In this thesis, coupled inductors have been applied to multiphase DC-DC converters. Detailed analysis has been done to investigate the benefits of directly coupled inductors and inversely coupled inductors, compared to conventional uncoupled inductors. In general, coupled inductors for multiphase DC-DC converters have inherent benefits such as excellent current sharing characteristics, immunity to component tolerance and reduction in current control complexity. Specifically, by employing directly coupled inductors for multiphase DC-DC converters, overall current ripple can be effectively reduced, compared to that of uncoupled inductors. For inversely coupled inductors, phase current ripple can be reduced if operating points and coupling coefficients are carefully chosen. As for small-signal characteristics, inversely coupled inductors have the advantages of broadening the bandwidth of multiphase DC-DC converters and being more immune to load variation at low frequencies. On the other hand, directly coupled inductors have the benefit of low sensitivity to input variation at high frequencies. In addition, the proposed new structure for multiphase DC-DC converters has excellent current sharing performance and reduced current ripple. Computer simulations have been done and hardware prototypes have been built to validate the concepts.

Power Factor Correction rectifiers are widely adopted as the first stage in most grid-tied power conversion systems. Among all PFC converts for single phase system, Boost PFC is the most popular one due to simplicity of structure and high performance. Although the efficiency of Boost PFC keeps increasing with the evolution of semiconductor technology, the intrinsic feature of high output voltage may result cumbersome system structure with multiple power conversion stages and even diminished system efficiency. This disadvantage is aggravated especially in systems where resonant converters are selected as second stage. Especially for domestic induction cooker application, step-down PFC with wide range output regulation capability would be a reasonable solution, Conventional induction cooker is composed by input filter, diode-bridge rectifier, and full bridge or half bridge series resonant circuit (SRC). High frequency magnetic field is induced through the switching action to heat the pan. The power level is usually controlled through pulse frequency modulation (PFM). In such configuration, first, a bulky input differential filter is required to filter out the high frequency operating current in SRC. Second, as the output power decreases, the operating point of SRC is moved away from the optimum point, which would result large amount circulating energy. Third, when the pan is made of well conducting and non-ferromagnetic material such as aluminum, due to the heating resistance become much smaller and peak output voltage of the switching bridge equals to the peak voltage of the grid, operating the SRC at the series resonant frequency can result excessive current flowing through the switch and the heating coil. Thus for pan with smaller heating resistance, even at maximum power, the operating frequency is pushed further away from the series resonant point, which also results efficiency loss. To address these potential issues, a PFC circuit features continuous conducting input current, high power factor, step-down capability and wide range output regulation would be preferred. The Analysis and design work is present in this article for a non-isolated hard switching DCM SEPIC PFC. Due to DCM operation of SPEIC converter, wide adjustable step-down output voltage, continuous conduction of input current and elimination of reverse recovery loss can be achieved at same time. The thesis begins with circuit operation analysis for both DC-DC and PFC operation. Based on averaged switching model, small signal model and corresponding transfer functions are derived. Especially, the impact from small intermediate capacitor on steady state value are discussed. With the concept of ripple steering, theoretic analysis is applied to SEPIC converter with two coupled inductors. The results indicate if the coupling coefficient is well designed, the equivalent input inductance can be multiple times larger than the self-inductance. Because of this, while maintaining input current ripple same, the two inductors of SEPIC can be implemented with two smaller coupled inductors. Thus both the total volume of inductors and the total number of windings can be reduced, and the power density and efficiency can be improved. Based on magnetic reluctance model, a corresponding winding scheme to control the coupling coefficient between two coupled inductors is analyzed. Also the impact of coupled inductors on the small signal transfer function is discussed. For the voltage follower control scheme of DCM PFC, single loop controller and notch filter design are discussed. With properly designed notch filter or the PR controller in another word, the closed loop bandwidth can be increased; simple PI controller is sufficient to achieve high power factor; THD of the input current can be greatly reduced. Finally, to validate the analysis and design procedure, a 1 kW prototype is built. With 120 Vrms AC input, 60V to 100V output, experimental results demonstrate unity power factor, wide output voltage regulation can be achieved within a single stage, and the 1 kW efficiency is around 93%. / Master of Science

