Abstract: To eliminate the need for power cords and bulky chargers when charging multiple devices, portable devices placed near a wireless transmitter can be charged by changing their position, angle, or distance to the transmitter. To address long-standing compatibility challenges, capacitive power transfer (CPT) is proposed, using AC-DC and DC-AC conversion techniques to charge mobile devices. By improving the design of the rectifier bridge and the bias network and adopting switching power supplies, the system efficiency is increased. This architecture is more suitable for integrated circuit implementation. Experimental results show the system can improve wireless power transfer efficiency by 90%.
Keywords: wireless power transfer; magnetic coupling; power transmitter; AC/DC
Introduction
Driven by user demand for convenient charging, many companies joined the Wireless Power Consortium (WPC), founded in 2008, and established the Qi wireless power transfer standard. That standard enables portable devices of different models, regions, and power requirements to share a single power source, removing power cords and bulky chargers for multiple devices and solving long-standing compatibility issues. However, power transfer efficiency under the WPC standard is limited, and research focuses on reducing wireless transfer losses for charging low-power portable device batteries.
1 Electromagnetic coupling and its advantages
Although acoustic and optical coupling approaches are being developed, electromagnetic coupling remains the most efficient. Traditional inductive power transfer (IPT) has limitations: it cannot reliably penetrate metal barriers, has lower transfer efficiency, and is susceptible to electromagnetic interference. Resonant magnetic-field coupling establishes a transfer channel between transmitter and receiver through resonance, enabling longer-distance power transfer. By optimizing power-conversion circuits and compensating reactive power, energy-transfer efficiency can be maximized. Experiments have studied the effects of coil diameter and the switching timing of transmitter circuit switches on delivered power. In this work, the team proposes capacitive power transfer (CPT) and uses AC-DC and DC-AC conversion techniques to charge mobile devices.
CPT is a recently proposed alternative to contact power transfer. The CPT interface is a pair of coupled structure capacitors. The rest of the power-conversion system, including inverter and rectifier structures, remains similar to other architectures. Because electric-field characteristics can reduce losses and, at certain power levels, allow minimized capacitive isolation components, CPT's most significant advantages are lower power loss, cost, and size. For high-power applications CPT is less suitable, so most CPT solutions focus on low-power and portable electronics, such as wireless toothbrush chargers or wireless phone chargers that couple through a capacitive matrix at the power-transfer interface.
Compared with inductive transfer, CPT has these characteristics:
- CPT is based on electric-field coupling, so high-frequency AC voltage must be applied across the coupling plates.
- A complete CPT system requires at least two pairs of coupling plates to provide a full current loop between the power source and the receiver.
- When a metal barrier exists between coupling plates, the coupling can be modeled as two capacitors in series, which means CPT can transfer energy through metal barriers.
- Because most of the electric field is confined within the plate volume, electromagnetic radiation interference and power loss are greatly reduced compared with IPT.
- By removing heavy and costly magnetic materials and coils, the circuit size can be reduced.
The power transmitter connects to the mains and the power receiver is integrated into the mobile device. Power is transmitted and received via coupling. After AC-DC and DC-AC conversion, transmitted power is reconverted to DC to charge the battery under controlled resonance conditions. Resonant-coupled wireless power can extend effective charging distance to the meter range, separating physical connection between powered devices and power sources, which can improve product appearance, usability, and safety.
Advances in printed and MEMS technologies for CPT applications show promise; these enabling technologies are being adopted in consumer electronics products.
2 Improving power efficiency with switching power supplies
In earlier architectures, power efficiency was regulated using resistive adjustment, which introduced significant losses and presented a resistive load in parallel with the bridge rectifier. AC-DC rectification was typically implemented with a full-wave rectifier. In such circuits, efficiency is largely affected by conduction and switching losses, as well as power losses in the bias network. Magnetic resonant wireless power technology therefore has clear application value for scenarios requiring convenient remote power, such as underwater inspection, oilfield and mine environments, high-altitude or desert installations, and chemical plants.
In the improved design, both the rectifier bridge and the bias network use switching power supplies to increase efficiency. This approach is better suited to IC implementation. Additionally, the MOSFETs on the receiver board have very low on-resistance, so power loss is greatly reduced compared with conventional systems that rely on diodes and resistors.
