Design of a compact and highly efficient energy harvester system suitable for battery-less low cost on-board unit applications

This study addresses the general problem regarding the power supply in specific on-board unit (OBUs) solutions. In detail, this paper refers to a subset of the so-called electronic toll collection (ETC) applications such as assets control and vehicle identification, where simplicity, low costs, and maximum compactness represent the most important features. In this context, the next generation of OBUs, developed specifically with reference to such applications, will involve energy harvester-based battery-less techniques. Previous studies have mainly concentrated on performance optimization by achieving maximum energy transmission to the OBUs. This study discusses a technique suitable for both maximizing performance and minimizing the dimensions of transponder energy harvesters suitable for assets control and vehicle identification operating at 5.8 GHz. The technique assumes that an optimal source impedance exists that maximizes the energy transfer to the transponder, thus enabling its power supply in a battery-less configuration. We discuss a solution based on a compact patch antenna designed to exhibit this optimal source impedance to the RF-to-DC rectifier. This approach avoids the use of a lossy matching network. For the sake of comparison, the same function is compared with an equivalent development, which includes the interstage matching network between the antenna and the RF-to-DC rectifier. We introduce experimental results demonstrating that the ultracompact energy harvester optimized at −5 dBm of impinging power is capable of increasing both the charge current and energy efficiency from 340 to 450 μA and from 37% to 47%, respectively