Utilizing Flexible Printed Circuit Board (FPCB) to realize passives integration in LLC resonant converter

There are altogether four passive components in the resonant tank of LLC resonant converter: the series resonant inductor, the resonant capacitor, the parallel inductor and the transformer. A new method utilizing the flexible printed circuit board (FPCB) to realize passives integration in LLC resonant converter is presented. The design process is provided, and the experimental results verify its feasibility. However, due to the severe high frequency loss of the dielectric material used in FPCB technique, the converter efficiency decreases greatly at heavy load condition after the resonant capacitor is integrated, and the switching frequency range to realize output voltage regulation against load variation also becomes wider. We use another two kinds of dielectric material with better characteristic replace the substrate dielectric material in FPCB technique, then the efficiency improves and the switching frequency range shrinks. As a result, the fully integrated structure can reduce the total passive components size by 36% compared to the discrete structure with even better performance. Hence the power density is increased remarkably.