A novel and high fidelity simulation-design tool has been proposed for optimizing the design of DC-DC resonant converters. The design tool uses a generalized time-domain analysis applicable to multiple resonant tank configurations. This tool has been made using a MATLAB app designer. The main features of this tool are as follow: 1) fast and accurate steady-state analysis, 2) user-friendly GUI features, 3) design curves for performance optimization and 4) state-of-the-art design examples. The tool is demonstrated for solar PV applications using the LLC resonant converter in this paper. A comparative analysis is shown between LLC and CLL resonant tanks using design curves. Experimentation has been done to verify the theoretical findings from this tool on the 320W prototype.