An Inductive Power Transfer Charging System With a Multiband Frequency Tracking Control for Misalignment Tolerance

Abstract

A charging strategy that transitions from constant current (CC) mode to constant voltage (CV) mode is typical for lithium batteries. This presents design challenges for inductive power transfer (IPT) chargers to achieve high efficiency. In this article, a multiband frequency tracking control is proposed to match CC and CV modes and to mitigate misalignment. As in prior work, two load-independent frequencies <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CC</sub> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</sub> are used for respective CC and CV outputs. Unity power factor can be achieved when fully aligned. For misalignment cases, two narrow frequency bands around <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CC</sub> are selected for control to deliver target output currents and limit reactive power. The duty ratio of the inverter is controlled to set target output voltages. A single-stage IPT system with a series <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> topology is analyzed. The proposed control has advantages of narrow operating frequency ranges, high misalignment tolerance, direct design procedures, and high efficiency in both CC and CV modes. The method is verified on a 4.4 kW prototype with a 150 mm air gap. Experimental results show that the target CC and CV charging profile can be achieved with up to 120 mm of lateral misalignment. When fully aligned, the maximum system conversion efficiency reaches 97.5%. It stays above 95.5% and 93.3% over the entire CC and CV mode ranges.