Optimal Efficiency Operation of Non-Isolated DC/DC Converter for High Voltage Ratio Applications A. Shahin, Member, IEEE Electrical dept., Faculty of Engineering, Mansoura University, Egypt ahshein@mans.edu.eg J.-P. Martin, Babak Nahid-Mobarakeh, Senior Member, IEEE, S. Pierfederici GREEN, University of Lorraine, Nancy, France serge.pierferedici@univ-lorraine.fr Abstract— In this paper, a global optimal study of the efficiency of a cascaded structure with synchronous rectification composed of two sub-converters has been presented, this converter allows obtaining high voltage ratio and consists of two- interleaved boost converter which was chosen as 1 st sub-converter and three-level boost converter which was chosen as 2 nd sub- converter. An optimal efficiency operation as a function of the intermediate voltage and the branch number of the first converter is studied by the aid of an adaptive losses estimation algorithm to estimate the components losses for each DC/DC converter. The estimated power losses of the two converters are modeled by two estimated resistances. These estimated resistances are used to study and optimize the relation between the intermediate voltage and the global efficiency and also the influence of the number of branches of the interleaved converter on the total global losses. The algorithm is theoretically analyzed, developed and compared with the experimental results. Experimental results allow validating the proposed analysis for maximizing global efficiency. Index terms— DC/DC converters, Losses Estimation, Flatness Control. I. INTRODUCTION In order to maximize the efficiency of the power supply like the fuel cell power system, the operating point of a fuel- cell stack should be chosen close to the maximum power- delivery point [1]-[3]. However, maximizing the power- delivery and the efficiency demands evaluation of the losses of the conversion system. A converter for this application has to meet the prevalent automotive requirements, such as being a low-cost design, and minimizing the component size, a highly compact design and a low overall weight are required without forgetting stability issues [4], [5]. An efficiency optimization of the DC/DC converter over a wide input and output power range is also a critical issue [6], [7]. For investigating the performance of converter, an accurate mathematical model of the losses in the DC/DC power converter is needed. In this paper, we present a method for calculating all losses types for a cascaded DC/DC converter, this converter is composed of two sub-converters; two-interleaved boost converter which was chosen as 1st sub-converter and three- level boost converter which was chosen as 2nd sub-converter. The converter works in the synchronous rectification mode, this converter allows obtaining high voltage ratio. all the losses of this converter are analysed and modelled, we can define the Joule and magnetic losses in the non ideal inductance, switching and conduction losses in the semiconductors of the cascaded converter, as detailed in [8]. For the studied converter, the switching is achieved by synchronous rectification. The proposed estimation method can effectively deal with the issues caused by variation of the inductance, input voltage drift, and voltage drops caused by inductance DCR or semiconductors parasitic ON-state resistances. The online parameters estimation method is based upon voltages and currents sampled between the input and output. Mathematical modeling is carried out by careful consideration of all the significant depending parameters, namely the inductance current, the switching frequency and the temperature of the magnetic core. An analytical model of the losses in the semiconductor device like MOSFET is used which is derived from the mathematical equations of the waveforms [9]- [12]. The aim of this paper is to study, for cascaded structure of converter, the influence of the intermediate voltage amplitude on the global efficiency of the system. This optimization is based on the losses analysis for the entire converter components like inductance [13], magnetic core [14]-[16] and semiconductors. II. LOSSES MODELING FOR THE PROPOSED CONVERTER Fig.1 shows the structure of the proposed bidirectional converter which works in the reversible mode with synchronous rectification using MOSFET instead of diodes. A simplified general model for the inductance and the MOSFET transistor ON-state is used to evaluate the conduction losses. The equivalent circuit is shown in fig. 2 for each part and its parameters for the conduction model. The inductance model used to evaluate conduction losses is based on the parasitic resistance and magnetic core losses, and for the transistor, the losses model is based on the ON-state resistance. In the following part, a general analysis for all types of losses will be achieved to model these losses by two resistances; serial and parallel, for each converter. The estimated resistances will be used to model the losses to search the optimal value of the intermediate voltage for the maximum global efficiency and the optimal the number of branches for the interleaved converter. For both converters, interleaved and three-level, all the details for the losses calculations and the parameters for the modeling are detailed in [17], [18]. A. Losses Modeling for the Proposed Converter Based on Figs. 1 and 2, the total conduction losses of the interleaved converter is the sum of the inductor resistances r L11 loss P rLInter for M branches and the MOSFET parasitic resistance loss   with the average current I L11 , and the parasitic resistance r c of the output capacitor is neglected. ＹＷＸＭＱＭＴＷＹＹＭＰＲＲＳＭＱＯＱＳＯＤＳＱＮＰＰ＠ﾩＲＰＱＳ＠ｉｅｅｅ ＱＱＰＴ