Traditional LLC circuit analysis is based on frequency domain analysis method and adopts the fundamental harmonic approximation(FHA)

which just considers the fundamental component

for equivalent. It ignores the influence of higher harmonics on power conversion

which is an approximation to the resonance transformation

rather than an accurate model. According to the actual voltage and current waveforms of full bridge LLC circuit

the analysis method and steps of LLC resonant converter are firstly given

and the nonlinear time domain equations are listed. Then

the gain curve containing all information is plotted based on the time domain equations

the gain of the entire operating frequency band of the LLC circuit is analyzed

and the FHA simplified method is compared. Finally

the time domain model and gain analysis are experimentally verified on a 20 kW charging module circuit of electric vehicle.