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-08-28T20:15:07Z No. of bitstreams: 1 DIEGO ALBERTO ACEVEDO BUENO – DISSERTAÇÃO (PPGEEI) 2015.pdf: 8647076 bytes, checksum: c16b323a227c1c3224d8f023324db513 (MD5) / Made available in DSpace on 2018-08-28T20:15:07Z (GMT). No. of bitstreams: 1 DIEGO ALBERTO ACEVEDO BUENO – DISSERTAÇÃO (PPGEEI) 2015.pdf: 8647076 bytes, checksum: c16b323a227c1c3224d8f023324db513 (MD5) Previous issue date: 2015-08 / CNPq / Esta dissertação examina seis topologias hibridas assimétricas com acoplamento magnético. O termo topologia hibrida assimétrica se refere a qualquer topologia que combine os princípios das topologias clássicas com duas fontes de tensão isoladas ou mais com tensões diferentes. Das topologias propostas, quatro possuem saída única conectada ao ponto intermediário do barramento e as restantes dispõem de saída diferencial usando outro braço de dois níveis. Para simplificar o estudo destas topologias, desenvolveu-se um modelo elétrico e magnético detalhado dos indutores acoplados usando componentes de modo comum e diferencial. Posteriormente, esse modelo foi inserido no modelo do conversor para o desenvolvimento de uma estratégia de controle da corrente de saída e da corrente de modo diferencial garantindo a operação em modo de condução continua com número reduzido de componentes. Os modos de operação e as respectivas estratégias de modulação são apresentados. Além disso, o efeito da diferença das tensões de polo máximas sobre o desempenho dos conversores e na escolha do indutor acoplado foi analisado. Para avaliar o desempenho destas topologias, foram realizadas simulações com os modelos completos das chaves e dos diodos usando o software PSIM fixando a potência de saída para todas as topologias em 1 kVA. Também, alguns resultados experimentais em malha aberta são apresentados para validar os modelos desenvolvidos / This paper examines six asymmetric hybrid topologies with magnetic coupling. The term “asymmetric hybrid topology” refers to any topology that combines the principles of classical topologies with two isolated voltage sources or more with different voltages. From these topologies, four have single-ended output connected to the midpoint of dc-link and the remaining has differential output employing another two-level arm. To simplify the analysis, a detailed electric and magnetic model of the coupled inductors was developed appealing to common- and differential-mode components. Subsequently, this model was inserted into the model of the converter for developing a control strategy for the output and the differential mode currents, ensuring the operation in continuous conduction mode with a reduced number of components. The modes of operation and the modulation strategies are presented. In addition, the difference of the maximum pole voltages was analyzed for determining its effect on the converter performance and the coupled inductor selection. To evaluate the performance of these topologies, simulations were performed with the complete models of switches and diodes using the PSIM software by setting the output power at 1 kVA for all topologies. Also, experimental open loop results are presented to validate the developed models.

This dissertation presents the derivation, analysis and application issues of advanced topologies with coupled inductors. The proposed innovative solutions can achieve significant performance improvement compared to the state-of-the-art technology. New applications call for high-efficiency high step-up DC-DC converters. The basic topologies suffer from extreme duty ratios and severe rectifier reverse recovery. Utilizing coupled inductor is a simple solution to avoid extreme duty ratios, but the leakage inductance associated with the coupled inductor induces severe voltage stress and loss. An innovative solution is proposed featuring with efficient leakage energy recovery and alleviated rectifier reverse recovery. Impressive efficiency improvement is achieved with a simple topology structure. The coupled inductor switching cell is identified. Topology variations and evaluations are also addressed. The concept that utilizes coupled inductors to alleviate rectifier reverse recovery is then extended, and new topologies suitable for other applications are generated. The proposed concept is demonstrated to solve the severe rectifier reverse recovery that occurs in continuous current mode (CCM) boost converters. Significant profile reduction and power density improvement can be achieved in front-end CCM power factor correction (PFC) boost converters, which are the overwhelmingly choice for use in telecommunications and server applications. This dissertation also proposes topologies to realize the single-stage parallel PFC by utilizing coupled inductors. Compared to the state-of-the-art single-stage PFC converters, the proposed topologies introduce a new power flow pattern that minimizes the bulk-capacitor voltage stress and the switch current stress. / Ph. D.

