Integrated magnetics is applied to replace the threediscrete transformers by a single core transformer in a threephase LLC resonant converter. The magnetic circuit of the integrated transformer is analyzed to derive coupling factors between the phases; these coupling factors are intentionally minimized to realize the magnetic behavior of the three discrete transformers, with the benefit of eliminating the dead space between them. However, in a practical design, the transformer parameters in a multi-phase LLC resonant converter are never exactly identical among the phases, leading to unbalanced current sharing between the paralleled modules. In this regard, a current balancing method is proposed in this paper. The proposed method can improve the current sharing between the paralleled phases relying on a single balancing transformer, and its theory is based on Ampere’s law, by forcing the sum of the three resonant currents to zero. Theoretically, if an ideal balancing transformer has been utilized, it would impose the same effect of connecting the integrated transformer in a solid star connection. However, as the core permeability of the balancing transformer is finite, the unbalanced current can’t be completely suppressed. Nonetheless, utilizing a single balancing transformer has an advantage over the star connection, as it keeps the interleaving structure simple which allows for traditional phase-shedding techniques, and it can be a solution for the other multi-phase topologies where realizing a star connection is not feasible. Along with the theoretical discussion, simulation and experimental results are also presented to evaluate the proposed method considering various sources of the unbalance such as a mismatch in: 1) Resonant and magnetizing inductances 2) Resonant capacitors. 3) Transistor on-resistances of the Mosfets. 4) Propagation Delay of the gate drivers.

Powder cores have been gaining much attention as one of the attractive magnetic cores used in power converters due to their superior features such as high saturation flux density and their capability to suppress fringing flux due to the distributed airgaps. However, powder cores have a unique feature that the relative permeability of the magnetic core varies depending on the magnetic field intensity. The comprehensive modeling of variable relative permeability, design method of powder cores, and computer simulation methods are not well discussed in the relevant literature. This paper proposes a novel modeling, magnetic design method, and simulation technique considering dc superimposition characteristics of powder cores. The modeling method relies on a simple novel model equation representing the behavior of the variable relative permeability under the dc current superimposition condition, which is helpful to evaluate the performance of powder cores and to properly design various magnetic components. In this evaluation, five different magnetic powder cores are used to show the accuracy and properness of the proposed method. Theoretical analysis has been presented and the effectiveness of the proposed methods has been evaluated through simulation and experimental tests.

Multi-phase topologies are preferably employed in power conversion systems to lessen the per phase circuit current, conduction losses, devices thermal stresses, and to reduce the output current ripples. Multi-phase LLC resonant dc/dc converter usually possess a number of magnetic cores equal to the number of phases. These magnetic cores are the major contributors to supply volume, weight, and size. For these reasons, circuit designers tend to select the topologies that have a minimal number of magnetic cores. In this paper, the authors aim to promote the industrial applications of the three-phase LLC resonant converter by integrating three transformers into a single, commercially available, magnetic core to reduce the volume, weight, and cost of the power converter. A comprehensive magnetic analysis for the three-phase integrated transformer is conducted. FEM Simulation and experimental tests are carried out to validate the proper operation of the integrated transformer utilized in a 390/12V-500W prototype. Furthermore, the power losses distribution has been presented. The proposed integrated transformer has been proven efficient, and it realized a uniform thermal distribution along the core compared to the three discrete transformers.

High power density and high efficiency in dc/dc converters are required in various applications such as the automotive application. Interleaved multiphase circuits with integrated magnetic components can fulfill these requirements because passive components occupying significant space in power converters can be downsized without high-switching frequency driving of power devices. However, dc-biased magnetization is a drawback of integrated magnetic components because of unbalanced inductor average currents. This imbalance arises from the tolerance among the phase components. To overcome this problem, inductor average current control is implemented in interleaved multiphase dc/dc converters. Nevertheless, the imbalance cannot be completely eliminated because the current sensors inserted into each phase have gain errors. The purpose of this paper is to present a magnetic design method to improve the immunity to unbalanced currents. A comprehensive analysis is carried out with two main objectives: to prevent magnetic saturation, which may arise due to the current unbalance and to downsize the magnetic components by selecting the optimal coupling coefficient taking into consideration the maximum permissible percentage of unbalanced currents. Simulation case studies are presented to support the analysis. Finally, a 1-kW prototype of the interleaved boost converter is built to validate the accuracy of the design method.

It is quite often to utilize the transformer leakage inductance in the resonant tank of the LLC resonant converter to allow for a drastic reduction in the converter cost, weight, size and volume. The effects of the secondary leakage inductance on the operation of the LLC resonant converter are not well-discussed in the relevant literature, and it is the purpose of this paper to give an insight into these effects. The contribution of this paper lies in the following: (1) highlighting that it is not always an accurate assumption to consider that the values of the primary and secondary leakage inductance are identical, specifically in asymmetric magnetic core structures. (2) it has been disclosed that the well-known coupling factor $(k_{12})$ cannot properly express the unequalized leakage inductance distribution in the proposed transformer. Therefore, the authors bring the primary coupling factor $(k_{1})$ and secondary coupling factor $(k_{2})$ into practice to appropriately express the unequalized leakage distribution on the primary and secondary windings, which can be controlled by: (i) allocation of the relevant winding with respect to the air gap. (ii) utilizing noise absorber. (iii) changing the distance between the winding. Several transformer prototypes had been built and experimentally tested to validate these hypotheses (3) it has been observed that the transformer voltage gain and efficiency can be improved when the transformer leakage inductance is concentrated on the secondary side to avoid the voltage drop inflicted by the relatively large value of the magnetizing current (im), especially at light load condition (4) it has been reported that in a transformer structure with concentrated value of leakage on secondary side would decrease the resonant tank input impedance, vertically widen the voltage-gain curve of the converter, and eventually increase the frequency control bandwidth with respect to the load variation. Transformer prototypes had been constructed and tested in 390V/12V-220W LLC resonant converter to evaluate the proposed analysis.