1 μW-3.75 W Dual-Mode Near/Far-Field Wearable Wireless Power Transfer using a Hybrid Rectenna

Abstract

While far-field wireless power transfer (WPT) promises a long range of operation, it falls short on the DC power which can be provided. On the other hand, resonant near-field WPT suffers from a short wireless range. In this work, a dualmode high/low-power near/far-field rectenna is demonstrated for the first time based on a flexible rectifier and a textile antenna. Based on a high-efficiency single-series rectifier, the far-field rectenna achieves a peak efficiency of 42.7% from an ultralow power density of 0.43 µW/cm 2 , with a 1 V DC output from 4 µW/cm 2 . A high-power 6.78 MHz bridge rectifier is designed and implemented on a flexible textile-based substrate with a 10 dB return loss for inputs above 15 dBm. As a near-field receiver, the antenna could receive 3.75 W DC power at 5 cm from the transmitting coil, with an end-to-end efficiency of 30% inclusive of rectifier, coils, and power amplifier losses. Based on the proposed dual-mode antenna, textile-based wearables could be wirelessly powered dynamically using resonant near-field and radiative near-field WPT.

A Wearable All-Printed Textile-Based 6.78 MHz 15 W-Output Wireless Power Transfer System and its Screen-Printed Joule Heater Application

“…The higher temperature increase over the rectifier's surface is attributed to the diodes' series resistance being confined to their very small footprint, resulting in lower heat dissipation. However, the proposed rectifier and tuning capacitor reduce the temperature rise over prior flexible rectifiers, which could not operate above 4 W due to the temperature rising above 180 • C [24].…”

“…The coils and their tuning capacitors are realized using an inexpensive screen printing and lamination process, detailed in the next section, enabling them to achieve higher power handling than prior work utilizing discrete components [24]. The rectifier is seamlessly integrated on the same textile substrate and encapsulated for mechanical reliability.…”

“…11(e), the temperature across the surface of the heater with the 2 cm coil separation results in a peak temperature exceeding 60 • C. At higher separation distances, the power output drops to mW levels which can only be used to power smaller sensors and wearable devices. Beyond 12 cm, the DC power output is comparable to that of radiative farfield WPT systems implying that a dual-mode near/far-field implementation could be utilized [17], [24]. The final step in characterizing the WPT system is evaluating its safety for use in a standard unregulated setting.…”

“…Printed textile-based coils were integrated with rigid FR4-based Qi-standard circuitry in [23] and demonstrated receiving around 1.5 W with a 37% DC-DC efficiency. In [24], we demonstrated a 3.75 W 6.78 MHz WPT receiver based on a flexible rectifier and resonant embroidered coils. However the WPT system in [24] had several limitations which cannot be solved using existing flexible and wearable WPT implementations including: (a) low power-handling capability of compact (<1 cm) surface-mount tuning capacitors, (b) low efficiency of the rectifier due to the high switching speed and the small form-factor, (c) high thermal losses with a peak temperature exceeding 150 • C, and (d) unreliable packaging due to using multiple fabrication processes (embroidered coils, copper-based rectifier), and the surface mismatch caused by the flexible-rigid interface between the tuning rigid capacitors and the coils.…”

Microwave-Enabled Wearables: Underpinning Technologies, Integration Platforms, and Next-Generation Roadmap

“…This effort builds upon existing applications in HF RFID systems integrated with UHF antennas [76]. The near-field WPT receiver could generate over 3 W DC output, with the potential for over 50 W reception with an SAR under 1.7 W/kg [77], based on a simple bridge rectifier implemented on a flexible substrate, representing over 100% improvement in the DC power output compared to prior Qi-based implementations [61].…”