The switching noise in a hard-switched point-of-load (POL) converter may result in false turn on, electromagnetic interference issues, or even device breakdown. A resonant cross-commutated buck (rccBuck) converter operates with low noise since all MOSFETs are turned on with zero voltage within a wide load range. A state-space model was developed to calculate the voltage gain, voltage stresses, and current stresses. Design guidelines for the rccBuck converter operating at continuous voltage mode or discontinuous voltage mode are provided. The design methodology of a one-turn inductor with significant ac and dc fluxes is given. Four fabricated one-turn inductors achieved 2.1% higher efficiency and 50% smaller total magnetic volume than the commercial inductors in the same rccBuck converter. The Omni-coupled inductors (OCI), composed of a twisted E-E core and PCB windings, further improve power density and efficiency. The core loss and inductances were modeled from a complex reluctance network. According to the loss-volume Pareto fronts, the total inductor loss was minimized within a smaller volume than that of discrete inductors. The expectations were validated by an OCI-based rccBuck converter switched at 2 MHz with 12 V input, 3.3 V at 20 A output, and peak efficiency of 96.2%. The small-signal model with a good accuracy up to half switching frequency was developed based on the averaged equivalent circuit. The transient performance of an rccBuck regulator is comparable to that of a second-order buck regulator with the same switching frequency, output capacitance, and closed-loop bandwidth. / Ph. D. / The switching noise in a hard-switched point-of-load (POL) converter may result in false turn on, electromagnetic interference issues, or even device breakdown. A resonant cross-commutated buck (rccBuck) converter operates with low noise since all MOSFETs are turned on with zero voltage within a wide load range. A state-space model was developed to calculate the voltage gain, voltage stresses, and current stresses. Design guidelines for the rccBuck converter operating at continuous voltage mode or discontinuous voltage mode are provided. The design methodology of a one-turn inductor with significant ac and dc fluxes is given. Four fabricated one-turn inductors achieved 2.1% higher efficiency and 50% smaller total magnetic volume than the commercial inductors in the same rccBuck converter. The Omni-coupled inductors (OCI), composed of a twisted E-E core and PCB windings, further improve power density and efficiency. The core loss and inductances were modeled from a complex reluctance network. According to the loss-volume Pareto fronts, the total inductor loss was minimized within a smaller volume than that of discrete inductors. The expectations were validated by an OCI-based rccBuck converter switched at 2 MHz with 12 V input, 3.3 V at 20 A output, and peak efficiency of 96.2%. The small-signal model with a good accuracy up to half switching frequency was developed based on the averaged equivalent circuit. The transient performance of an rccBuck regulator is comparable to that of a second-order buck regulator with the same switching frequency, output capacitance, and closed-loop bandwidth.

High power multi-level converters are deemed as the mainstay power conversion technology for renewable energy systems including the battery storage system, PV farm and electrical vehicle charge station. This thesis is focused on the study of three-level DC/DC converter with multi-phase interleaved structure, with coupled and integrated magnetics to achieve high power density. The proposed interleaved phased legs offer the benefit of output current ripple reduction, while inversed coupled inductors can suppress the circulating current between phase legs. Compared with conventional non-interleaving three-level DC/DC converter with non-coupling inductors, both inductor current ripple and output current ripple are largely reduced by interleaving with inverse-coupled inductors. Because of the non-linearity of the inductor coupling, the equivalent circuit model is developed for the proposed interleaving structure. The model identifies the existence of multiple equivalent inductances during one switching cycle. A combination of them determines the inductor current ripple and dynamics of the system. By virtue of inverse coupling and means of controlling the coupling coefficients, one can minimize the current ripple and the unwanted circulating current. To further reduce the magnetic volume, the four inductors in two-phase three-level DC/DC converter are integrated into one common structure, incorporating the negative coupling effects. The integrated magnetic structure can effectively suppress the circulating current and reduce the inductor current ripple and it is easy to manufacture. This thesis provides an equivalent circuit model to facilitate the design optimization of the integrated system. A prototype of integrated coupled inductors is assembled with nano-crystalline C-C core and powder block core. It is tested with both impedance analyzer and single pulse tester, to guarantee proper mutual inductance for inductor current ripple and output current ripple target. With a two-phase three-level DC/DC converter hardware, the concept of integrated coupled inductors is verified, showing its good performance in high-voltage, high-power conversion applications. / Master of Science / With the demanding energy consumption globally, there is an increasing trend for the requirement of high efficiency power converters with high power density. The application for renewable energies including the PV farm, battery storage system and electrical vehicle become more and more important for the sustainable development of society. High-power, high-voltage DC/DC converters can fulfill the role for such renewable energy power conversion. In this paper, a multi-phase multi-level DC/DC converter solution suitable for high-power, high-voltage application is analyzed and designed. With the techniques including interleaving operation and inverse coupled inductors, the power density for the power conversion is increased while keeping a high system efficiency. The discussed power converter in this thesis demonstrated a solution for high-power, highvoltage DC/DC power conversion with high efficiency. And the concept is verified with real hardware experimental results, showing its good performance for a 200kW power conversion system.

