AC to DC conversion improvement techniques in switching converters and their applications

Abstract

AC/DC conversion is usually found in two locations in a switching converter. The first one is at the input of the converter, the purpose being to convert the low-frequency AC voltage into a DC voltage which can be used by the subsequent high-frequency switching converter stage. The other AC/DC conversion is found at the output of the converter and its purpose is to rectify the high-frequency switched AC voltage and to provide suitable filtering in order to produce a smooth DC output. Usually, significant power loss and unwanted current harmonic distortions in these two locations are undesirably found under certain conditions, such as low input AC voltage, high input mains frequency and high output current. In this thesis, two new design approaches for AC/DC conversion are described, namely, "input rectifierless converter" and "output filterless converter". The goal is to have a higher conversion efficiency, lower current harmonics and reduced ripple in the two AC/DC conversion locations mentioned above. The input rectifierless converter approach has the advantages of eliminating rectifiers and avoiding crossover distortions. A general method for synthesizing input rectifierless converters using a minimum of two DC/DC converters is proposed. A detailed analysis on the requirement of the constituent converters is presented. A specific converter structure has been developed for aircraft power system applications using the basic configuration. The proposed basic configuration is also found suitable and proven to be a minimum configuration for general impedance synthesis. Prototypes have been tested and experimental results for synthesizing impedances are presented in this thesis. For the high-frequency AC/DC conversion at the output of a switching converter, a filterless AC/DC conversion technique based on overlapping AC voltage sources is proposed. A new topology has been created with high conversion efficiency and low output ripple voltage, which is suitable for high output current and low output voltage applications. The contributions of this thesis are summarized as follows: I AC/DC conversions at the input (low frequency) and the output (high frequency) of switching converters have been studied. II A general synthesis method for input rectifierless AC/DC converters based on a two-converter configuration has been proposed. The method forms the basis for systematic generation of new input rectifierless topologies that can eliminate the otherwise unavoidable distortions caused by the input rectifiers and the input current phase lead. III Using the synthesis method for input rectifierless AC/DC converters, a specific application circuit has been developed for aircraft power supply applications. IV The proposed two-converter configuration for rectifierless AC/DC conversion has been extended to general impedance synthesis. V A filterless AC/DC conversion technique has been studied. The technique eliminates the problems caused by conventional output filters, such as high conduction loss in high output current applications. In addition, a technique for reducing switching loss of the output rectifiers has been proposed. VI A specific converter topology has been derived to implement the filterless AC/DC conversion technique as well as the proposed switching loss reduction technique that improves both the converter efficiency, the ripple and noise specifications.