Abstract— A switching control strategy to extend the soft-switching operating range of the dual active bridge (DAB) dc-dc converter under the zero-voltage-switching (ZVS) operating mode is proposed. The converter topology consists of two active bridges linked by a high-frequency transformer. One drawback of this strategy is that soft-switching is only possible in a restricted converter operating region. A novel pulse width modulation strategy to extend the conventional soft-switching operating mode region and its analysis are presented in this paper. Experimental results are given in order to validate the theoretical analysis and practical feasibility of the proposed strategy. I. INTRODUCTION The dual active bridge converter (DAB) was originally proposed in [1]. It is a buck and boost, bidirectional dc-dc converter, based on two active bridges interfaced trough a high-frequency transformer. Several soft-switched high power-density dc/dc converter topologies for high-power applications were compared in [2], where the authors concluded that the DAB topology is an interesting alternative for high power applications, in which high efficiency, buck and boost voltage operation, and bidirectional power flow capabilities are needed [3]. The DAB topology can operate in the soft-switching mode in all the power switches, but only within a limited region restricted by its operating point. Full control range in the soft-switching mode is achievable only when the converter voltage ratio is equal to one ( ) 1 d = [4]. A novel switching control strategy to extend the operating range of the DAB topology in the soft-switching mode of operation is proposed in this paper. This strategy allows optimizing the use of the power semiconductor switches needed to implement the converter, improving the whole converter efficiency. An experimental converter prototype was built and experimental results are given to validate the practical feasibility of the proposed ideas. II. DUAL ACTIVE BRIDGE TOPOLOGY AND SWITCHING CONTROL ANALYSIS The DAB topology is shown in Fig. 1. This topology comprises of a full bridge working as a dc-ac converter that feeds a high-frequency transformer. The transformer supplies a second full bridge that works as an ac-dc converter. The power flow is controlled by switching both active bridges, in order to obtain a proper phase shift, δ , between the voltages imposed by the bridges in both sides of the transformer. The direction of the power flow is determined by the sign of δ . For given input and output dc voltages, the maximum power that can be transferred corresponds to 90º δ ± [1]. In the Fig. 1 each switch xx S is performed by xx T , xx D and xx C . The innovation proposed in this paper consists of adding one pulse modulation in one of the converter bridges. This modulation adds a new degree of freedom; a new variable that allows controlling the current waveform with the objective of extending the soft-switching operating mode region. The converter analysis can be simplified by considering that the transformer turns ratio is unity and its magnetizing inductance is much greater than its leakage inductance. In this way, the magnetizing inductance can be considered an open circuit. Therefore, the converter can be represented by a simplified scheme comprised of two active bridges linked by an inductance, L , that represents the transformer leakage inductances, as it is shown in Fig. 2. Extending the ZVS Operating Range of Dual Active Bridge High-Power DC-DC Converters C1 S11 S12 S13 S14 S21 S22 S23 S24 C2 TX V 1 I C1 I 1 I T2 V T1 V T2 V 2 I C2 I 2 I B1 I B2 I T1 B1 B2 Fig. 1 DAB converter topology. Germán G. Oggier 1 , Roberto Leidhold 2 , Guillermo O. García 1 , Alejandro R. Oliva 3 , Juan C. Balda 4 and Fred Barlow 4 1 National University of Río Cuarto, School of Engineering. Argentina. Email: goggier@ing.unrc.edu.ar 2 Darmstadt University of Technology, Department of Power Elec. and Control of Drivers. Germany. Email: leidhold@srt.tu-darmstadt.de 3 Universidad Nacional del Sur, Department of Electrical Engineering. Argentina. Email: aoliva@uns.edu.ar 4 University of Arkansas, Department of Electrical Engineering. USA. Email: jbalda@uark.edu 37th IEEE Power Electronics Specialists Conference / June 18 - 22, 2006, Jeju, Korea 1-4244-9717-7/06/$20.00 2006 IEEE. WeA4-2 - 2471 -