Efficient Wireless Power Transfer With Capacitively Segmented RF Coils

Abstract

Wireless power transfer systems have been widely applied in the field of portable and implantable devices, featuring contact-free and reliable energy supply. Novel implant systems, such as brain-computer interfaces, impose the challenges of strong miniaturization and operation under loosely coupled conditions. Therefore, maximizing power transfer efficiency while decreasing the size of transmitter and receiver structures becomes a central research question. This paper presents a unified design strategy of modeling, analyzing and optimizing planar spiral coils with integrated capacitive elements, so-called capacitively segmented coils, for operation in wireless power transfer interfaces. It mathematically analyzes and experimentally verifies that the combination of capacitive coil segmentation, increased operational frequencies and geometrical coil optimization can be used to establish wireless power transfer links with comparatively high efficiency, small size and limited detuning effects in lossy dielectric environments. The paper embraces the formulation and verification of a broadband analytical link model based on partial element equivalent circuits, which is subsequently used to determine dominant coupling and loss mechanisms and to optimize the coils' geometries for high efficiency. Moreover, an extended analysis shows how the capacitive coil segmentation can effectively suppress dielectric losses and non-uniform current distributions by canceling the inductive contribution of every coil segment at the frequency of operation. Utilizing these methods, an exemplary 40.68 MHz wireless power link with a 30 mm primary and a 10 mm secondary coil is designed and evaluated: With a maximum efficiency of up to 31 % in biological tissue at 20 mm separation distance, it features efficiency levels which are up to ten times higher and a specific absorption rate which is up to five times lower compared to non-segmented systems. When operated at 150 MHz in air, efficiency levels are up to 1.5 times higher than in state-of-the-art systems of the same size.

Very High Bit Rate Near-Field Communication with Low-Interference Coils and Digital Single-Bit Sampling Transceivers for Biomedical Sensor Systems

“…Decoupling energy and data interface regardless of a close placement of the corresponding coils is the prerequisite to establish a stable and resilient data communication. The energy inductors used in this work are two capacitively segmented planar spiral coils with radii of 15 mm and 5 mm, respectively, which are designed by the authors of this paper in [ 28 ] for a high-efficiency wireless power transfer link operating at 40.68 MHz. Due to the capacitive elements included into the traces, the current distribution is uniform along the conductor and parasitic and lossy capacitive displacement currents are limited, so that high-frequency operation in biological tissue is possible without sacrificing efficiency: For coil separation distances of 20 mm, coil efficiency levels of up to 32% are achievable, while the maximum receivable power at the receiver is approximately 30 mW to fall below the regulatory limit of the specific absorption rate (SAR) of 1.6 W/kg.…”

“…As the transient signal shall settle within a fraction of a period , i.e., , we get from ( 18 ): Solving ( 19 ) for and substituting the result into ( 10 ) yields the optimal series loss resistance and resistive circuit load to achieve both fast transient settling and simultaneous impedance matching: Recalling from [ 28 ] that the maximum power efficiency of any inductive link system scales positively with the figure of merit we see that increasing the number of turns does not improve the output power and therefore signal-to-noise ratio in this high-bandwidth inductive link: a higher number of turns N corresponds to higher mutual inductance , but also increases the self-inductance of the coils with . As the loss resistances have to scale with the inductance as given by ( 20 ) to provide fast transient settling, the effective figure of merit will remain more or less constant with N .…”

“…To assure that the optimal load resistance lies in the range from 50 to 200 Ω, which is feasible to be matched to a receiving amplifier, the coils should have a self-inductance in the range from 170 to 680 nH according to ( 21 ), which can be realized with three turns each (recall the self-inductance from ( 7 )). The coils’ pitch was arbitrarily chosen to be 0.3 mm for the primary and 0.15 mm for the secondary coil, while the trace width was dimensioned with the resistance model from [ 28 ] to obtain an ohmic coil resistance resistance well below 1/10 of the required optimal series resistance , which is then defined by an additional lumped resistor on the circuit board of the coil. The final design parameters of the resonators are compiled in Table 1 .…”

“…The data coils specified in Table 1 were implemented on a commercially available two-layer printed circuit board (PCB) of 1.6 mm thickness as shown in Figure 15 , together with the optimized and segmented data coils from [ 28 ]. The series resistors and parallel capacitors were included as lumped elements with a component size of 0201 to yield the values of Table 1 .…”

Wireless Power-Data Transmission for Industrial Internet of Things: Simulations and Experiments

“…The time consuming and labour intensive operation of manual replacement of these batteries poses practicality challenges. Several articles have appeared over the implementation of wireless power transfer technology(WPT) to charge the batteries of the IoT devices [5], [6].…”