A Study on High-efficiency AC-DC Converters with Single Power-conversion

Abstract

This thesis proposes high-efficiency ac-dc converters with single power-conversion and their respective control algorithms. Since the converter with single power-conversion consists of only one dc-dc converter without a power factor correction (PFC) circuit, it has a simple structure, high power density, and low cost. Three high-efficiency ac-dc converters with single power-conversion are proposed for off-line power supplies and a battery charger. The first converter is derived by integrating a full-bridge (FB) diode rectifier and a series-resonant active-clamp dc-dc converter. To obtain a high power factor (PF), a novel control algorithm is proposed. The role of the proposed control algorithm is PFC and output power control. The active-clamp circuit clamps the surge voltage of switches and recycles the energy stored in the leakage inductance of the transformer. Also, it provides zero-voltage turn-on switching of the switches. A series-resonant circuit of the output-voltage doubler removes the reverse-recovery problem of the output diodes. The second converter is composed of a step-down ac chopper and the output-voltage doubler. The second converter provides high PF and an isolated dc output voltage without using both a FB diode rectifier and a PFC circuit. The step-down ac chopper using an active-clamp mechanism serves to clamp the voltage spike across the main switches and provides zero-voltage turn-on switching. The resonant-current path formed by the leakage inductance of the transformer and the resonant capacitor of the output-voltage doubler achieves the zero-current turn-off switching of the output diodes as the first converter. The third is a bidirectional dual-active-bridge (DAB) inverter using the novel hybrid modulation. This inverter consists of a DAB dc-dc converter and a synchronous rectifier (SR) for unfolding. The DAB dc-dc converter transforms the low dc voltage into a rectified sine wave that pulsates twice the ac line frequency. The rectified sine wave is unfolded into the ac line voltage by SR. The proposed hybrid modulation combines a phase shift control and a variable frequency control. The variable frequency control converts the nonlinear function of the phase shift angle into a linear function and controls the output power. This leads to simple closed-loop control for the sinusoidal current waveform, a low harmonic distortion, and a high voltage conversion ratio without an increase of the transformer turn ratio. All proposed converters are experimentally verified to evaluate the performance of the proposed converters. The experimental results show the system have high efficiency and low total harmonic distortion (THD).