Ziwei Ouyang

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

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.

Analysis and Design of a Bidirectional Isolated DC–DC Converter for Fuel Cells and Supercapacitors Hybrid System

Electrical power systems in future uninterruptible power supplies or electrical vehicles may employ hybrid energy sources, such as fuel cells and supercapacitors. It will be necessary to efficiently draw the energy from these two sources as well as recharge the energy storage elements by the dc bus. In this paper, a bidirectional isolated dc-dc converter controlled by phase-shift angle and duty cycle for the fuel-cell hybrid energy system is analyzed and designed. The proposed topology minimizes the number of switches and their associated gate driver components by using two high-frequency transformers that combine a half-bridge circuit and a full-bridge circuit together on the primary side. The voltage doubler circuit is employed on the secondary side. The current-fed input can limit the input current ripple that is favorable for fuel cells. The parasitic capacitance of the switches is used for zero voltage switching (ZVS). Moreover, a phase-shift and duty-cycle modulation method is utilized to control the bidirectional power flow flexibly and it also makes the converter operate under a quasi-optimal condition over a wide input voltage range. This paper describes the operation principle of the proposed converter, the ZVS conditions, and the quasi-optimal design in depth. The design guidelines and considerations regarding the transformers and other key components are given. Finally, a 1-kW 30~50-V-input 400-V-output laboratory prototype operating at 100-kHz switching frequency is built and tested to verify the effectiveness of the presented converter.

Overview of Planar Magnetic Technology—Fundamental Properties

The momentum toward high efficiency, high frequency, and high power density in power supplies limits wide use of conventional wire-wound magnetic components. This paper gives an overview of planar magnetic technologies with respect to the development of modern power electronics. The major advantages and disadvantages in the use of planar magnetics for high-frequency power converters are covered, and publications on planar magnetics are reviewed. A detailed survey of winding conduction loss, leakage inductance, and winding capacitance for planar magnetics is presented so power electronics engineers and researchers can have a clear understanding of the intrinsic properties of planar magnetics.

The analysis and comparison of leakage inductance in different winding arrangements for planar transformer

The coupling of the windings can be easily increased by using multiply stacked planar windings connection. Interleaving is a well-known technique used to reduce leakage inductance and minimize high-frequency winding losses. The paper aims to analyze leakage inductance based on magneto motive force (MMF) and energy distribution in planar transformer and correct the formula of leakage inductance proposed by previous publications. The investigation of different winding arrangements shows significant advantages of interleaving structure. In this work, a novel half turn structure is proposed to reduce leakage inductance further. Some important issues are presented to acquire desired leakage inductance. The design and modeling of 1 kW planar transformer is presented. In order to verify the analytical method for leakage inductance in this paper, finite element analysis (FEA) and measurement with impedance analyzer are presented. Good matching between calculation, FEA 2D simulation and measurement results is achieved.

Planar-Integrated Magnetics (PIM) Module in Hybrid Bidirectional DC–DC Converter for Fuel Cell Application

In most power electronics converters, the overall volume is mainly determined by the number of parts and the size of passive components. Integrated magnetics and planar magnetics techniques, therefore, have been an excellent option in order to reduce the counts and the size of magnetic components, hereby increasing the power density of converters. In this paper, a new planar-integrated magnetics (PIM) module for a phase-shift plus duty-cycle-controlled hybrid bidirectional dc-dc converter is proposed, which assembles one boost inductor and two transformers into an E-I-E core geometry, reducing the number of parts, the total volume of converter, as well as the total core loss of the magnetic components. AC losses in the windings and leakage inductance of the transformers are kept low by interleaving the primary and secondary turns of the transformers. To verify the validity of the design approach and theoretical analysis, a laboratory prototype employing the PIM module is implemented for a fuel cell application with 20-40-V input voltage and 400-V output voltage. Detailed results from the experimental comparisons demonstrate that the PIM module is fully functional and electromagnetically equivalent to the discrete magnetics and a significant reduction of size can be achieved by using the PIM module.

