Performance Analysis of an Input-Series-Output-Parallel LLC Resonant Converter with Parameters Mismatch

Input-series-output-parallel (ISOP) converters enable the use of low voltage devices in high voltage applications. Additional benefits include modularity and increased efficiency due to improved device figure-of-merit (FOM) for low voltage devices. A ISOP system typically consists of many converter modules. In order to ensure proper system operation, voltage and current among each module should be equal. However, parameter mismatch is inevitable between converter modules, causing input voltage and current unbalanced. In this paper, performance of an ISOP LLC converter with resonant tank parameter mismatch is analyzed. Impedance model analysis of system input voltage sharing (IVS) is established and conducted under first harmonic approximation (FHA) assumption. Analysis shows that the ratio of resonant inductor Lr and magnetizing inductor Lm has a major impact on input voltage sharing, while voltage sharing is not affected significantly by L
m
value. To achieve an ISOP LLC with good input voltage sharing, small $L_{r}/L_{m}$ ratio is expected. A 3kW, 400V to 48V four-module ISOP LLC is developed to verify the conclusion and good input voltage sharing property is demonstrated by simulations and experiments.