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A planar wireless power transfer (WPT) system with printed square spiral resonators is reported. To facilitate its fabrication, both the spiral resonator and its coupling loop are printed on the top and bottom layer of a single FR4 substrate, such that the spiral resonator is driven by the magnetic field of the coupling loop. The utilised coupling loop along with the appropriate matching circuit will improve the overall efficiency. The proposed WPT system which features a measured efficiency of more than 40% is not only compact but also low cost.

A highly efficient wireless power transfer using a metamaterial slab with a zero refractive property is proposed. The proposed metamaterial slab consists of a spiral structure, a lumped element and a via through the ground. A prototype of a wireless power transfer system and a metamaterial slab was designed and fabricated. Experiment results prove the transfer efficiency improvement with the proposed metamaterial slab. The power transfer efficiency is improved by reducing the radiation loss through focusing the magnetic field. The power transfer efficiency is above 45% at a distance of 100–150 mm, and the resonant frequency is 6.78 MHz. The sizes of the transmitting coil and the metamaterial slab are 140 × 210 mm and 50 × 70 mm, respectively.

A family of single-switch SEPIC-derived converters, which apply a tapped inductor (TI), a charge pump (CP) and an inductorless regenerative snubber to attain a high DC step-up at high efficiency is introduced. The proposed converter family has the advantage of continuous input current, which can be helpful in attaining accurate tracking of the maximum power point of solar panels. The feasibility of the proposed topology is verified by simulation and experimental results.

A constant frequency control of an LLC resonant converter is presented. A switched capacitor is introduced to change the resonant capacitance, which enables the control of the voltage gain at a fixed switching frequency. The operation principle of the proposed converter with a switched capacitor is discussed. The experimental results are provided to show the validity of the proposed method.

An automatic frequency-tuned wireless charging system is proposed and successfully demonstrated to enhance the power transfer efficiency for different vertical spacings between the charging coils in the presence of proximal metallic objects. The frequency characteristics of wireless power transfer efficiency have been investigated through simulation and experimental measurements. It has been observed that the resonance-based wireless charging system is very sensitive to the imperfect positioning of charging coils and nearby metallic objects. At the operating frequency, the power transfer efficiency of the charging system reduces drastically. However, by allowing the transmitting frequency to adapt to the position of the coils and metallic objects through the automatic frequency-tuned system, the deteriorated power transfer efficiency is further improved.

A new energy harvesting technology that extracts energy from the stray electric field around a three-wire AC power line is presented. It is observed that when 20 cm of the insulated power line is surrounded with a conductive sheet, about 20 mJ of energy is harvested in 15 min with a 1 μF storage capacitor. An autonomous harvesting circuit was designed and built adopting an MEMS switch as a low leakage, low power consumption and hysteretic switch. It is demonstrated that the harvested energy is utilised to drive a commercial Zigbee-based wireless sensor module autonomously. Since it is easy and safe to install, the proposed stray electric field harvesting technology should greatly expand the field in which wireless sensors are required, such as home automation, smart grid, building energy management and structural health monitoring, as long as power lines are available nearby.

A planar coil of multiple loops connected in series and in parallel is proposed for wireless power transfer (WPT). The current direction in each loop is mixed in forward and in reverse. The proposed coil gives uniform mutual inductance and enables simple impedance matching for free-positioning WPT.

Different types of capacitors have different parameters, e.g. equivalent series resistance (ESR) and capacitance. In switching DC–DC converters with voltage-mode (VM) hysteretic control, the output capacitor ESR has a significant effect on dynamic performance. In this reported work, two critical conditions of the output capacitor ESR for mode shifting and normal operation of the VM hysteretic controlled buck converter are derived. These results, verified by circuit simulations, illustrate that the larger output capacitor ESR is necessary for the converter operating normally; otherwise, the converter operates abnormally.

A single-ended primary inductor converter (SEPIC)-based light-emitting diode (LED) power supply, which can achieve power factor correction (PFC) and constant-current drive for LED in the critical conduction mode (CRM), is presented. The circuit principle is described in detail. Meanwhile the formulas for metal–oxide semiconductor field-effect transistor switch-on time, switching frequency and the main influences of power factor are given. Experimental results show that the power can drive the LED with high efficiency by virtue of its high power factor, low power loss and stable output. Furthermore, it can be applied to low-power lighting occasions with simple structure and high reliability.

A constant current control technique for a primary-side control system is proposed to realise high precision output current regulation. With the novel ring detect and demagnetisation portion control technique, the ending time of demagnetisation can be accurately determined and the portion of demagnetisation in a switching period remains precisely the same. The simulation results show that with this control technique, the variation of output current is less than ±1.5% of the target value.