This study investigates the use of a high frequency LLC resonant circuit with double loop control in an electric propulsion system's secondary power source. A detailed presentation and discussion of the LLC resonant converter topology design process are given. In order to minimize switching loss when operating at high frequencies, the power switching tube's primary side uses soft switching technology and its secondary side uses synchronous rectification technology. This study presents a thorough analysis of the harmonic converter LLC structure, utilizing pulse width modulation (PFM) in place of standard PWM. Simultaneously, a double loop voltage-current control technique is developed that can function at constant voltage throughout the load and voltage change range. Ultimately, the simulation results demonstrate that the controller performs well and that the suggested circuit converter can achieve a constant voltage across a wide range of input voltages and load steps.