Operation analysis and numerical approximation for the LLC DC-DC converter

The LLC resonant converter has the advantages of achieving high power efficiency for wide input voltage and load range, which makes it a popular candidate in high power density and high efficiency power converter applications. However, the complex interaction between resonant components makes it hard to analysis. The fundamental harmonic approximation (FHA) is widely adopted as a simplified analysis model. Yet, it has poor accuracy in gain prediction and cannot explain LLC's various operation modes. In this paper, a detailed mode analysis is developed that can offer highly accurate description on the resonant behavior, and the mode distribution under different load and frequency is illustrated and discussed. Moreover, a numerical approach is developed based on the mode model to provide precise peak gain points estimation. An experimental prototype is built to verify the analysis.