Bi-directional phase-shifted DC-DC converter with an improved superconducting transformer design concept

Abstract

High conversion efficiencies of power converters can be usually obtained at full load conditions; however, the current trend is to achieve high efficiency even at light load conditions. This is especially important when the energy star program was proposed by the Environmental Protection Agency, USA (EPA) a few years ago. Operating the converter at high efficiency under all load conditions is gaining importance because of the wide variation in loading condition in an actual working environment. Therefore, one of the stringent requirements on the modern converters is to maintain a high efficiency over a large load range, typically from 15 percent to full load. The search for an efficient converter having bi-directional power flow capability is receiving much attention in the research community with the rise in the usage of such converters in applications such as power system controllers, uninterruptible power supplies (UPS) and motor driver. The present research work has concentrated on developing a modified phase-shift controlled DC-DC converter having bi-directional power flow capability. The converter maintains a high efficiency over wide range of load. A detailed study of the operations of the converter has been made to formulate design guidelines. The theoretical predictions were verified by simulation and also by conducting experiments on the prototype built in the laboratory. The input and the output stages of the converter are isolated by a transformer. An air-core transformer built with superconducting coils has been designed and developed to enhance the efficiency of the converter. A simple mathematical model of the transformer was developed to formulate user-friendly design guidelines. The simulation and experimental results were provided to validate the model. The proposed converter has the following advantages compared to the other topologies in the literature. - Constant switching frequency - Soft switching - Bi-directional power flow - High efficiency over wide range of load - No core loss of transformer - Reduced winding loss - Less number of turn of transformer winding The original contributions of the present research work are summarized below: - Develop a modified bi-directional phase-shift controlled DC-DC converter - Extend the load range of the modified converter - Work out an improved design of high frequency power transformer - Derive a simplified Neumann's Formula for inductance calculations - Model the improved transformer using the simplified formula