Asymmetrical loosely coupled transformer for wireless laptop charger with higher misalignment tolerance

Abstract

The proposed inductive power transfer (IPT) system for laptop charging 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 relatively large air gap, the magnetic coupling coefficient of the IPT system is significantly lower than that with tightly coupled transformer. As a result, the efficiency of the IPT system is always a major concern for applications with possible misalignment variation condition. To ensure high power transfer efficiency, the IPT system should have high tolerance for different misalignment conditions. In this paper, the effect of coupling coefficient deviation to compensation network efficiency is analyzed, and asymmetrical LCT which has significant higher misalignment tolerance is proposed. By using finite element analysis (FEA) simulation method, the performance of different transmitter and receiver coil dimensions are compared. In order to validate the performance of the proposed design, a 100-W hardware prototype with two sets of LCT is built and the corresponding experiments are carried out. As compared to the symmetrical LCT architecture, the proposed asymmetrical LCT prototype improves the coupling coefficient reduction from 70% to 12.5% when the misalignment ranges from 0 to 40 mm. Therefore, the efficiency deviation for the asymmetrical LCT is maintained nearly constant over the entire tested misalignment variation ranges.