Motivated by the recent commercialization of gallium-nitride (GaN) switches, an effort was initiated to determine whether it was feasible to switch the flyback converter at 5 MHz in order to improve the power density of this versatile isolated topology. Soft switching techniques have to be utilized to eliminate the switching loss to maintain high efficiency at multi-megahertz. Compared to the traditional modeling of zero-voltage-switching quasi-square-wave converters, a numerical methodology of parameters design is proposed based on the steady-state model of zero-voltage switching quasi-square-wave flyback converter. The magnetizing inductance is selected to guarantee zero-voltage switching for the entire input and load range with the trade-off design for conduction loss and turn-off loss. A design methodology is introduced to select a minimum core volume for an inductor or coupled inductors experiencing appreciable core loss. The geometric constant Kgac = MLT/(Ac2WA) is shown to be a power function of the core volume Ve, where Ac is the effective core area, WA is the area of the winding window, and MLT is the mean length per turn for commercial toroidal, ER, and PQ cores, permitting the total loss to be expressed as a direct function of the core volume. The inductor is designed to meet specific loss or thermal constraints. An iterative procedure is described in which two- or three-dimensional proximity effects are first neglected and then subsequently incorporated via finite-element simulation. Interleaved and non-interleaved planar PCB winding structures were also evaluated to minimize leakage inductance, self-capacitance and winding loss. The analysis on the trade-off between magnetic size, frequency, loss and temperature indicated the potential for a higher density flyback converter. A small-signal equivalent circuit of QSW converter was proposed to design the control loop and to understand the small-signal behavior. By adding a simple damping resistor on the traditional small-signal CCM model, it can predict the pole splitting phenomenon observed in QSW converter. With the analytical expressions of the transfer functions of QSW converters, the impact of key parameters including magnetizing inductance, dead time, input voltage and output power on the small-signal behavior can be analyzed. The closed-loop bandwidth can be pushed much higher with this modified model, and the transient performance is significantly improved. With the traditional fix dead-time control, a large amount of loss during dead time occurred, especially for the eGaN FETs with high reverse voltage drop. An adaptive dead time control scheme was implemented with simple combinational logic circuitries to adjust the turn on time of the power switches. A variable deadtime control was proposed to further improve the performance of adaptive dead-time control with simplified sensing circuit, and the extra conduction loss caused by propagation delay in adaptive dead-time control can be minimized at multi-megahertz frequency. / Ph. D. / With the fast development of telecom, computer and network systems, high efficient and small volume power supplies are highly desired. A typical method for achieving high power density involves increasing the frequency and implement soft-switching techniques to minimize loss. Thanks to the recent commercialization of the advanced semiconductor gallium-nitride (GaN) switches, it is feasible to design high density power supplies and cost effective power system. Several challenges including optimization of power converter, high frequency magnetics and implementation of control architecture have been addressed in this dissertation which helps to realize this compact power system. With the implementation of proposed circuit model and seminumerical design procedures for magnetics, a 30W high-frequency isolated DC/DC converter with planar inductor is fabricated to verify the theoretical analysis, which also demonstrates much improved performances.

