Abstract
Step-down DC converters are widely used in various applications such as VRMs of CPU boards and battery chargers. For higher step down ratio case, the resulting output current becomes rather large. Naturally, an interleaved two-phase buck converter can be employed to reduce the output ripple. However, additional control effort should be made for equal current sharing of two phases. Recently, an improved two-phase step-down converter is proposed to provide higher step-down ratio and with automatic current sharing capability by one more capacitor. However, for even higher step down ratio and power level, the new two-phase DC converter is not sufficiently good enough to maintain high efficiency. Hence, a novel four-phase high step-down ratio DC converter is presented in this thesis for further improvement. Basically, the contributions of this thesis may be summarized on follows. First, a novel four-phase DC converter is proposed to achieve high step down voltage ratio, low voltage stress of power devices, reduced conduction losses and output ripples, and automatic current sharing capability together with automatic uniform voltage distribution of input series capacitors. Second, modeling and analysis of the new converter are also made in the context. Third, two prototypes namely 400V/24V input/output with 500W rating and 400V/48V input/output with 1kW rating, are constructed for verifying the validity of the proposed converter. It is seen that the maximum efficiencies of 95.2% and 97.23% can be achieved in sequence respectively for the previous two prototypes. Keywords：High step down voltage ratio, interleaved control, four phase DC converters, high efficiency.