Partial inductance modeling and series integrated capacitance with optimization for thin and flexible planar transformers in LLC power converters

Abstract

This thesis presents two novel devices: a novel integrated multilayer Flexible Printed Circuitry (FPC) transformer and a bendable transformer. Models of these transformers are established to analyze the characteristics of the proposed transformers. Furthermore, a multi-physics design and optimization method is suggested for the proposed transformer in high frequency LLC resonant converter. The LLC resonant converter is popular but a discrete series resonant inductor and a capacitor in the resonant tank are needed, which increases the component count. In order to eliminate these two components, a novel multilayer FPC (Flexible Printed Circuitry) planar transformer is proposed in this thesis. The proposed transformer successfully integrates the resonant capacitor and the resonant inductor with the transformer such that one can reduce the amount of converter components. In order to study the characteristics of the transformer, a model is established to accurately predict the equivalent series resonant capacitance of the transformer, as verified by the experimental results. A 170 V to 5 V, 4 A output LLC converter prototype with the multilayer FPC planar transformer is built, which achieves 94% efficiency at full load. In a typical LLC converter, ferrite-based transformers are commonly used but remain inflexible. This restricts applications that need physical flexibility such as wearable electronics. In this thesis, a bendable transformer with air core is presented for such applications. The winding of the bendable transformer is printed on a thin, bendable film, allowing the transformer to be wrapped around body limbs such as forearm. A model is developed to study the characteristics of this transformer based on the partial equivalent circuit theory (PEEC). A converter with 5 V, 500 mA output is included to confirm the usefulness of the transformer and the validity of the model. An air core planar transformer is an attractive alternative to a typical ferrite core transformer, not only because of its physical flexibility, but also for its zero ferrite loss property while operating at high switching frequency. Although there are many design methods for LLC converters in literatures, most are based on traditional ferrite core transformers. There is a lack of effective methods to design air-core planar transformers for high frequency LLC converter. Therefore, an optimal design strategy that combines different modeling and optimization techniques is proposed for the high frequency air core LLC converter in this thesis. In this method, an accurate transformer model and loss model are built respectively for transformer design and efficiency evaluation. The bisection method and Bayesian optimization algorithm are both applied to accelerate the design process. An air core LLC converter with 2 MHz switching frequency is built to demonstrate the design method. Finally, the experimental results match well with the transformer models and loss model.