DC micro-grids and AC micro-grids are usually connected with a Power Electronic Transformer (PET) to operate as a Hybrid AC/DC Micro-grid (HADM) and reduce energy conversion. However, the resonant parameters of a High Frequency Transformer (HFT) limit the efficiency of a PET, and a slight variation of the topology poses a big challenge on parameter design. Therefore, a generalized parameter design approach is proposed for a bidirectional high frequency DC transformer. It is derived that the maximum transmission power of an HFT is ensured by keeping the resonant current in phase with the fundamental part of the input voltage. Meanwhile, the resonant HFT with CLLC, LLC or CLL will successfully transmit power bi-directionally and satisfy the requirement of normalized gain and efficiency of an HADM system, when the ratio between the magmatic inductance and the leakage inductance is larger than a certain value. The CLLC topology is conducive to the power transmission. Finally, a 6 kW experimental prototype with SiC is established to verify the feasibility and effectiveness of the proposed approach. The maximum efficiency is more than 97% in the bidirectional operating mode. This work is supported by the National Natural Science Foundation of China (No. 51677151 and No. 51507138) and the Science and Technology Project of Sichuan Province (No. 2021YJ0490).