Research On High Frequency And Low Power Wireless Charging System

Wireless charging is quickly progressing from a novelty to a standard feature on mobile devices, particularly on mobile phones that may need to be charged more than once a day. Wireless power systems are typically classified as either inductive or resonant. Multiple industry consortia such as the Wireless Power Consortium(WPC)(sponsor of the Qi specification) and Alliance for Wireless Power(A4WP)(sponsor of the Rezence specification) have proposed specifications for wireless charging, but none has yet been adopted as a standard. The WPC specifications are commonly described as inductive power transfer, while A4 WP are developing specifications for resonant charging. In this paper, we will take a comprehensive research on the power transmitting unit(PTU) of a high frequancy and low power wireless charging(WC) system which is partially based on commercial Rezence solutions, and the operational characteristics of the WC system is also discussed.We describe a rigorous theoretical approach to the circuit-level design of an entire magnetic resonant coupling wireless charging(RC-WC) system, including the PTU and power receiving unit(PRU) subsystems. Starting from a novel analytical characterization of the resonant coupling link based on the coupled mode and two-port network model theory, the system efficiency is parametrically computed as a function of a set of circuital parameters, including the power levels to be transferred. These quantities are then used as design goals inside the optimization of the PTU and PRU blocks. By adopting the last generation miniaturized enhanced-mode Ga N-power field-effect transistor, a Class-E power amplifier which acts as the transmitter of the WC system is designed with the operating frequency of 6.78 MHz and obtaining a 92% efficiency at output power of 8 W.A method to detect and control various operating states of PTU is proposed. The three operating states are off state, low power state, and power transfer state. The detection scheme probes the transmitter circuitry periodically to determine the operating states. For safety and power saving, the transmitter is powered off when there is no valid receiver placed on the transmitting coil. The PTU shall enter the low power state to make sure the PRU is proper and ready for the system before it enters the power transfer state. The power transfer state is the nominal operation state when the power transmission efficiency is high with minimum power loss and zero-voltage switching operation of the Class-E transmitter is achieved. The determination of the operating states is achieved by analyzing the power amplifier’s DC supply voltage and current and the forward and reverse power of its output terminal which are detected by using of a Bi-Directional coupler, requiring no communication link between the transmitter and receiver.A WC system is developed for charging a smart phone with a Class-E amplier and a Class-D rectifier followed by a switching DC/DC Buck converter. The transmitter and the receiver are directly connected by the RC two-port network, and a A4 WP Class 3 spiral type power transmitting coil is adopted. With the proposed RC-WC system, up to 2.3 W of transferred power and a peak of 53% DC-to-DC efficiency were obtained with an input DC voltage VDC = 15 V and a regulated 5-V output at a link distance d = 35 mm. Numerical and experimental results are discussed, demonstrating the operational characteristics of the WC system.