Practical Design Considerations for a LLC Multi-Resonant DC-DC Converter in Battery Charging Applications Fariborz Musavi, Marian Craciun, Murray Edington Department of Research, Engineering Delta-Q Technologies Corp. Burnaby, BC, Canada fmusavi@delta-q.com, mcraciun@delta-q.com, medington@delta-q.com 1 Wilson Eberle and 2 William G. Dunford 1 School of Eng. | 2 Dept. of Electrical and Computer Eng. University of British Columbia | 1 Okanagan | 2 Vancouver 1 Kelowna, BC, Canada | 2 Vancouver, BC, Canada 1 wilson.eberle@ubc.ca | 2 wgd@ece.ubc.ca Abstract—In this paper, resonant tank design procedure and practical design considerations are presented for a high performance LLC multi-resonant dc-dc converter in a two-stage smart battery charger for neighborhood electric vehicle applications. The multi-resonant converter has been analyzed and its performance characteristics are presented. It eliminates both low and high frequency current ripple on the battery, thus maximizing battery life without penalizing the volume of the charger. Simulation and experimental results are presented for a prototype unit converting 390 V from the input dc link to an output voltage range of 48 V to 72 V dc at 650 W. The prototype achieves a peak efficiency of 96 %. I. INTRODUCTION Neighborhood Electric Vehicles (NEVs) are propelled by an electric motor that is supplied with power from a rechargeable battery [1], [2]. Presently, the performance characteristics required for many electric vehicle (EV) applications far exceed the storage capabilities of conventional battery systems. However, battery technology is improving and as this transition occurs, the charging of these batteries becomes very complicated due to the high voltages and currents involved in the system and the sophisticated charging algorithms [3]. Quick charging of high capacity battery packs causes increased disturbances in the ac utility power system, thereby increasing the need for efficient, low-distortion smart chargers. The accepted charger power architecture includes an ac-dc converter with power factor correction (PFC) [4], followed by an isolated dc-dc converter, as shown in Fig. 1 [5]. This architecture virtually eliminates the low- and high- frequency current ripple on the battery, thus maximizing battery life without penalizing the volume of the charger. The front end ac-dc PFC converter is a conventional CCM boost topology [6], [7]. The following dc-dc section is a half- bridge multi-resonant LLC converter. The criteria for choosing these topologies includes high reliability, high efficiency and low component cost. The half-bridge resonant LLC converter is widely used in the telecom industry for its high efficiency at the resonant frequency and its ability to regulate the output voltage during the hold-up time, where the output voltage is constant and the input voltage might drop significantly [8]-[11]. The output voltage requirement for a battery charger is drastically different and challenging compared to telecom applications. Fig. 2 illustrates a simplified battery charging profile for a 48 V system. As it indicates, the battery voltage, at the dc-dc converter output, can vary from as low as 36 V and as high as 72 V. Therefore the design requirements for selecting the resonant tank components are different of those for telecom application with constant output voltage [12], [ 13]. The resonant tank design principle is extracted from the lead acid battery V-I plane, as shown in Fig. 3. The V-I plane for lead acid battery dictates the design criteria for the half- bridge multi-resonant LLC converter, in particular the resonant tank components, L r , L m and C r .                          !    "  #$ % % Figure 1. Typical battery charging power architecture. Figure 2. Simplified battery charging profile. This work has been sponsored and supported by Delta-Q Technologies Corporation.