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Combining the auxiliary DAB converter and the bidirectional LLC converter operating at a fixed frequency in an input-series output-parallel (ISOP) structure can achieve bidirectional power regulation and high efficiency in DC-DC transformer (DCX) applications. However, for bidirectional DCX applications, the synchronous rectification (SR) operation becomes challenging since that the instantaneous-transferred power is difficult to obtain, and LLC-DCX will operate in discontinuous current mode (DCM) under light load conditions. In addition, the rectifier-bridge has to be swapped under different power flows. To achieve on-line SR modulation for LLC-DCX under bidirectional power application, a new solution to achieve accurate SR operation, based on instantaneous-transferred power estimation is proposed under full load range. The proposed method uses the added auxiliary DAB converter as a power detector to estimate the power value without input voltage sampling and any auxiliary current sensor or SR detection circuit. Besides, the mathematical derivation of power estimation, DCM operation conditions and the SR conduction period for the rectifier-bridge, are analyzed in depth. Finally, the effectiveness of the proposed method and the correctness of analysis are verified by the simulation and experimental results.