Wireless power transfer (WPT) systems are becoming ubiquitous with applications in powering medical implants and a range of portable consumer electronic devices such as smart phones and wearable devices. Wireless power transferring methods can be classified into two types: inductive and resonant. For the resonant type, wider-range power transfer is possible, and multiple devices with different power requirements can be charged at the same time. The Alliance for Wireless Power (A4WP) has chosen the 6.78MHz ISM band as the power-transfer frequency [1]. At 6.78MHz, the associated switching loss is an order of magnitude larger than that in a typical wireless receiver based on an inductive coupling, with a carrier frequency of around 200kHz. Besides the two fundamental aspects of switching frequency and power, there is a third important parameter, notably the higher input voltage range needed for the `loosely coupled' resonant type, which is 25V maximum for this work. Ref. [2] appears to be one of few works that can be entirely related to the current work, targeting the same application. However, it does not integrate the most critical parts of the receiver, such as the AC-DC rectifier. Other works in the same frequency range are either limited to low-power applications [3] or the AC-DC rectifier is a stand-alone chip [4-5].
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