A High Boost Ratio Bidirectional Isolated dc-dc Converter for Wide Range Low Voltage High Current Applications Cong Li*, Luis Herrera, Jizhou Jia, Lixing Fu, Yi Huang, and Jin Wang Department of Electrical and Computer Engineering The Ohio State University 205 Dreese Lab, 2015 Neil Ave. Columbus, Ohio 43210, USA *li.1012@buckeyemail.osu.edu Alexander Isurin*, Alexander Cook Vanner Inc. 4282 Reynolds Drive Hilliard, OH 43026, USA *sashai@vanner.com Abstract — This paper introduces a high boost ratio, bidirectional, isolated, dc/dc converter designed as an interface between low voltage, high current dc sources and 24 V dc systems. This converter realizes bidirectional power flow with two main switches and operates under a wide input voltage range, from 1.5 V to 12 V, with an input current as high as 300 A. An active snubber circuit is utilized to recycle the energy stored in the leakage inductor of transformer and realize zero voltage switching (ZVS) off for one main switch. The zero current switching (ZCS) on of this switch is achieved by using transformer leakage inductor, thus no additional components are needed. Furthermore, the power loss of another main switch is minimized by operating it in the Synchronous Rectification mode. Complete circuit description and operation principles are provided in this paper. A 1.2 kW lab prototype has been built and tested with full power rating. Both simulation results and experimental results are presented to verify proposed functions. I. INTRODUCTION Many low voltage high current dc sources, like ultra capacitor banks, Li - Ion batteries and NiMH batteries require high boost ratio dc/dc converters to transfer their energy into higher dc systems. However, it is challenging to design a universal interface between these types of dc sources and higher voltage systems because of the large variation of input voltage and the high current requirement. Based on the survey of existing products of ultra capacitors, Li - Ion and NiMH batteries, the designed targets of the converter include:  input voltage range: 1.5 V to 12 V;  maximum current: 300 A (from 2 V to 4 V);  minimum output voltage at 24 V, which is a common voltage rating in electric power steering systems.  bidirectional power flow capability. There are a lot of applications in low voltage high current dc/dc energy conversion area, but most of them are designed for fuel cell and automotive electrical systems, which have different operation conditions. For fuel cell applications, many papers are focused on the Solid Oxide Fuel Cell which can operate from 22 V to 41 V, with 5 kW continuous power [1- 2]. In automotive area, due to the increasing demand for electrical power, 42 V electrical distribution systems have been seen in hybrid electric vehicles together with the traditional 14 V system. Thus, a 42 V /14 V, 2 kW – 5 kW dc/dc converter is needed and a lot of researches have been carried out on this topic [3-5]. However, for the lower voltage range addressed in this paper, only limited publications have been presented [6-8]. One of the major challenges in this topic is the difficulty to realize high efficiency under such a low voltage and high current rating. Taking the example of Energy Star Program’s 80 PLUS standard, till now many manufacturers still do not have products that could be certified for 80 PLUS Silver level, which requires efficiency higher than 85% [9-11]. Compared with server power supplies, the proposed converter design is facing tougher challenges: 1) extremely low operation voltage, down to 1.5 V; 2) large current capability, up to 300 A; 3) high boost ratio as high as 16; and 4) wide input voltage range; from 1.5 V to 12 V, an eight fold of change. In practical designs, it is often difficult to use transformerless converters to realize high boost ratio without adding component counts [12]. Furthermore, the large current rating excludes traditional isolated circuit topologies, such as the flyback and forward converters. For commonly used full bridge based converters, each switch is required to handle the full load current. To deliver 300 A, the high switch count and the number of devices in parallel for each switch, will make the circuit economically not feasible. As shown in [13-15], coupled inductor based converters can meet part of the requirements, but at the price of either high circuit complexity or limited current rating. This paper proposes a cost effective circuit based on isolated Cuk converter [16]. The proposed converter has 978-1-4577-1216-6/12/$26.00 ©2012 IEEE 539