A Novel Transformer-less Uninterruptible Power Supply

Uninterruptible Power Supplies (UPS) are finding increased applications with the advent of sophisticated electronic equipment. Low power equipment like FAX machines, Personal Computers, etc., form the major portion of the applications. There is a need for cost-effective, light and sleek UPS in the low power range. In low power UPS, the presence of line frequency magnetic components makes the system bulky and costly. In this thesis, a circuit configuration suitable for low power UPS is proposed in which the line frequency magnetic components are eliminated. Elimination of the line frequency magnetic components necessitates the use of a high voltage DC bus. Cost considerations keep the battery voltage low. These factors prompt the use of a bi-directional interface between the high voltage DC bus and the battery. The design and the development of a bi-directional interface is the highlight of this work. The front end of the proposed UPS is a bridge rectifier followed by a Boost converter. Unity power factor operation is achieved by programming the current through the boost inductor to be a rectified sinusoid. The output of the boost converter is the DC bus voltage which is controlled by controlling the magnitude of the current through the inductor. The model of the converter and the design of the controller to achieve unity power factor operation and DC bus voltage regulation are presented. The inverter is a single phase H-bridge inverter. The switching pattern used is generated by using unipolar Sinusoidal Pulse Width Modulation (SPWM) technique. The harmonic content in the switching pattern is presented. The switching harmonic content in the output of the inverter is attenuated by using an appropriate passive filter at the output of the inverter. The bi-directional interface is realised using the tapped inductor boost converter circuit. The operation of the converter and the sequence of events in one switching period are explained. The mathematical model of the converter is obtained by circuit averaging technique. Instantaneous flux programming control scheme (similar to instantaneous current programming) is used to control the converter. The model of the converter in this control scheme is derived. The design of suitable compensators to achieve Battery charge control (when the Mains power is present) and the DC bus voltage control (when the Battery is supplying power to the critical load) are presented. The mathematical model of the different converters are simulated using SIMULINK, a platform in MATLAB. The simulation results of the complete UPS, constructed by interfacing the model of the various converters, is presented. The design strategies developed in this thesis were used to construct a 250 VA prototype UPS. The steady state and dynamic tests were performed. The performance evaluation results are presented. The results confirm the model and the design methodology.