A novel secondary-side phase-shift-controlled (SS-PSC) LLC resonant converter is proposed for applications requiring hold-up time operation such as distributed power systems and server power supplies. High efficiency at the normal input voltage is achieved because the proposed SS-PSC LLC converter always operates at the series-resonant frequency of the resonant tank. What's more, the magnetizing inductor of the proposed converter doesn't need to be reduced to provide desired voltage boost ratio for the hold-up time operation, which results in reduced conduction losses and improved efficiency. Sufficient voltage boost ratio for the hold-up time operation is provided by using secondary-side phase-shift control strategy. In comparison with the conventional variable-frequency-controlled LLC resonant converter, the main advantages are that the circulating current caused by the magnetizing inductor is effectively suppressed and the efficiency of normal operation is significantly improved. The operation principles, output characteristics and design considerations of the proposed converter are presented in detail. Experimental results are given to verify the effectiveness and the advantages of the proposed solutions.

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Surface modification and characterization of titanium dioxide (TiO2) nanoparticles and their roles in thermal, mechanical, and accelerated aging behavior of foamed wheat straw fiber/polypropylene (PP) composites are investigated. To improve the dispersion of nanoparticles and increase the possible interactions between wheat straw fiber and the PP matrix, the surface of the TiO2 nanoparticles was modified with ethenyltrimethoxy silane (A171), a silane coupling agent. The grafting of A171 on the TiO2 nanoparticles’ surface was characterized by Fourier transform infrared spectroscopy (FTIR). The wheat straw fibers treated with A171 and modified TiO2 nanoparticles were characterized by FTIR and thermogravimetric analysis (TGA). FTIR spectra confirmed that the organic functional groups of A171 were successfully grafted onto the TiO2 nanoparticles and wheat straw fibers, and the modified TiO2 nanoparticles were adsorbed onto the wheat straw fibers. Thermogravimetric analysis showed that a higher thermal stability of the wheat straw fiber was obtained with the modified TiO2 nanoparticles. The flexural, tensile, and impact properties were improved. A higher ultraviolet (UV) stability of the samples treated with modified TiO2 nanoparticles was exhibited by the study of the color change and loss in mechanical properties.

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