1.2-mm Thin and Compact Direct AC-DC Converter in Wireless Power Receiver Suitable for Wearable Devices

This paper describes a small power converter used for wireless power transfer (WPT) systems in mobile devices. We propose two techniques as part of a new architecture. The first is a control method for combining a rectifier and a DC-DC step-down converter while properly maintaining the regulation and rectification, and the second involves incorporating a clock generator synchronized with an AC input. These techniques enable controlling a power converter while eliminating the need for a clock oscillator, and reducing the size of an inductor by a generated fast-frequency clock. A prototype was implemented with discrete components, and the power efficiency was 64% at 20 V input and 5 V/100 mA output. The efficiency was almost the same value as a conventional converter using an external clock. The active area is only 20 mm × 20 mm with a 1.2 mm height. This size is suitable for wearable devices.