Analysis and Design of Fully Integrated Planar Magnetics for Primary–Parallel Isolated Boost Converter

A high efficient planar integrated magnetics (PIM) design approach for primary parallel isolated boost converters is presented. All magnetic components in the converter including two input inductors and two transformers with primary-parallel and secondary-series windings are integrated into an E-I-E core geometry. Due to a low reluctance path provided by the shared I-core, the two transformers as well as the two input inductors can be integrated independently, reducing the total ferrite volume and core loss. AC losses in the windings and the leakage inductance of the transformer are kept low by interleaving the primary and secondary turns of the transformers. To verify the validity of the design approach, a 1-kW prototype converter with two primary power stages is implemented for a fuel cell fed battery charger application with 20–40 V input and 170–230 V output. An efficiency of 96% can be achieved during nominal operating conditions. Also experimental comparisons between the PIM module and two separate cases have been done in order to illustrate the advantages of the proposed method.

Four Quadrants Integrated Transformers for Dual-Input Isolated DC–DC Converters

A common limitation of power coupling effect in some known multiple-input dc-dc converters has been addressed in many literatures. In order to overcome this limitation, a new concept for decoupling the primary windings in the integrated multiple-winding transformers based on 3-D space orthogonal flux is proposed in this letter. And thus, a new geometry core and relative winding arrangements are proposed in accordance with the orthogonal flux decoupling technology. Due to the four secondary windings are arranged in a quadratic pattern at the base core plate with the two perpendicular primary windings, a name of “four quadrants integrated transformers” (FQIT) is, therefore, given to the proposed construction. Since the two primary windings are uncoupled, the FQIT allows the two input power stages to transfer the energy into the output load simultaneously or at any time-multiplexing scheme, which can optimize the utilization of input sources, simplify the system structure, and reduce the overall cost, so they are attractive for the hybrid renewable power system. Section IV initiates a discussion for the advantages of the FQIT. In order to verify the feasibility of the FQIT in multiple-input converter, a dual-input isolated boost dc-dc converter with the FQIT is designed and tested. The results have excellently demonstrated that the two input power stages can be operated independently and the correctness of all the analysis in the letter.

Calculation of Leakage Inductance for High-Frequency Transformers

Frequency-dependent leakage inductance is often observed. The high-frequency eddy current effects cause a reduction in leakage inductance. The proximity effect between adjacent layers is responsible for the reduction of leakage inductance. This paper gives a detailed analysis of high-frequency leakage inductance and proposes an accurate prediction methodology. High-frequency leakage inductances in several interleaved winding configurations are also discussed. Interleaved winding configurations actually give a smaller degree of reduction of leakage induction at high frequency. Finite-element analysis simulation and measurement validate the models.

Leakage Inductance Calculation for Planar Transformers With a Magnetic Shunt

The magnetic shunt is generally inserted in a planar transformer to increase the leakage inductance, which can be utilized as the series inductor in resonant circuits such as the LLC resonant converter. This paper presents a calculation methodology for the leakage inductance of the transformer with a magnetic shunt by means of the stored magnetic energy in the primary and secondary sides of the transformer using the magnetomotive force (MMF) variation method, as well as the stored energy in the shunt based on the reluctance model. The detailed calculation method is described. Both the finite-element analysis simulation and the experimental results have proven the validity of the proposed calculation method for leakage inductance.

Planar integrated magnetics design in wide input range DC-DC converter for fuel cell application

In the most power electronics converters, the overall volume is mainly determined by the number of parts and the size of passive components. Integrated magnetics and planar magnetics techniques therefore have been an excellent option in order to reduce the count and the size of magnetic components, hereby increasing the power density of converters. A new planar integrated magnetics (PIM) technique for a phase-shift plus duty cycle controlled hybrid bi-directional DC/DC converter is presented and investigated in this paper. The main magnetic components including one boost inductor and two independent transformers are integrated into an E-I-E core geometry. Utilizing the flux cancellation as the principle of uncoupling, the transformers and the boost inductor are integrated, to reduce the total ferrite volume and core loss. The transformers and inductor are wound in the outer legs and the center legs respectively. The uncoupling effect between them is determined by the winding connections. The middle I-core provides a shared low reluctance flux path, uncoupling the two independent transformers. With the air gaps shift into the center legs, the magnetizing inductance of transformers will not be decreased due to there is no air gap throughout the flux paths generated by the two transformers. The new PIM structure can be extended to other topologies. To verify the validity of design approach and theoretical analysis, a lab prototype with PIM has been built, and tested. Comparing with the discrete structure, the result demonstrated a great improvement in profile and volume without sacrificing electrical performance.