Development of high frequency power conversion technologies for grid interactive PV systems

This thesis examines the development of DC-DC converters that are suitable for Module Integrated Converters, (MICs), in grid interactive photovoltaic (PV) systems, and especially concentrates on the study of the half bridge dual converter, which was previously developed from the conventional half bridge converter. Both hard-switched and soft-switched half bridge dual converters are constructed, which are rated at 88W each and transform a nominal 17.6Vdc input to an output in the range from 340V to 360Vdc. An initial prototype converter operated at 100kHz and is used as a base line device to establish the operational behaviours of the converter. The second hard-switched converter operated at 250kHz and included a coaxial matrix transformer that significantly reduced the power losses related to the transformer leakage inductance. The soft-switched converter operated at 1MHz and is capable of absorbing the parasitic elements into the resonant tank. Extensive theoretical analysis, simulation and experimental results are provided for each converter. All three converters achieved conversion efficiencies around 90%. The progressive increases in the operation frequency, while maintaining the conversion efficiency, will translate into the reduced converter size and weight. Finally different operation modes for the soft-switched converter are established and the techniques for predicting the occurrence of those modes are developed. The analysis of the effects of the transformer winding capacitance also shows that soft switching condition applies for both the primary side mosfets and the output rectifier diodes.