Beatriz Borges

Resonant Contactless Energy Transfer With Improved Efficiency

This paper describes the theoretical and experimental results achieved in optimizing the application of the series loaded series resonant converter for contactless energy transfer. The main goal of this work is to define the power stage operation mode that guarantees the highest possible efficiency. The results suggest a method to select the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that efficiency improvement. The research clarifies also the effects of the physical separation between both halves of the ferromagnetic core on the characteristics of the transformer. It is shown that for practical values of the separation distance, the leakage inductance, being part of the resonant inductor, remains almost unchanged. Nevertheless, the current distribution between the primary and the secondary windings changes significantly due to the large variation of the magnetizing inductance. An approximation in the circuit analysis permits to obtain more rapidly the changing values of the converter parameters. The analysis results in a set of equations which solutions are presented graphically. The graphics show a shift of the best efficiency operation zone, compared to the converter with an ideally coupled transformer. Experimental results are presented confirming that expected tendency.

New Optimized Full-Bridge Single-Stage AC/DC Converters

The inclusion of a few additional diodes and passive elements in the high-frequency full-bridge ac-dc converter with galvanic isolation permits one to achieve sinusoidal input-current wave shaping and output-voltage regulation simultaneously without adding any auxiliary transistors. Recently, this procedure, together with an appropriate control process, has been used to obtain low-cost high-efficiency single-stage converters. In an attempt to improve the performance of such converters, this paper introduces three new single-stage full-bridge ac-dc topologies with some optimized characteristics and compares them with the ones of the existing full-bridge single-stage topologies. The approach used consists in the definition of the operating principles identifying the boost function for each topology, their operating limits, and the dependence between the two involved conversion processes. Experimental results for each topology were obtained in a 500-W modular laboratory prototype that was built with the necessary flexibility to allow the realization of each different topology. Comparison was evaluated in terms of input-current distortion, efficiency, and output-voltage disturbances that result from the input-current wave-shaping process.

Analysis and Design of a High-Efficiency Full-Bridge Single-Stage Converter With Reduced Auxiliary Components

This paper presents a single-stage circuit topology consisting of the association of a full-bridge isolated dc-dc converter and two input inductors and two input diodes connected to the mains network, in order to obtain an isolated ac/dc switch mode power supply, with sinusoidal input current. The proposed topology does not use an input bridge rectifier, common in similar applications. The current in the two input inductors can, therefore, flow in both directions. Consequently, the proposed topology equally distributes the current by the four-bridge transistors that provide four input parallel boost power factor correctors (PFCs). The use of the four-bridge transistors to obtain the PFC function and regulate the output voltage with galvanic isolation is a new technique that makes this topology unique, which also contributes to improve the converter efficiency. The definition of appropriate control strategies permitting the accurate simultaneous regulation of output voltage and input current is hereby described. The interdependency between these two conversion processes is completely analysed, allowing for useful design rules. Experimental results were obtained in a 650-W laboratory prototype to verify the theoretical study. A maximum efficiency of 94% was obtained.

Single-stage DC-AC converter for photovoltaic systems

This paper presents a DC-AC converter that merges a DC-DC converter and an inverter in a single-stage topology to be used as an interface converter between photovoltaic systems and the electrical AC grid. This topology is based on a full bridge converter with three levels output voltage, where two diodes and one inductor have been added in order to create a Boost converter. The control system of the proposed converter is based on two hysteretic controllers: one for the grid injected current and the other for controlling the panel current. A prototype of the proposed converter including power and control circuits was developed. The MPPT algorithm is not yet implemented and, therefore, to obtain experimental results an additional power supply is used to emulate the PV panel. Theoretical analysis and design criteria are presented together with simulated results to validate the proposed concepts. Experimental results are obtained in a lab prototype to evidence the feasibility and performance of the converter.

Single stage inverter for PV applications with One Cycle Sampling technique in the MPPT algorithm

This paper introduces a single-stage, single-phase, inverter, for maximum efficient photovoltaic grid connection. The inverter is based in a full-bridge converter where a few diodes and one inductor have been added to enable single stage operation. The new topology provides an additional buck-boost converter function at the bridge level in order to extract the maximum power from photovoltaic (PV) panel. Non linear control is used to regulate the sinusoidal current injected in the grid and the PV voltage, so that maximum power point tracker, MPPT, is obtained. The new MPPT algorithm uses one cycle sampling technique to obtain the average value of the voltage and current in the panel with a small time delay. This technique eliminates errors introduced in the MPPT calculation, due to the switching ripple. The proposed algorithm also allows the operation of the control circuit at a lower frequency than the high switching frequency of the bridge converter transistors.

