Design optimization of off-line power converters: from PWM to LLC resonant converteres

Abstract

High power conversion efficiency is desirable in power supplies. Design optimization of on-line power converter is presented in this thesis. High efficiencies over a wide load range, for example 20%, 50% and 100% load, are often required. It is a challenge for on-line pulse-width modulation (PWM) converters to maintain good efficiencies with light load as well as full load. A two-stage multi-objective optimization procedure is proposed to optimization power converter efficiencies at 20%, 50% and 100% load. Two-FET forward prototype converters are built to verify the optimization results. The LLC (abbreviation of two resonant inductor L and one resonant capacitor C ) series resonant converter can provide high power conversion efficiency because of the resonant nature and soft switching. The design of LLC resonant converter is more difficult than that of PWM converters since the LLC resonant converter has many resonant modes. Furthermore, the LLC resonant converter does not have analytical solution for its resonant operation. In this thesis, a systematic optimization procedure is proposed to optimize LLC series resonant converter efficiency. A mode solver technique is developed to solve LLC resonant converter operations. The proposed mode solver employs non-linear programming techniques to solve a set of LLC state equations and determine the resonant modes. Loss models are provided which serve as the objective-function to optimize converter efficiency. Optimization results show outstanding efficiency performance and experimental agreement with optimization. The optimization work extends to the LLC resonant converter with power factor correction (PFC) circuits where the effect of LLC converter input voltage variation cased by the PFC circuit is considered. Detail comparisons of PWM converter and LLC resonant converter loss profiles are also presented. The reasons that LLC resonant converter has higher efficiency are given and supported by quantitative data. Converter lifetime is highly related to component losses and temperature. The lifetime analysis is presented. The analysis reveals that the LLC resonant converter output capacitor is the weakest component concerning life.