Design considerations of LLC resonant converter for contactless laptop charger

In this paper, a contactless charging system for laptop battery is proposed. The system consists of three parts: a high frequency power supply from a full-bridge inverter with frequency modulation, a LLC resonant tank that incorporates a loosely coupled transformer (LCT) and a resonant capacitor series connected with the primary winding, and a rectification output circuits that uses a full-bridge diode rectifier. Due to the characteristics of the LCT, the design of proposed contactless charging system is different from traditional LLC converter. Several design considerations are studied thoroughly to help improve the system efficiency. With carefully designed system parameters, the LLC converter could operate at or slightly before the series resonant point within the full load range. Therefore, zero-voltage-switching (ZVS) can be ensured for all the primary switches, and zero-current-switching (ZCS) can be achieved by all the secondary diodes. The design considerations based on the effect of the LCT turns number to the winding and core loss are analyzed and verified by experimental results. For a 100-W hardware prototype employing secondary synchronous rectification, the peak DC-DC efficiency reaches 92%, and from 20% load to full load the efficiency maintains higher than 88.5%.