J Electr Eng Technol.2016; 11(?): 1921-718 http://dx.doi.org/10.5370/JEET.2016.11.1.1921 1921 Copyright ⓒ The Korean Institute of Electrical Engineers This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. A 3 kW Bidirectional DC-DC Converter for Electric Vehicles Arsalan Ansari*, Puyang Cheng** and Hee-Jun Kim † Abstract – A bidirectional DC-DC converter (BDC) is an indispensable electrical unit for the electric vehicles (EVs). High efficiency, high power density, isolation, light weight and reliability are all essential requirements for BDC. In this paper, a 3 kW BDC for the battery charger of EVs is proposed. The proposed converter consists of a half-bridge structure on the primary side and an isolation transformer and a synchronous rectifier structure on the secondary side. With this topology, minimum number of switching devices are required for bidirectional power flow between the two dc buses of EVs. The easy implementation of the synchronous rectification gives advantages in terms of efficiency, cost and flexibility. The proposed BDC achieves high efficiency when operating in both modes (step-up and step-down). A 3 kW prototype is implemented to verify theoretical analysis and the performance of the proposed converter. Keywords: Bidirectional DC-DC converter, Battery charger, Electric vehicles, Efficiency 1. Introduction With the global energy crisis the conventional vehicles (internal combustion engines) face the increasingly serious problems of energy. In contrast, the EVs especially battery electric vehicles (BEVs) depend on variety of options for its driving power. BEVs offer the advantages of safety, silent operation and no emissions when powered by renewable energy sources such as wind or solar which are virtually emission free [1]. These vehicles can also make efficient use of energy by storing energy recovered during braking or deceleration cycle in the batteries. The storage or charging process of the battery is achieved by a BDC, which is the key block in EV energy system to link high voltage (HV) dc bus and low voltage (LV) dc bus as shown in Fig. 1. This BDC should have high power density and high efficiency to meet the desired goals for EV’s battery charger. When the EV is parked, the battery can be charged by the household utility outlet from the grid through the BDC. For the other case when the EV is in the driving state, the BDC provides the electrical power from LV battery to the motor through DC-AC inverter and also DC loads in the EV. BDCs are broadly classified into isolated and non- isolated types. The conventional non-isolated buck/boost BDC cannot operate in the wide voltage conversion range [2]. The isolated BDC are preferred for EVs due to the advantages of high voltage conversion ratio and safety. Many different types of isolated BDCs [3-6] have been proposed due to these advantages, some full-bridge BDCs [7-10] have also been proposed in recent years. However, full-bridge converters have the disadvantage of high voltage ripples if not employing an extra voltage clamping circuit [11]. By contrast, half-bridge converters [12-14] have a simple structure and a better anti-imbalance ability in the transformer. In some topologies of half- bridge converters, MOSFET body diodes are applied for synchronous rectification in both buck/boost modes [15], but high conduction losses result in low efficiency, thus limiting the use of these converters to only low power applications. This paper describes the development of a 3 kW BDC for EVs. The converter consists of a half-bridge topology, an isolation transformer and a synchronous rectifier. The isolation transformer provides the advantages of wide conversion range and safety, and the easy implementation of synchronous rectification offers the benefits in terms of efficiency, cost and flexibility. However, this structure has been mostly proposed for less than 1.5 kW application [3, 11-13, 16-17], so the operation of more than 3 kW in both step-up and step-down modes has the practical significance for the EV battery charger products. † Corresponding Author: Dept. of Electronic Systems Engineering, Hanyang University, Korea. (hjkim@hanyang.ac.kr) * Dept. of Electronic Systems Engineering, Hanyang University, Korea. (arsalan.07el@gmail.com, chengpuyang@outlook.com) Received: June 9, 2015; Accepted: October 29, 2015 ISSN(Print) 1975-0102 ISSN(Online) 2093-7423 Fig. 1. The energy system of EV