Optimal air-gap design in high-frequency foil windings

High-frequency AC losses are normally induced in transformer and inductor windings due to skin, proximity, fringing and other AC effects. In addition, the winding structure greatly affects the distribution of losses within the windings. Air gaps are usually placed in the core of magnetic devices to support the high magnetomotive force (MMF). Fringing fields can cause additional AC winding losses, and care must be taken to minimize these losses. In this paper, the effect of air-gap design on the induced losses is investigated. In particular, three air-gap designs-lumped, discretely distributed and uniformly distributed-are investigated and evaluated. Both one-dimensional (1-D) and finite-element analyses (FEAs) are used to investigate the different design structures     

A novel approach for minimizing high-frequency transformer copperlosses

A graphical and numerical method of calculating and minimizing losses in windings, that generalizes previous findings, has been introduced. Using electromagnetic theory and MMF diagrams in both space and time a method is proposed that provides insight into the mechanism of skin and proximity effect losses and that also yields quantitative results. Using this method, several winding geometries for various topologies are covered. The analysis and optimization process is experimentally verified using an interleaved flyback transformer. The mathematical treatment justifying the use of the field method and which is essential in arriving at any numerical result is presented are more general equations for the calculation of copper losses are derived. The relation between the fields in the transformer and copper losses is emphasized. Also, the tools necessary to derive optimization diagrams are provided     

High-Efficiency Isolated Boost DC–DC Converter for High-Power Low-Voltage Fuel-Cell Applications

A new design approach achieving very high conversion efficiency in low-voltage high-power isolated boost dc–dc converters is presented. The transformer eddy-current and proximity effects are analyzed, demonstrating that an extensive interleaving of primary and secondary windings is needed to avoid high winding losses. The analysis of transformer leakage inductance reveals that extremely low leakage inductance can be achieved, allowing stored energy to be dissipated. Power MOSFETs fully rated for repetitive avalanches allow primary-side voltage clamp circuits to be eliminated. The oversizing of the primary-switch voltage rating can thus be avoided, significantly reducing switch-conduction losses. Finally, silicon carbide rectifying diodes allow fast diode turn-off, further reducing losses. Detailed test results from a 1.5-kW full-bridge boost dc–dc converter verify the theoretical analysis and demonstrate very high conversion efficiency. The efficiency at minimum input voltage and maximum power is 96.8%. The maximum efficiency of the proposed converter is 98%.      

A lumped winding model for use in transformer models for circuitsimulation

A lumped circuit model is derived for a winding in a multiwinding transformer. The model is intended to be used in transformer models for circuit simulation using electrical-network simulators. A hybrid (partly electrical, partly magnetic) modeling approach is adopted in which magnetic components are described using the capacitance-permeance analogy instead of the widespread resistance-reluctance analogy. The network correctly models energy storage and power dissipation due to DC series wire resistance and to eddy current losses, independent of the way of excitation of the winding (electrical and/or magnetic). All component values are frequency independent and are parameterized by geometrical parameters, winding data and material parameters. The mathematical continued-fraction approximation technique is applied to derive approximating circuits to model eddy current losses. A fourth-order circuit shows acceptably small errors up to a frequency of about a factor of 1500 above the frequency at which eddy-current losses become apparent. The model is applied in a six-layer two-winding transformer model. Calculations both in the frequency domain and in the time domain show good agreement with measurements     

Analysis of magnetic field and circulating current for HTS transformer windings

In a high-temperature superconducting (HTS) transformer, the leakage magnetic field decreases the critical current and increases the ac loss in the tapes. Moreover, because of nearly zero resistance of HTS tapes, a slight unbalance of the branch inductances of the windings might result in heavy circulating current. So, the numerical analysis of the leakage magnetic field and circulating current is especially necessary for an HTS transformer design. In this paper, the influence of the winding configurations on the stray field and circulating current is studied. That is, the magnetic field distribution is analyzed by finite-element method and then, based on the inductance matrix obtained after a magnetic field analysis, the circulating current is calculated by circuit analysis. Some measures for improving the leakage field and circulating current distribution are also proposed to make HTS transformers more efficient.     

Reduction of high-frequency conduction losses using a planar litz structure

A new trend in power converters is to design planar magnetic components that aim for low profile. However, at high frequencies, ac losses induced in the planar inductor and transformer windings become significant due to the skin and proximity effects. A planar litz conductor can be constructed by dividing the wide planar conductor lengthwise into multiple strands and weaving these strands in much the same manner as one would use to construct a conventional round litz wire conductor. Each strand is then equally subjected to the magnetic fields in the winding window, thereby equalizing the flux linkage and improving the current distribution. Three-dimensional finite-element modeling was performed for simple models. The simulation results showed that the planar litz conductor can result in lower ac resistance than a solid conductor over a specific frequency range. The performance of the planar litz winding was also verified with measurements on two experimental prototypes.     

