A Study on High Efficiency Direct-single-power-conversion AC-DC Converters for Battery Charger

Abstract

This dissertation proposes high efficiency direct-single-power-conversion AC-DC converters and their respective control algorithms. Since the AC-DC converters with direct-single-power-conversion scheme directly convert AC grid voltage to DC battery voltage using only one power conversion stage, these converters have simple structure, high power density, and low manufacturing cost. Firstly, a high efficiency bridgeless AC-DC converter that uses a single-power conversion scheme and a control method for it are introduced. The proposed converter dose not requires full-bridge diode rectifiers in grid-side and converts AC grid voltage directly to DC battery voltage. The proposed AC-DC converter requires two active switches, but only one switch operates at high frequency depending on the polarity of the grid voltage. Also, the series-resonant circuit consisting of leakage inductance of transformer and resonant capacitor provides zero-current-switches for output diodes and reduces the reverse recovery problem. The proposed control method makes it possible to simultaneously control the power factor collection on the grid side and the charging power control on the battery side with one switch. Secondly, a high efficiency bidirectional grid-connected inverter for battery charger and a control method for it are introduced. The proposed bidirectional inverter interfaces directly with a low-voltage battery and grid with only one power conversion stage and performs a bidirectional power conversion. The proposed inverter has a higher reliability compared with the conventional DAB-based inverters, including bidirectional switches, because the commutation capacitor provides commutation paths in any switching case. The proposed frequency compensation method linearizes the power transfer characteristic of the proposed inverter. The linearized transfer function makes the grid current control easy and facilitates the easy implementation of the proposed dual-mode control. In addition, the three control variables are interdependent; therefore, the control algorithm implementation is simple. The proposed dual-mode control enables high power capacity without increasing the turns ratio and reducing the leakage inductance of the transformer, thereby making the proposed inverter reduce the conduction losses. Furthermore, the zero-voltage-switching capability of all switches reduces the switching losses. These features enable the proposed inverter to achieve high reliability, simple control, and high efficiency with a simple circuit structure. All proposed converters are analyzed theoretically, and implemented practically to evaluate their performance. Finally, the experimental results show that the proposed converters improve overall performance such as high power conversion efficiency, power density, power quality, and manufacturing cost.