diverse core shapes. In addition, the gradient descent method is used to calibrate the theoretical and experimental results. Moreover, temperature deviations of the transformer due to the fringing losses are measured and evaluated both experimentally and theoretically to show the accuracy of the proposed loss formula. Due to the proposed model's higher accuracy, an improved design procedure for planar transformers is obtained, adding substantial value for design engineers. [ABSTRACT FROM AUTHOR]

In past years, LLC resonant converters have become a mainstream topology for dc–dc power conversion due to their advantages, such as the superior efficiency obtained with the soft switching of MOSFETs. In order to further improve the efficiency of the converter, synchronous rectification (SR) can be implemented as an alternative for diode rectification. As a result, the vast majority of the literature related to this field of study presents different LLC SR control algorithms, which aim to improve the operation of the rectification. Unlike prior work on SR controllers, this paper contributes to the area of power loss estimation using rectifier current equations (RCE). The developed method based on time-domain analysis of SR currents provides a new analytical framework to characterize the behavior of SR. Implications in SR power losses of different time delays are investigated using the developed loss estimation method. In addition, different converter design parameters, such as choice of inductance ratio, can be analyzed. The RCE captures the true discontinuous and complex behavior of SR, which is often oversimplified by the conventional first-harmonic approximation (FHA). As a result, the proposed method facilitates the design of LLC resonant converters and provides increased precision in SR power loss estimation when compared to FHA, and in a considerably faster fashion when compared with precise yet computationally intensive simulation software. This paper is validated with simulation and experimental results. [ABSTRACT FROM AUTHOR]

Boost-based converters are used in a variety of non-isolated step-up applications, such as power factor correctors, because of their simplicity. The power density of these converters can be increased in higher switching frequencies, which reduces the size of the magnetic elements. This increases the switching losses, but that can be solved by soft-switching techniques. This paper proposes a resonant bridgeless power factor correction converter that provides soft switching for all of the semiconductors. The proposed structure can provide zero voltage switching for the switches and zero current switching for the diodes. In the proposed structure, the input current is inherently sinusoidal with low total harmonic distortion, even with small inductances. Therefore, a high input power factor is achieved without requiring a current control loop in the circuit. This reduces the complexity of the control circuit. An experimental prototype has been constructed to investigate the validity of the claims. Experimental results show near-unity power factor, as well as $\text{2}\%$ efficiency improvement at full load when compared to a conventional interleaved boost converter with the same components. [ABSTRACT FROM AUTHOR]

Three-phase solar inverters can be operated with different modulation strategies such as sinusoidal pulsewidth modulation (SPWM), space vector modulation (SVM), and third-harmonic injection PWM (THIPWM). The selection of the modulation strategy can significantly affect photovoltaic (PV) leakage currents and power losses and compromise the inverter performance. This paper presents a detailed analysis of traditional and modified modulation effects on power semiconductor losses and PV leakage current. The research focuses on a three-phase three-level T-type neutral point clamped (3L-TNPC) solar inverter. A detailed exploration of traditional SPWM and SVM is presented, and a new modified modulation strategy referred to as two-triplen harmonic injection PWM (2THIPWM) is proposed, which can substitute the SVM scheme. Analytical expressions are derived for the proposed 2THIPWM strategy, which can be used for the SVM power-loss approximation. The comparison between 2THIPWM and SPWM shows a conduction-loss difference greater than the 5% normalized range. The modulation strategies are discussed with respect to the PV common mode current signature, and specific design techniques in modulation lead to reduced leakage currents for safety compliance. The theoretical and experimental results obtained using a 3L-TNPC solar inverter are presented to evaluate the modulation strategies and their impact on performance. [ABSTRACT FROM AUTHOR]

LLC resonant converters have gained popularity in a wide number of industrial applications due to their high efficiency and power density. Common applications of these converters are battery chargers, electric vehicles, and high-efficiency power supplies, which require tight output voltage regulations. However, traditional averaging techniques employed in pulse width modulation converters cannot be employed for LLCs. As a consequence, designing linear controllers for these types of converters requires complex analysis or applying empirical methods. This paper proposes a simple and straightforward methodology for designing linear controllers for LLC resonant converters. A dual-loop control scheme including an inner current loop and an outer voltage loop is introduced. A simplified second-order equivalent circuit is employed to derive all the relevant equations for designing the compensators. Simulations and experimental results using a 150-W platform are employed to validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

