Optimal Trajectory Control Scheme for Dual-Bridge Series-Resonant Converters With Constant and Small Low-Voltage Side Turn-Off Current

With the emerging of large-capacity battery cells, the demand of high-current, high-conversion-ratio bidirectional dc-dc converters in energy storage systems (ESS) has also increased. Similarly, electric vehicle on-board chargers (OBC) face the same requirement when charging or discharging low-voltage (LV) batteries. Although conventional approaches such as inductor-inductor-capacitor converter (LLC) have provided good performance, their LV transistors still suffer from large switching stress during conversions from the LV side to the high-voltage (HV) side. In addition, due to the resonance, load transition response is slow and resonant amplitude oscillates during transients, which further increases the LV switching stress. To overcome these two shortcomings, an optimal trajectory control (OTC) based control for dual-active-bridge series-resonant converter (DBSRC) is proposed. It uses state-plane analysis (SPA) to find mode length solutions with the following two properties: First, for all the steady-state and transition solutions, zero-voltage switching (ZVS) always holds and the LV switching stress is constant and small; Second, transitions converge within one cycle and the resonant amplitude does not oscillate or overshoot. To validate the proposed solution, steady state tests, open-loop power transition tests and closed-loop load transition tests are conducted on a 3 kW prototype. In all tests, ZVS and small turn-off current are observed in every cycle, which is consistent with expectations. Besides, single-cycle convergence and fast load transition response are also verified.