薄膜解耦集成磁件的结构设计与研究

文中分析了磁件解耦集成的基本原理,得出了磁件实现解耦集成所需的基本条件;根据抵消绕组间耦合作用的方法设计了一个解耦的集成磁件,将一个小功率变压器和一个电感集成在一起;采用薄膜化技术设计制作该集成磁件的薄膜磁芯;为了消除采用传统磁件结构时磁件两端的漏磁,设计了特殊的磁芯结构和与其相配套的绕组结构;对该磁件进行了仿真研究,验证了设计的有效性和可行性。     

Control algorithms and circuit designs for optimal flyback-chargingof an energy-storage capacitor (e.g., for flash lamp or defibrillator)

A flyback-type of a transformer-coupled DC/DC power converter supplies a train of current pulses to charge an energy-storage capacitor to a desired high voltage, converting input DC power obtained from a lower voltage DC source. The energy-storage capacitor is charged to a specified voltage within a specified time with minimum peak and RMS currents in the transistor, the rectifier diode, the transformer windings and the DC power source, minimizing the i²R losses. This is done by generating: (1) energy-storage current pulses in the power transistor and the transformer primary winding in which the current increment from the beginning to the end of a pulse is only a small fraction of the final (peak) value; and (2) energy-delivery flyback current pulses in the capacitor and the transformer secondary winding in which the current decrement from the beginning to the end of a pulse is only a small fraction of the initial (peak) value. Recommended methods are: (1) hysteretic current-mode control with current sensing in both transformer windings; (2) peak-current-commanding current-mode control with switching frequency or transistor-nonconducting time varying in a prescribed way during the charging; or (3) valley-current-commanding current-mode control with switching frequency or transistor-conducting time varying in a prescribed way during the charging. Compared with one nonoptimal method, peak currents are reduced by a factor of about 2 and i²R power losses are reduced by a factor of about 1.33     

A leakage-inductance-based ZVS two-inductor boost converter with integrated magnetics

A two-inductor boost converter topology has conduction loss and transformer utilization advantages in converting low-voltage higher current inputs to high output voltages. In this letter, a new zero-voltage switching (ZVS) two-inductor boost converter with integrated magnetics is proposed. In the new topology, the two current source inductors, a resonant inductor and a two-winding transformer, are integrated into one single magnetic core with three windings. Two windings simultaneously perform the functions of the current source inductors and the transformer primary. The transformer leakage inductance forms the resonant inductance. This leads to a much more compact converter design with a significant reduction in the number of core and winding components. A theoretical analysis establishes the operating point of the ZVS converter. Both of the theoretical and experimental waveforms, including flux waveforms for the legs of the integrated core structure, are presented at the end of the letter.     

Introduction to modeling of transformers and coupled inductors

A tutorial paper is presented on modeling and design of transformers and coupled inductors. Beginning with a brief review of electromagnetic laws and magnetic circuit models, the magnetic and electric models of transformers and coupled inductors are developed, including both magnetizing and leakage effects. It is shown that while the voltage waveforms on the windings are primarily related by the turns ratio for both devices, the winding currents of transformers and coupled inductors are determined by very different mechanisms. An integrated structure with both transformer and coupled inductor on the same core is also discussed, as well as the special case of the coupled inductor used on a multiple-output transformer-isolated converter     

Design issues for the transformer in a low-voltage power supplywith high efficiency and high power density

Circuit model, design feasibility, and design tradeoffs are investigated for the transformer in 1.5-5 V power supplies with high efficiency and high power density. The transformer is constructed from a single or a matrix of pot cores and from interleaved planar windings. It has been determined theoretically and verified experimentally that such a transformer is realizable as long as the loss constraint is not severe (e.g. less than 0.5 W transformer loss per 100 W output). The primary source of loss is the winding, not the core, in 1.5 V/turn design. Measures to reduce the transformer height tend to increase transformer loss or volume     

金属线宽及线间距渐变的射频螺旋电感

为减少射频螺旋电感的金属导体损耗,提出了一种电感金属线宽及金属间距从外到内逐渐变小的新颖结构.与传统的固定金属线宽和间距的电感相比,该渐变结构电感涡流效应的影响较小,金属导体损耗减小,从而降低其串联电阻,品质因子Q值提高.实验结果确证了所提方法的正确性.对一个高阻硅衬底上6nH电感,优化设计的渐变结构电感Q值在2.46GHz处可达到14.25,比版图面积相同、固定线宽及间距的传统电感高11.3%.因此,在无线通信系统的射频前端,采用这种电感与射频集成电路结合,能获得更好的射频电路性能.     