In the past two decades, the operating voltage of photovoltaic (PV) installation has increased in order to reduce current ratings and system installation costs. Such a large number of PV panels in series (e.g., 1000–1500 V systems) lead to wide variations of PV voltage due undesirable effects such as shading, soiling/dust, aging, and hot surface temperature. A partial shade would normally shut down solar inverters, unless a boost stage is added to the system input to ensure a proper dc-link operating voltage for the inverter. A boost input stage can double the input voltage operating range to extract maximum power under any possible shading and temperature condition. In this paper, a new PV string boost topology arrangement is proposed in the form of a miniboost used for three-phase grid-connected converters. The string miniboost increases the dc voltage range to extend power extraction under shading and low irradiance conditions and only requires to process a fraction of the rated power. The proposed cost-effective solution is further improved by using the inverter peak power envelope at boundary operation, resulting in expanded energy extraction beyond existing techniques. A design procedure to optimally size the miniboost and the peak-power envelope is presented along with a comparative analysis, under different irradiance levels, to illustrate the advantages in energy extraction obtained with the proposed PV string miniboost solution under low voltage and boundary operation. Furthermore, improved efficiency operation schemes and issues of common-mode current are discussed, and three mitigation design techniques intended for three-phase, three-level inverter transformerless topology are presented. Simulations and experimental results using a dual miniboost dc–dc stage and three-phase three-level neutral point clamped inverter are presented to validate the proposed dc-bus extension range and system design. [ABSTRACT FROM AUTHOR]

The use of $LLC$ resonant converters has gained popularity in multiple applications that require high conversion efficiency and galvanic isolation. In particular, many applications like portable devices, flat TVs, and electric vehicle battery chargers require demanding slim-profile packaging and enforce the use of planar transformers (PTs) with low-height, low leakage inductance, excellent thermal characteristics, and manufacturing simplicity. The main challenge in successfully designing $LLC$ converters with PT resides in controlling high-parasitic capacitances produced by large overlapping layers in PT windings. When the parasitic capacitances are not controlled, they severely impair the converters’ performance and regulation, and limit the application of PTs in high-frequency $LLC$ converters. This paper characterizes the PT capacitance issue in detail and proposes mitigation strategies to improve the performance of $LLC$ converters with PTs. A systematic analysis is performed, and six PT winding layouts are introduced and benchmarked with a traditional design. As a result of the investigation, an optimized structure is obtained, which minimizes both the interwinding capacitance and ac resistance, while improving the regulation performance of $LLC$ converters. Experimental measurements are presented and show a significant reduction of parasitic capacitance by up to 21.2 intra- and 16.6 interwinding capacitances, without compromising resistance. This substantial capacitance reduction has a tangible effect on the regulation performance of $LLC$ resonant converters. Experimental results of the proposed PT structure in a 1.2 kW $LLC$ resonant converter show a reduction in common-mode noise, extended output voltage regulation, and improved overall efficiency of the converter. [ABSTRACT FROM AUTHOR]

Compensation capacitors are important components of any resonant wireless power transfer (RWPT) systems and significantly affect the output power, operating frequency, and efficiency. Depending on the compensation network, these capacitors can be placed in series or parallel with the coils. The physical capacitor reduces reliability and increases the size and cost of RWPT systems. Self-resonant RWPT systems have been proposed to remove the physical capacitor, but have to work with high frequency, several megahertz, as reported in existing literatures. In this article, a new planar coil that utilizes parallel-connected multilayer Printed Circuit Boards (PCB) to generate large parasitic capacitance is proposed for RWPT. Both transmitter and receiver coils are made using PCB coils and a spiral winding layout is used to achieve the required inductance and capacitance. Benefit from large parasitic capacitance, the resonant frequency of the proposed RWPT system is reduced to below 200 kHz. Meanwhile, because of the elimination of physical capacitance, the proposed RWPT system has a compact structure, improved reliability, and reduced size, weight, and cost. A coil prototype is designed, constructed, and tested. Simulation and experimental investigations validate the effectiveness of the proposed design. [ABSTRACT FROM AUTHOR]

