DESIGN AND IMPLEMENTATION OF POWER CONVERTERS FOR CHARGING APPLICATIONS WITH IMPROVED PFC

Abstract

With increasing popularity of electric vehicles and the growing need for fast and efficient charging stations, there exists a necessity for designing a power converter system that will provide a highly regulated DC output voltage at low voltage levels in response to a standard AC supply voltage. In this thesis, a two-stage power converter system consisting of an interleaved boost power factor correction converter, which is followed by a full bridge LLC resonant converter, will be designed and analyzed. The proposed power converter system has been designed for a rated power of 3 kW, which provides a highly regulated output voltage of 48 V. For providing high efficiency and reduced switching losses, the first stage consists of an interleaved boost power factor correction converter, which directly converts the mains AC voltage (230 V) to regulated DC link voltage of 400 V. This stage ensures regulation of the DC link voltage to 400 V and provides a high-power factor due to shaping of input current waveforms to follow the input sinusoidal voltages. Interleaving is done by means of 1800 out-of-phase gating signals applied to two identical boost converters operated in parallel with one another. The second stage consists of a full-bridge LLC resonant converter that accepts the 400 V regulated DC link voltage and delivers a controlled output voltage of 48 V at 3 kW. The operating principles of the LLC converter are examined across three frequency regions: at resonance, below resonance, and above resonance. The first harmonic approximation method is employed to derive the voltage gain expression, which forms the basis of a systematic resonant tank design procedure covering the selection of Qmax, optimization of the inductance ratio m, and explicit calculation of the resonant component values Lr, Cr, and Lm. A closed-loop voltage controller based on pulse frequency modulation is designed to regulate the 48 V output under both load-side and input-side disturbance conditions. The complete system is modelled and validated through simulation studies in MATLAB Simulink. Results confirm stable output voltage regulation at 48 V, high power factor at the AC input, satisfactory dynamic response under load and input voltage variations, and maintained soft-switching operation across the intended operating range of the full-bridge LLC resonant converter.