An LLC resonant converter includes a switching circuit for converting a DC voltage into switching signal, a resonant tank coupled to the switching circuit to receive the switching signal and to provide a primary current, a transformer circuit coupled to the resonant tank. The transformer circuit includes a plurality of separated transformers, each has a primary side and a secondary side windings disposed on the PCB, where the primary side winding of each transformer can be selected to couple in series or in parallel with the primary side winding of other transformers to form a dynamically varied equivalent primary side winding, maintaining the turns ratio to fine-tune the resonant tank. The gain curve of the LLC converter can be adjusted by electrically coupling an external excitation inductor, a resonant capacitor or a resonant inductor to the resonant tank, according to the demand of output current.