The widespread use of the LLC converter in dc–dc applications has increased the importance of small-signal modeling to control the converter below- to above-resonant frequency. The analysis of LLC converters is complicated, since they process the electrical energy through a high-frequency resonant tank that causes excessive nonlinearity. As a result of this complexity, small-signal modeling of the LLC converter is traditionally performed using empirical methods, iterative simulation approaches, or theory limited to the vicinity of the resonant frequency. Often, such approaches may lead to limited insight (just empirical trends) or low accuracy below and above resonance. This article proposes a new average small-signal modeling technique for the LLC converter, and it is performed in the time domain. The proposed technique is based on the analysis of the homopolarity cycle and accurately predicts not only the small-signal dynamic behavior of the LLC converter at resonance, but also below and above resonance. By using the homopolarity cycle, the theoretical analysis of the LLC converter is significantly simplified to a level that the small-signal dynamic behavior of the LLC converter is expressed by two second-order circuit models. Experimental and simulation results of a 650-W LLC converter are provided to verify the theoretical analyses and accuracy of the circuit models. The results have shown that the proposed small-signal circuit models can accurately predict the small-signal dynamic behaviors of the LLC converter from below- to above-resonant operations. [ABSTRACT FROM AUTHOR]

DC microgrids interconnect load-end converters and distributed renewable energy sources within efficient and reliable networks that can operate independently from the main grid. When load-side converters tightly regulate their output voltages, they behave as constant power loads (CPLs) from the standpoint of the source-end converters. CPLs can cause instability within the network, including large voltage drops or oscillations in the dc bus during transients, which can lead to the collapse of the dc bus. Traditionally, the stability of CPL-loaded dc microgrids relies on the addition of passive elements, usually leading to increase in dc-bus capacitance. In these scenarios, source-end converter controllers are usually linear dual-loop proportional–integral compensators, which exhibit a limited dynamic response. State-plane-based controllers have been proposed to improve the dynamic response of stand-alone power converters loaded by CPLs. However, the operation of these converters in the context of a microgrid, where they interact with other converters of slower response, has not been studied thoroughly. This work proposes the use of a fast state-plane controller to replace one of the system's source-side controllers in order to improve three aspects of the microgrid operation: resiliency under CPL's changes, load transient voltage regulation, and voltage transient recovery time. Since the converter is operating within a microgrid, the controller incorporates a traditional droop rule to enable current sharing with the rest of the converters of the network. The system performance improvement is analyzed mathematically for a linear model, and a parametric analysis is performed for a more detailed model. Simulations and experimental results of a microgrid with three converters feeding a CPL are provided for different transients. [ABSTRACT FROM AUTHOR]

This paper presents a fast selective harmonic current mitigation strategy for inverters with active power filter capabilities based on synchronous reference frames and two degrees-of-freedom internal model controllers. The advantage of this control strategy over the conventional proportional integral (PI) control solution is a significant increase in the speed of harmonic detection and mitigation. Furthermore, this control strategy reduces the computational burden when applied in a digital controller. These characteristics make this strategy desirable for applications, where fast/harmonic detection and mitigation are needed. Mathematical analysis and simulations are presented to corroborate the performance of the proposed controller strategy. Finally, the results of this proposal are verified in a 1 kW three-phase multifunctional inverter with harmonic compensation capabilities up to the 17th harmonic. [ABSTRACT FROM PUBLISHER]

Small wind energy conversion systems (WECSs) are becoming an attractive option for distributed energy generation. WECSs use permanent-magnet synchronous generators (PMSGs) directly coupled to the wind turbine and connected to the grid through a single-phase grid-tie converter. The loading produced on the dc link is characterized by large ripple currents at twice the grid frequency. These ripple currents are reflected through the dc bus into the PMSG, causing increased heating and ripple torque. In this paper, the PMSG inverter is used to control the dc-link voltage. In order to avoid reflecting the ripple currents into the PMSG, the feedback dc-link voltage is passed through a filter. The Butterworth filters, notch filters, antiresonant filter (ARF) and moving average filter (MAF) are considered. For a fair comparison, formulas are provided to tune the filter parameters so that dc-link voltage control will achieve the selected bandwidth. The different filtering options produce different levels of torque ripple reduction. The notch filter, ARF, and MAF obtain the best results and there is a tradeoff between the filter implementation complexity, bandwidth, overshoot, and the torque ripple reduction. Simulations and experiments using a 2.5-kW PMSG turbine generator validate the proposals. [ABSTRACT FROM AUTHOR]