Fully ZVS, Minimum RMS Current Operation of Isolated Dual Active Bridge DC-DC Converter Employing Dual Phase-Shift Control

Abstract

This paper proposes the least possible transformer rms current control strategy to operate an isolated dual active bridge (DAB) dc-dc converter using dual phase-shift (DPS) control. Close-form expressions of converter parameters e.g. average and rms current, active and apparent power are derived for four possible operating modes under DPS control. The Lagrangian objective function is formulated such that apparent power and hence reactive power are truly minimized for the desired active power transfer. As the output power is varied, optimal phase-shift ratios are deduced numerically, which constitute the minimum rms current trajectory. Using the optimal trajectory, a closed-loop controller is devised which employs feedforward control. It obviates the use of a lookup-table-based approach, requiring off-line precomputation of optimal duty ratios. In addition, necessary boundary conditions are also derived to ensures fully zero-voltage switching (ZVS) operation in both active H-bridges of DAB converter. A 4.0 kW, 400/400 V, 25 kHz, isolated DAB converter is simulated in MATLAB/Simulink. At rated output, a maximum reactive to active power ratio (Q/P) of 1.158 and a minimum efficiency of 98.77% are observed, which validates the performance of proposed minimum rms current control strategy.