The simplicity and low cost of capacitive interfaces makes them very attractive for wireless charging stations. Major benefits include low electromagnetic radiation and the amenability of combined power and data transfer over the same interface. We present a capacitive power transfer circuit using series resonance that enables efficient high frequency, moderate voltage operation through soft-switching. An included analysis predicts fundamental limitations on the maximum achievable efficiency for a given amount of coupling capacitance and is used to find the optimum circuit component values and operating point. Automatic tuning loops ensure the circuit operates at the optimum frequency and maximum efficiency over a wide range of coupling capacitance and load conditions. An example interface achieves near 80% efficiency at 3.7 W with only 63pF of coupling capacitance. An automatic tuning loop adjusts the frequency from 4.2 MHz down to 4MHz to allow for 25% variation in the nominal coupling capacitance. The duty cycle is also automatically adjusted to maintain over 70% efficiency for light loads down to 0.3 W.

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Capacitive power transfer for contactless charging

The development of capacitive power transfer (CPT) as a competitive wireless/contactless power transfer solution over short distances is proving viable in both consumer and industrial electronic products/systems. The CPT is usually applied in low-power applications, due to small coupling capacitance. Recent research has increased the coupling capacitance from the pF to the nF scale, enabling extension of CPT to kilowatt power level applications. This paper addresses the need of efficient power electronics suitable for CPT at higher power levels, while remaining cost effective. Therefore, to reduce the cost and losses single-switch–single-diode topologies are investigated. Four single active switch CPT topologies based on the canonical Cuk, SEPIC, Zeta, and Buck-boost converters are proposed and investigated. Performance tradeoffs within the context of a CPT system are presented and corroborated with experimental results. A prototype single active switch converter demonstrates 1-kW power transfer at a frequency of 200 kHz with >90% efficiency.

Single Active Switch Power Electronics for Kilowatt Scale Capacitive Power Transfer

Capacitively coupled power transfer (CCPT) technology has been proposed and investigated recently as an alternative contactless power transfer method. A CCPT system has the advantage of being able to transfer power across metal barriers with low standing power losses, so it is potentially good for charging consumer electronics such as mobile phones, laptops, lamps, etc. However, a big challenge in designing a CCPT charging platform is the output voltage of a CCPT power supply varies greatly with the positioning and alignment of the pickup on a charging pad due to the change in power flow. This paper proposes a CCPT charging platform with a matrix charging pad and a dynamically soft switched transformer to control the power flow and regulate the output voltage under variable coupling conditions. The proposed charging platform is verified by simulation and experimental results.

A Capacitively Coupled Contactless Matrix Charging Platform With Soft Switched Transformer Control

This paper introduces a new capacitive wireless power transfer approach with the potential to significantly enhance power transfer density in large air-gap applications. This enhancement is achieved through the use of multiple phase-shifted capacitive plates that reduce fringing fields in areas where field levels must be limited for safety reasons. The effectiveness of the proposed approach is evaluated using an analytical framework and validated using finite-element and circuit simulations. It is shown that a 6.78-MHz eight plate-pair system based on the proposed approach reduces fringing fields by a factor of five relative to a two plate-pair system while retaining the same power transfer density.

Investigation of power transfer density enhancement in large air-gap capacitive wireless power transfer systems

Capacitive power transfer (CPT) has been investigated as an alternative wireless power transfer technology based on electric field coupling. The coupling interface of CPT is formed by a pair of “capacitors” in series with the power source and load. The effective capacitance ranges from tens to a few hundreds of picofarads, yielding high impedance. Therefore, in most CPT systems, a tuning inductor is connected in series with the coupling interface for circuit compensation and power transfer capability enhancement. However, this compensation method suffers from high voltage spikes from the inductor if the secondary side load is removed suddenly causing electrical and health hazards. To address the issue, this paper proposes a CPT system based on a Z-impedance compensation network with inherent open-circuit and short-circuit immunity. It also has the voltage boost capability as a Z-source inverter. Its operating principle is described and a set of design equations are given. Both simulations and experimental results from a 5 W low power design have demonstrated that the proposed compensation method using the Z-impedance matching network exhibited open-circuit and short-circuit immunity, could boost up the output voltage by 50% with power efficiency exceeding 80%.

Z-Impedance Compensation for Wireless Power Transfer Based on Electric Field

Requirements for patient safety and a high interference rejection ratio in medical equipment create a demand for effective isolation devices. A system scale approach that uses capacitive coupling for power and signal isolation is presented. In addition, we describe the development of an instrumentation system prototype that applies microwaves for power exchange and bidirectional data transfer across the isolation barrier. The system consists of an isolated transducer unit, a central unit, and a single coaxial cable between the units. The isolation capacitance is as low as 1.6 pF, inclusive of the digital data transfer and power exchange up to 600 mW of isolated direct current (dc) power. The system is suitable for line-powered biopotential measurements and it is shown that reducing the isolation capacitance from 180 to 1.6 pF improves the power line rejection by 30 dB in a typical electrocardiogram (ECG) measurement setup.

P. Eskelinen

Papers

A Biosignal Instrumentation System Using Capacitive Coupling for Power and Signal Isolation