The proposed approach integrates an optimized transformer in an LLC resonant circuit for onboard charging in electric or hybrid electric vehicles (EVs or HEVs). The transformer is designed using an advanced optimization method, Horse Herd Optimization Algorithm (HOA), to minimize core loss, winding loss, and overall cost. A 3D model of the transformer is created and its performance is validated using MATLAB and ANSYS software. The proposed model achieves high power quality, low temperature, and low manufacturing cost. It outperforms other existing algorithms in terms of core loss and cost. The proposed model is well-suited for onboard charging applications in EVs or HEVs.