27.1 A Differential Hybrid Class-ED Power Amplifier with 27W Maximum Power and 82% Peak E2E Efficiency for Wireless Fast Charging To-Go.

Portable power banks with wireless charging are popular on the market, targeting a wireless charging to-go experience. However, the low input voltage from the battery and the large equivalent series resistance (ESR) of the coupling coils limit the charging power and efficiency of device-to-device (D2D) wireless charging, making it difficult to achieve high power and high efficiency simultaneously [1]. The winding loss of the coupling coils is one major power loss due to the large ESR of hundreds of $\mathrm{m}\Omega$ [1–4]. Moreover, thicker winding wire or multi-strand Litz wire at 6.78MHz cannot effectively reduce the ESR due to the skin and proximity effects. Therefore, we should limit the AC current through the coupling coils for thermal performance. Consequently, the current wireless charging to-go suffers from thermal issues, slow charging speed, and discounted energy-sharing percentage.