Design and Development of High Step-Up and Step-Down Voltage Ratio Converters for Electric Vehicle Application

Abstract

Transportation electrification helps to mitigate the effect of the emissions along with saving valuable foreign exchange. The development of Electric Vehicle(EV) will bring in requirement of capacity expansion of the existing system. The need for clean and efficient energy can be achieved throughthe migration to charging schemes based on photovoltaic sources or other renewable energy systems. Study of the nature of charging characteristics of the battery with the nature of the Photovoltaic(PV) based source profile will give an estimation of the charging time and help in the design. Using the National Solar Radiation Database(NSRDB) data viewer, the irradiance, temperature profile of a 200 W panel was studied and presented. The observations reveal that the PV source can be used to charge the EV effectively for 8 months of the year. With the change in irradiance and temperature, maximum power point varies and the terminal voltage would vary around (22-26 V) for a 200 W system. To charge a battery of 48 V, the conventional converters need to be operated at a very high duty cycle. This results in higher device ratings and lower switching frequency due to the inherent switch limitations on the turn-on/turn-off times. A high gain step-up converter is explored to address this issue. Investigations of available topologies of high gain converters in the literature reveals converters were designed with gain over 8 times for high voltage and low current appliations. The study of such converters for the medium voltage and current applications is not reported. In the present thesis, a modified boost converter was presented which yields a moderately higher voltage gain than that of the conventional boost converter. Simulation studies on the converters are done using MATLAB/Simulink. Hardware prototype models of conventional and modified boost converter were rigged up and tested. The sun shines for more than 300 days in many parts of the country. The irradiance and temperature vary with time of the day and seasons which limit the extraction of maximim power. Prototype model of a compact tracker using Particle Swarm Optimization(PSO) algorithm was developed and tested to extract the maximum power from the PV Source. To mitigate the effect of the varying irradiance and provide alternate charging schemes, conventional grid support would be required. To charge these EV batteries from the grid, AC-DC chargers are required, which meet the IEC 61000-3-2 standards. Conventional bridge rectifiers need Power Factor Correction (PFC) due to their poor power quality apart from drawing peaky currents from the input side. Conventional bridge rectifiers have over 55% Total Harmonic Distortion (THD) and a poor power factor.The absence of PFC would result in poor efficiency, higher current pulses being drawn at the source side, higher peak ratings of the devices, and hence increased losses. Converters need to operate at an extremely low duty cycle in applications that require a low output from higher input. Due to the limitation of the turn-on / turn-off of the devices, switching frequency needs to be reduced, and hence the size of the inductors and capacitors become bulkier. Conventional active power factor correction topologies employ boost-based correction schemes for unity power factor operation. This will require a still steeper step-down ratio and higher switch voltage stress apart from complexity in the control scheme with sensors. In this thesis, a high step-down ratio AC-DC converter employing a Quadratic Buck Converter(QBC) with PFC was designed and developed. The proposed topology presents an investigation and comparative evaluation of the conventional bridgeless buck system with the QBC. MATLAB R2020b is used for carrying out simulation studies. Xilinx FPGA-based controller using system generator is implemented for the generation of pulses of appropriate duty cycle. Simulation andexperimental results for a 150 W prototype are presented