The trend toward high power density, high operating frequency, and low profile in power converters has exposed a number of limitations in the use of conventional wire-wound magnetic component structures. A planar magnetic is a low-profile transformer or inductor utilizing planar windings, instead of the traditional windings made of Cu wires. In this paper, the most important factors for planar transformer (PT) design including winding loss, core loss, leakage inductance, and stray capacitance have individually been investigated. The tradeoffs among these factors have to be analyzed in order to achieve optimal parameters. Combined with an application, four typical winding arrangements have been compared to illustrate their advantages and disadvantages. An improved interleaving structure with optimal behaviors is proposed, which constructs the top layer paralleling with the bottom layer and then in series with the other turns of the primary, so that a lower magnetomotive force ratio m can be obtained, as well as minimized ac resistance, leakage inductance, and even stray capacitance. A 1.2-kW full-bridge dc-dc converter prototype employing the improved PT structure has been constructed, over 96% efficiency is achieved, and a 2.7% improvement, compared with the noninterleaving structure, is obtained.

https://orbit.dtu.dk/files/4566316/Ouyang.pdf

Optimal Design and Tradeoff Analysis of Planar Transformer in High-Power DC–DC Converters

Planar magnetic components using printed circuit board (pcb) windings are attractive due to their high repeatability, good thermal performance, and usefulness for realizing intricate winding patterns. To enable higher system integration at high switching frequency, more sophisticated methods that can rapidly and accurately model planar magnetics are needed. This paper develops a systematic approach to modeling impedances and current distribution in planar magnetics based on a lumped circuit model named as the modular layer model (MLM). Stacked pcb layers are modeled as repeating modular impedance networks, with additional modular impedances representing the magnetic core, air gaps, and vias. The model captures skin and proximity effects, and enables accurate predictions of impedances, losses, stored reactive energy, and current sharing among windings. The MLM can be used to simulate circuits incorporating planar magnetics, to visualize the electromagnetic fields, and to extract parameters for magnetic models by simulations, among many other applications. The modeling results are checked with results of previous theories and finite-element-modeling approaches, with good matching presented. A group of planar magnetic devices, including transformers and inductors with various winding patterns, are prototyped, and measured to validate the proposed approach and clarify the boundaries of its applicability.

/pdf/a-systematic-approach-to-modeling-impedances-and-current-edo1uqt39e.pdf

A Systematic Approach to Modeling Impedances and Current Distribution in Planar Magnetics

A general expression for winding loss (copper loss) including harmonics for an inductor carrying periodic non-sinusoidal current is presented. The skin and proximity effects are taken into account. Expressions for amplitudes of inductor current harmonics are derived and illustrated as functions of duty cycle for selected non-isolated converters namely buck, boost and buck-boost converters. An example of inductor design procedure using the area-product method is shown for a buck converter operating in continuous conduction mode (CCM). The amplitude spectra of the inductor current, winding resistance and winding power loss are illustrated. Inductor winding losses in the designed inductor are analysed using MatLab simulations. Experimental results using the designed inductor are also presented.

Inductor winding loss owing to skin and proximity effects including harmonics in non-isolated pulse-width modulated dc-dc converters operating in continuous conduction mode

Planar transformers provide a light-weight and low-profile solution for power electronic converters with highly reproducible parameters and simple manufacturability. Parasitic inductances, capacitances, and resistances in planar magnetics are difficult to model due to the complex interactions between the physical winding arrangement of each layer and the core geometry (track width, air gap, clearance, etc.). These nonlinear and multivariate magnetic devices play a key role in defining the performance of traditional, soft-switching, and resonant converters by ruling behaviors such as ringing, self-resonant frequency, conduction losses, and the current rate of change in these converters. In this paper, a methodology for determining parametric models for leakage and magnetizing inductance, inter- and intrawinding capacitances, and the winding resistance of small planar transformers is presented using a variety of winding arrangements. The models are employed to shape the winding design to control parasitic elements in order to optimize soft-switching and resonant converters. A central composite design based on the design of experiment methodology is employed on finite element simulations to provide the comprehensive models. Results from physical verification on a planar Ferroxcube ER18/3.2/10-3F3 core set are provided and show excellent correlation between models and verification tests. The method is later employed to effectively design an LLC resonant converter, which is also experimentally verified to illustrate the benefits of the proposed method. The methodology can be employed to characterize and design planar transformers and to predict their performances as part of a variety of power electronic converters.

Applying Response Surface Methodology to Small Planar Transformer Winding Design

The more electric aircraft (MEA) has motivated aircraft manufacturers since few decades. Indeed, their investigations lead to the increase of electric power in airplanes. The challenge is to decrease the weight of embedded systems and therefore, the fuel consumption. This is possible thanks to new efficient power electronic converters made of new components. As magnetic components represent a great proportion of their weight, planar components are an interesting solution to increase the power density of some switching mode power supplies. This paper presents the benefits and drawbacks of high-frequency planar transformers in dc/dc converters, different models developed for their design and different issues in MEA context related to planar’s specific geometry and technology.

/pdf/planar-magnetic-components-in-more-electric-aircraft-review-2ammnc98gr.pdf

Planar Magnetic Components in More Electric Aircraft: Review of Technology and Key Parameters for DC–DC Power Electronic Converter

In today's high-frequency low-profile switching-mode power converters, paralleling winding in planar transformers is needed, especially for low voltage and high current applications. Although this practice is believed to be effective to increase the current handling capacity of the windings, the effect might be offset due to the unequal current sharing within parallel layers if the winding layers are not properly arranged and connected. In this work, the particular problems raised from parallel winding are addressed and modeled based on a one-dimensional field model. The factors affecting winding losses with parallel connection are analyzed. Software to predict the current density distribution and AC loss for planar windings has been developed.

Model and design of PCB parallel winding for planar transformer

An inductor with air-gap used in high-frequency switch-mode power supply has high extra eddy current winding losses due to the air-gap, resulting in low efficiency and high temperature rise in the inductor. In order to reduce the extra winding losses, many techniques are proposed. But all these techniques make the winding and core structures much more complicated. In this work, by 2D eddy current finite element analysis (FEA) field simulation, eddy current winding loss mechanism in the inductor with air-gap was deeply investigated and then a new simple inductor structure was proposed to reduce the winding losses greatly. Moreover, analysis and design for the new structure was also performed.

Winding loss mechanism analysis and the design for a new structure high-frequency gapped inductor

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Model and design of PCB parallel winding for planar transformer

Winding loss mechanism analysis and the design for a new structure high-frequency gapped inductor