A Novel Three-phase LLC Resonant Converter with Integrated Magnetics for Lower Turn-off losses and Higher Power Density Mostafa Noah, Shota Kimura, Shun Endo, Masayoshi Yamamoto Shimane University Matsue, Shimane, Japan yamamoto@ecs.shimane- u.ac.jp Jun Imaoka Kyushu University Fukuoka, Japan imaoka@ees.kyushu- u.ac.jp Kazuhiro Umetani Okayama University Okayama, Japan umetani@okayama-u.ac.jp Wilmar Martinez Universidad Nacional de Colombia Bogota, Colombia whmartinezm@unal.edu.co Abstract— The aim of this work is to present a novel topology of a three-phase LLC converter with integrated magnetics. The converter operation and the comprehensive theoretical analysis are presented; this analysis follows the first harmonic approximation (FHA) approach to simplify the system model. Usually, LLC converter achieves zero voltage switching (ZVS) as long as it working in the inductive region. Therefore, the turn off losses are considered as the main source of the switching losses in the converter. In this paper the design in optimized to minimize the switching losses. On the other hand, adapting three discrete transformer cores in this topology will definitely increase the size and volume of the converter. As a result, a novel magnetic integration concept is introduced where all magnetic components of the three-phases are advantageously combined into a single magnetic core to increase the converter power density. Finally, the experimental results are presented to verify the optimized design by showing a reduction in the turn-off losses and the effectiveness of adapting the proposed integrated transformer, in which an increment of 56% in the power density of the converter could be attained. Keywords—Three phase LLC resonant converter; integrated magnetics; steady state analysis; optimization. I. INTRODUCTION In a typical electric vehicle (EV) auxiliary system, a DC- DC converter is usually connected between the high voltage DC bus (200V in case of Toyota Prius 3 rd generation) to feed the low voltage non-propulsive loads. In such DC-DC converters, galvanic isolation is required to protect the low voltage electronic system from any hazardous high voltage; therefore, the converter shall incorporate a transformer [1],[2]. Nowadays, SiC and GaN semiconductor devices are adapted in the EV applications as they are capable of operating at high frequencies [3]. In the same way, resonant converters can operate at high frequency due to their soft switching characteristics. In fact, one of the most popular resonant topologies is the LLC resonant converter where soft switching and high efficiency can be attained over entire load range [4], [5]. Furthermore, Multiphase techniques have been proposed to offer many advantages over the single phase one [6]. Many studies [7], [8] have proposed a star connection on the primary and secondary sides to balance the currents between phases. Many studies focused on the analysis of three-phase resonant converter: in [9] the analysis of three-phase LCL series resonant is presented; the three-phase LCC-type resonant converter with inductive output filter has been proposed in [10]-[11] and the three-phase LCC-type resonant DC–DC converter with capacitive output filter was proposed in [12]- [13]. However, the analysis of the three-phase LLC resonant integrated transformer has not been reported in the literature. This paper introduces for the first time a comprehensive analysis of the three-phase LLC resonant converter depicted in fig.1. The equivalent AC circuit will be derived based on the first harmonic approximation (FHA) approach, which simplifies the system into a linear circuit. Moreover, an optimized design method is introduced based on the steady state output capacitance of the Mosfets in primary switching bridge. In addition, the concept of the magnetic integration is introduced, where the integrated transformer assembly the functions of the resonant inductors and magnetizing inductors of the three-phases on one single magnetic core. A methodology to design the three-phase integrated transformer is presented. Finally, experimental results are presented to validate the properness of the design. Fig. 1. Three-phase LLC converter star-delta connected with integrated magnetics. 978-1-5090-5366-7/17/$31.00 ©2017 IEEE 322