L'apparition de convertisseurs multicellulaires parallèles entrelacés et magnétiquement couplés a conduit ces dernières années à améliorer les performances des convertisseurs (en termes de densité de puissance, d'efficacité, de dynamique,...). Le pendant de ces améliorations successives résulte en une nécessité d'équilibrage précis des courants de phase, ce qui entraîne une complexification de la commande des ces convertisseurs. Une première étape de détermination de la fonction de transfert d'une boucle d'équilibrage des courants nous permet de déterminer la nature des correcteurs d'équilibrage de ces courants. Cette étude nous permet d'appréhender des systèmes plus complexes avec différentes topologies de couplage magnétique entre les bras du convertisseur parallèle. Suite à une étude bibliographique mettant en avant le manque de précision des techniques actuelles de mesure des courants de bras, nous proposons une technique d'émulation analogique précise de ces courants ne nécessitant qu'un seul capteur. Deux prototypes ont été réalisés et permettent de valider cette technique. Enfin, face à l'intérêt grandissant que portent les industriels pour des architectures modulaires, deux innovations permettant de s'affranchir d'un circuit spécifique de supervision sont proposées. Dans un premier temps, une technique modulaire d'équilibrage des courants est proposée et validée expérimentalement : elle permet, entre autres, une mesure différentielle précise des courants de bras. Ensuite, une méthode de génération modulaire de porteuses triangulaires auto-alignées est proposée et validée grâce à la réalisation d'une maquette de test. L'association de ces deux techniques nous permet de proposer une architecture entièrement modulaire ne nécessitant plus de circuit de commande superviseur. / Development of interleaved power converters with coupled inductors has enhanced converters performances (better power density, eciency, transient response. . .). Such improvements lead to the necessity of a precise current-sharing in the converter legs, and consequently to much more complex control strategy for those converters. First step is to determine current sharing loop transfer function in order to choose the kind of sharing corrector and calculate its parameters. State-space representation is used to consider any coupling topology. Because ux induced in coupled inductors must be controlled with accuracy, a bibliography study emphasizes the lack of precision in present current-sensing techniques. Then, a precise analogical emulation of currents in every leg, based on only one current sensor, is proposed. Two prototypes have been developed and validate this approach. Finally, because of growing interest of industrial in modular architectures, two innovations which avoid the use of central specic circuit are presented. First, a masterless and modular current sharing technique is proposed and tested : it allows a very precise dierential current measurement and regulation. Then a modular generation of self-aligned triangular carrier for interleaved converters is proposed and conrmed by test. The association of both techniques leads to a full masterless and modular approach for the control circuit of parallel converter with coupled inductors.

This Thesis addressed the challenge of ensuring balanced currents in the phases of a multi-kW, interleaved dc-dc converter by means of closed-loop digital control. The Thesis examines uniformly-sampled, valley-current, peak-current and average-current control for a dual interleaved boost converter with inter-phase transformer which might form part of the power train of an electric vehicle. Also, an enhancement of the average-current control is investigated in which the transistor duty-ratio is updated more rapidly, which allows an improvement of approximately ten times in the response speed of the system. Based on the theoretical analysis, the average-current control methodology was determined to be the most suitable technique for this type of converter as it ensures well-balanced phase currents over a wide range. To provide a basis for control system analysis and design for interleaved converters, a modelling methodology is developed based on a combination of multi-rate data-sampled theory and a small-signal averaged converter model. The model is shown to represent accurately the interaction between the interleaved phases, revealing a reduced stability range compared with a non-interleaved converter. The modelling and control methods are validated using switched and average value simulations obtained with the SABER software and by experimental results from a 25 kW, 30 kHz converter prototype. The control techniques were implemented on a Texas Instruments TMS320F28335 digital signal controller.

In sustainable energy systems those using fuel cells, high step-up gain converters are widely used to increase the output voltage to levels that can be used by other converters, such as inverters for grid connection or powering other AC loads. In order to obtain a higher voltage gain, in comparison to a traditional boost converter, often different topologies techniques are involved. In this project a new topology is studied, cascading to half bridges, each working similar to a boost converter but with magnetic coupled inductors in-between.    The converter design is modeled in PLECS Blockset and MATLAB Simulink to simulate and evaluate the performance with proper design procedure. The results showed that a high step-up gain was achieved, and the gain could easily be adjusted by changing the duty cycle and/or the coupling factor. The converter design showed similarities and differences to a traditional boost converter. The coupled inductor has its advantage reducing the fuel cell current ripple with the effect of the duty cycle.