Power factor correction in single phase AC-DC conversion: control circuits for performance optimization

AC-DC conversion with power factor correction may use several well known power circuits (boost, half-bridge and full-bridge), or some single stage converters, in the case of switch mode power supplies. All of them present the same control problem: an accurate control of the input current and the simultaneous control of the voltage across the energy storage capacitor. The control of the voltage at the storage capacitor presents a high response time or a reduced gain, in order to avoid a significant distortion of the input current, with a dominant harmonic at the double of line frequency. In the case of switch mode power supplies, the storage capacitor is designed to perform a required stand-on time and, consequently, is bulky and expensive. This paper presents a survey of the more recent pulse width modulators used in the PFC circuits, namely UCI modulators and the design objectives of those control circuits and their main limitations. It is also presented a new technique for capacitor voltage sampling that permit an important reduction of the response time of the voltage regulators. The stability conditions for PI regulators with sampled feedback are presented. Experimental results obtained with a full-bridge single stage converter are also presented.

High-Performance Voltage-Fed AC–DC Full-Bridge Single-Stage Power Factor Correctors with a Reduced DC Bus Capacitor

Full-bridge single-stage (FBSS) ac-dc converters allow to regulate both the output voltage and the input current that achieves a near sinusoidal waveform using only the four bridge transistors. Independent of this feature, these converters still need to be optimized in order to become an interesting and attractive solution for modern switch mode power supplies with power factor corrector (PFC) function. One of the most important improvements needed is the downsizing of the dc bus capacitor C b with the inherent cost reduction. However, this action introduces complex issues in the regulation of the input current and it is also responsible for the generation of high output voltage ripple. The new contribution of this paper consists in the introduction of a set of power circuit optimizations and control techniques in a FBSS PFC converter that solves the referred issues in order to enable the reduction of the dc bus capacitors size and cost. These procedures are based in the use of a free wheeling circuit that improves the light load operation and in the application of one shot nonlinear modulator, in order reduce the output voltage ripple even when the dc bus ripple is high. The possibility of using, in the proposed topology, a reduced ratio capacitance/watt lower than the typical values used in commercial applications (0.7-0.5 μF/W for 385-450 V, respectively), while maintaining the accurate input current regulation, is also theoretically proved. The developed concepts, solutions and design criteria are detailed described in the paper. The correspondent theoretical study is verified trough experimental results token in an optimized FBSS topology prototype.

Solving Technical Problems on the Full-Bridge Single-Stage PFCs

AC-DC conversion with power factor correction using full-bridge single-stage (FBSS) converters may introduce various complex operating limits and control problems that are not usually described in literature. This is mainly due to the fact that they devaluate and restrict the number of applications of those topologies. These problems are identified as primary transformer saturation and output voltage perturbations. The operation at light loads and with wide load variation is also a complex issue that leads to uncontrollable input current and dc bus voltage. This paper clarifies such technical problems referring their impact in the FBSS topologies operation, and presents design criteria and control solutions to minimize their effects. A low cost free wheeling circuit (FWC) that allows null input current to solve the light load operation problem is also presented.

EMI reduction by optimizing the output voltage rise time and fail time in high-frequency soft-switching converters

In order to ensure the compatibility between devices, several electromagnetic compatibility (EMC) standards specify the maximum permitted levels of radiated noise. To comply with these standards, hard switching power converters need to use expensive solutions, namely shielding, Filtering techniques and snubbers, because the voltages have low values of rise and fall times. This fact leads into significant harmonic content at high frequencies. In this case, the most cost effective way to deal with radiated noise is to reduce it at the source. A way to achieve this goal is to impose moderate values of output voltage rise and fall times. This paper theoretically and experimentally explains, that the harmonic content at high frequencies can be significantly reduced with a selection of a suitable value of voltage rise time and fall time. Based on the study of a generic trapezoidal waveform, simple but precise equations, that permit to easily evaluate the envelope of the frequency spectrum, are derived. These equations also permit to analyze the importance and the influence of the rise and fall times in the spectrum envelope. Finally, with this simple analysis, the influence of the values of rise time and fall time in more complex waveforms are shown, as the output voltage waveform of a high frequency cycloconverter. The validity of the theoretical analysis is confirmed by experimental results.

Contactless battery charger with high relative separation distance and improved efficiency

This paper presents the analysis and the design of a contactless energy transfer system to be used as a battery charger, for an electronic portable device. The separation distance between transmitter and receiver is of the same order as the coils dimensions, leading to a low coupling factor and imposing, therefore, a coreless transformer. The geometrical parameters of diverse transformer coils configurations were calculated in order to obtain the one that shows the higher magnetic coupling coefficient. The compensation system is composed by a series resonant converter working slightly above resonance, which is equivalent to an LLC resonant converter, when supplying the contactless loosely coupled transformer. The theoretical analysis is based in the development of a model that permits to obtain a closed set of equations enabling the definition of the output regulation characteristics. Simulated and experimental results were obtained to verify the accuracy of the theoretical study and also the circuit performance for each transformer configuration.