The Design Of : Front - End DC - DC COllverters Of Distributed Power Supply Systems With Improved Efficiency And Stability

This the~is explores the design of front-end converters in the 1 kW to 1.5 kW power range to be used in distributed power supply systems. The intent is to maximize the efficiency, power density and the stability of the overall system. The topologies used for these front-end converters are square-wave bridges switching a.t frequencies in the sub-MHz range. The thesis starts by reviewing the switching behavior of these bridges, paying particular attention to quantifying each significant power loss mechanism. The thesis then shows how the operation of the bridges can be modified to reduce the losseG due to the parasitic elements. To this end it d~scribes phase-shifted PWM, a mode of operating the full bridge that allows the recovery of most of the energy associated with the parasitic elements that would otherwise be lost. A 1 kW, 500 kHz phase-shifted full-bridge front-elld converter acconlmodating an input voltage range of 190-380 V and delivering a regulated bus voltage of 40 V is presented that features improved efficiency over conventionally operated full-bridges. This front-end converter can be nlade even more efficient by splitting it into a boost preregulator that converts the variable input voltage to a constant voltage of 390 V and a safetJ' i~4Jlated stage that converts this voltage without regulation to a bus voltage of 40-50 V. The preregulator, which also functions as a power factor corrector, has been described in a related thesis. The constant input and output voltages of the isolation stage allow the use for it of a half-bridge topology operated without an output inductor at a duty ratio of unity. This lllode of operation offers all the advantages of a phase-shifted bridge plus the eliminat.ion of eveIl t.llose switching losses that phase-shifted PWM could not remove. Two isolation stage prototypes are presented, one using diode rectifiers, and the other with synchronous rectifiers. These prototypes switch in the 125-250 kHz frequency range, and are about 95% efficient at power densities of up to about 50 W /in • Together with the preregulator, they form a front-end converter with an overall efficiency of 90%, fronl the ae, source to the input of the point-of-load converters. This efficiency represents a halving of the total losses typical in conventional systetns.