Abstract
AbstractDue to consideration of the limited fossil fuel and impact of the environment, various alternative energy sources have now been explored and developed. Among them, fuel cell is considered as one of the most promising energy vector because of its high efficiency and low maintenance requirement. Because the low output voltage of fuel cell and its slow dynamic response, usually a DC converter is required for boosting the fuel cell voltage to a higher value. However, due to the inherent characteristic of switching-mode power supplies, high frequency ripple current cannot be avoided. In fact, it is well known that the magnitude of the high frequency ripple current have rather significant impact to the operating efficiency and the life time of the fuel cell.In view of the above problems, a high efficiency step-up converter with a ripple mirror (RM) circuit is proposed in this thesis for fuel cell systems. Basically, the contributions of this thesis can be summarized as follows. First, a RM circuit is proposed for the step-up converter to achieve zero ripple condition and enhance the operating efficiency and life time of the fuel cell. The proposed RM circuit technique provides much better flexibility than the two-phase interleaved boost converter for locating the zero ripple operating point in the design stage. This characteristic is especially suitable for applying to fuel cell systems where the duty ratio of the step-up converter is usually much larger than fifty percents. Moreover, smaller boost inductor can be adopted to reduce the conduction loss. Second, a boundary mode control is adopted to achieve both soft-switching and constant duty ratio control for the converter to further increase the efficiency. Third, both DC and small signal models are derived and analyzed for simplifying the design of the proposed converter. Finally, a 200 watts 48 volts input 200 volts output prototype is constructed. It is seen that the resulting peak to peak input current ripple is less than 6.67 percents as compared with the 69.88 percents ripple of the two-phase interleaved boost converter with the same boundary mode control and power capacity. The full load efficiency of the proposed converter is about 93.90 percents and the added RM circuit for processing the ripple power only consumes 0.04 percent of the total losses. Both simulation and experimental results indeed verify the effectiveness of the proposed converter.