Development of Input Switched Hybrid AC to DC Converters

Abstract

The thesis presents novel circuit topologies for single phase AC to DC conversion. The converters are used for step up, step down and step up/down applications. AC to DC converter with high conversion efficiency is of critical importance in applications in renewable energy sources or power supplies for modern equipment. Unavoidable system disturbances like parameter variations, effect of sudden load change and other noises are resolved by designing precise feedback control for both step-up and step- down applications. The proposed single phase rectifiers are designed to maintain high conversion efficiency at extremely low and high duty cycles. In order to tackle the deviation from the ideal behavior of the gain and attenuation property of the converter at extreme condition, hybrid diode-capacitor networks are introduced. The network provides voltage step-up or step-down prior to DC to DC or AC to DC conversion. In addition, in order to take the advantage of input switching, new topologies are designed by paralleling the outputs of two AC to DC conversion paths of positive and negative half cycles of input AC to output DC. A common single bi-directional switch is used for both positive and negative half cycle conversion. Direct input current switching ensures the current displacement with input voltage to be nearly unity. This control makes it easy to maintain high input power factor and low input current THD. Since the topologies used for DC to DC conversion cannot be used directly for AC to DC conversion, the topologies are modified before applying in AC to DC conversion. The modification causes high component count and circuit complexity. So attempt is taken to design not only using modified DC to DC topologies but also conventional input switched converter is modified by shifting the switch location and using split capacitors at the output. The investigated new single phase AC to DC converters offer higher conversion efficiency which is validated by simulation. Common two loop feedback control strategy for PFC is used with proposed converters to improve the input power factor and to keep the input current THD within prescribed limits. The proposed input switched boost and SEPIC converters perform better compared to conventional ones. Among the proposed converters modified input switched AC to DC boost and SEPIC converter offered best result.

Converters with best performance in the simulation have been implemented in the laboratory experimentations. The test results validated the performances of the proposed converters with the simulation results.