Multilayer Flexible Printed Circuitry Planar Transformer With Integrated Series Capacitance for an LLC Converter

The LLC resonant converter is popular but needs a discrete series resonant inductor and a capacitor. In order to cut down component count, a novel multilayer flexible printed circuitry (FPC) planar transformer with integrated series capacitance and inductance is proposed in this paper. This planar transformer consists of two sets of multilayer open-ended windings coupled with each other to produce the integrated resonant capacitor. The equivalent resonant capacitance can be controlled by the number of the layers without changing the number of primary winding turns or pattern. The proposed transformer successfully integrates the resonant capacitor and the resonant inductor, into a single component. A model is established to accurately predict the equivalent series resonant capacitance of the transformer from the physical structure. Several multilayer FPC planar transformers are built to test the model. Practical measurements and theoretical results show good agreement and validate the proposed model. Finally, an LLC power converter with 94% efficiency is built to demonstrate the proper functioning of the proposed multilayer FPC planar transformer.