Performance Analysis of RF Spiral Inductor with Gradually Changed Metal Width and Space

To decrease the metal losses of RF spiral inductor,a novel layout structure with gradually reduced metal line width and space from outside to inside is presented.This gradual changed inductor has less eddy-current effect than the conventional inductor of fixed metal width and space.So the series resistance can be reduced and the quality (Q) factor of the inductor relating to metal losses is increased.The obtained experimental results corroborate the validity of the proposed method.For a 6nH inductor on high-resistivity silicon at 2.46GHz,Q factor of 14.25 is 11.3% higher than the conventional inductor with the same layout size.This inductor can be integrated with radio frequency integrated circuits to gain better performance in RF front end of a wireless communication system.     

Integrated magnetic full wave converter with flexible output inductor

A new integrated magnetic full wave DC/DC power converter that provides flexible transformer design by incorporating an independent output inductor winding is introduced. The transformer is implemented on a traditional three-leg magnetic core. The inductor winding can be separately designed to control the output current ripple. The cross-sectional area of the inductor core leg can be reduced dramatically. The operation and performance of the proposed circuit are verified on a 100 W prototype converter.     

Fabrication method for a winding assembly with a large number ofplanar layers

Z-folding of flex circuits is presented as a method of fabricating a transformer winding assembly having a large number of conductive and insulating layers. Flex-circuit patterns. for practical winding configurations are described along with a synthesis procedure. The steps to assemble interleaved planar windings from flex circuits are described. Equations relating winding resistance to geometrical parameters are derived for design purpose     

Integrated planar L-C-T component: Design,Characterization and Experimental Efficiency analysis

In a resonant dc/dc converter, the major part of the volume is filled by passive components. Moreover, all these components have to support all the power flow. On the one hand, this leads to losses in these components and their connections and on the other hand to an important cost of the converter. Concept of integrated passive component, called inductor-capacitor-transformer (L-C-T), is now well known [1],[3] ,[4] ,[6],[8], and this paper will present a new prototype which permits to integrate in an unique part an inductor, a capacitor and a transformer. Because it uses only one core with three windings, this component will reduce the volume of the passive part of the converter. By reducing the number of connections it will, also, have a positive effect on losses. After a presentation of L-C-T components, this paper describes a design of this device, using new windings disposition. The proposed structure permits to adjust separately L and C values. Then, to validate the approach, a characterization is carried out, leading to a new model, including dielectric, iron and copper losses.     

DC-DC converter: four switches V/sub pk/=V/sub in//2, capacitive turn-off snubbing, ZV turn-on

A new four-switch full-bridge dc-dc converter topology is especially well-suited for power converters operating from high input voltage: it imposes only half of the input voltage across each of the four switches. The two legs of a full-bridge converter are connected in series with each other, across the dc input source, instead of the usual topology in which each leg is connected across the dc source. The topology reduces turn-off switching losses by providing capacitive snubbing of the turn-off voltage transient, and eliminates capacitor-discharge turn-on losses by providing zero-voltage turn-on. (Switching losses are especially important in converters operating at high input voltage because turn-on losses are proportional to the square of the input voltage, and turn-off losses are proportional to the input voltage). The topology is suitable for resonant and nonresonant converters. It adds one bypass capacitor and one commutating inductor to the minimum-topology full-bridge converter (that inductor is already present in many present-day converters, to provide zero-voltage turn-on, or is associated with one or two capacitors to provide resonant operation), and contains a dc-blocking capacitor in series with the output transformer, primary winding, and some nonresonant converters (that capacitor is already present in resonant power converters). The paper gives a theoretical analysis, and experimental data on a 1.5-kW example that was built and tested: 600-Vdc input, 60-Vdc output at up to 25A, and 50-kHz switching frequency. The measured performance agreed well with the theoretical predictions. The measured efficiency was 93.6% at full load, and was a maximum of 95.15% at 44.8% load.     

Coupled Inductor Incorporated Boost Half-Bridge Converter With Wide ZVS Operation Range

A new boost half-bridge (BHB) converter is presented. It is composed of an additional diode and a coupled winding to the boost inductor of the BHB converter. Using the transferring of a boost inductor current to the coupled winding, the cancellation of zero-voltage-switching (ZVS) current, which always occurs in a conventional one, is prevented. Therefore, the ZVS operation is easily achieved by the leakage inductor current of the transformer. Furthermore, since the negatively built-up leakage inductor current of the boost winding helps the ZVS operation throughout a wide load range, the ZVS operation is always guaranteed.      

Misconception regarding use of transformer resonators in monolithic oscillators

A misrepresentation prevalent in the recent literature concerning the selectivity of an integrated transformer resonator compared to an inductor equivalent, i.e. an inductor resonator constructed from the same metal windings, is clarified.