4. Energy harvesting circuit

combined effect of the input power and the termination load can be seen in the chart presented in Figure 14c, which presents the achieved efficiency when both

For most of the applications presented in the introduction, the rectifiers are not used as stand-alone components but they are part of a rectenna or a multi-stage energy harvesting system. In the literature, the term rectenna refers to the combination of an antenna cascaded with a rectifier which can convert a wirelessly received RF signal into DC voltage. This section presents a voltage doubler rectenna implementation, with a directive patch antenna suitable for targeted wireless power transfer. Another rectenna implementation with an omni-directional printed inverted F antenna (PIFA) suitable for collecting ambient RF power from random directions is presented as part of the energy harvesting discussed in the subsequent

The implemented rectangular microstrip patch antenna has 7.6 dBi gain and 97% simulated radiation efficiency radiating effectively at 5.6 GHz. Good matching in ensured using an inset-microstrip line that has characteristic impedance 50 Ω. The directive patch antenna is cascaded with a rectifier, which is first implemented as a stand-alone device on the same Duroid material. The two standalone devices with their S-parameter measurements can be seen in Figure 15a and b. For the integration of the two components, full-wave simulations using lumped ports at the common connection point were carried out in CST Microwave Studio to take into account the effect of the radiating element on the performance of the rectifier.

5.6 GHz rectenna system; simulated and measured |S11| (a) of rectangular patch antenna, (b) 5.6 GHz rectifier, (c) 5.6 GHz rectenna on Roger RT/Duriod5880, and (d) simulated and measured efficiency and

input power and termination load are varied logarithmically.

3. Rectenna design

Recent Wireless Power Transfer Technologies

Section 4.

Figure 15.

208

rectified voltage vs. input power [9].

When energy harvesting (EH) or wireless power transfer (WPT) is used, the rectified DC voltage can be either temporarily stored until it reaches certain level, or it can be used directly without the need of any storage device. The most common storage devices are either rechargeable batteries or capacitors. When the rectified DC voltage is not stored, usually some kind of booster is needed in order to increase the low DC voltage level to make it suitable for the device that needs to be powered. Furthermore, the addition of subsequent stages unavoidably decreases the overall RF-to-DC efficiency. However, in many cases, the use of a booster is necessary. Boosters can be either active which means that an external DC power source is used to bias the booster, or it can be entirely passive. The schematic of such an EH system is presented in Figure 16. In this section, the implementation of an energy harvesting circuit that consists of an omnidirectional PIFA antenna and a voltage doubler matched at 1.6 GHz, cascaded with an active DC-to-DC boost converter is discussed.

The module, which is presented in Figure 17, was built on a Rogers 4003 material as a system on package (SoP) using a milling machine for the traces and the landing pads. The required lumped components, inductors, capacitors, Schotkky diodes, and the IC module were manually soldered on the traces. The PIFA used is presented in the inset photograph of Figure 17a along with the measured |S11|, and the implemented voltage doubler presented in the inset photograph of Figure 17b along with its |S11|. Considering the space limitations, the rectifier was designed in a Γ-shape, and radial stubs were used to ensure wideband matching in order to overcome the |S11| resonance shift in correspondence to the input power variations. The rectifier was terminated with the DC-to-DC booster built around the Texas Instruments TPS60301 charge pump IC model [21] that can be seen in Figure 18a. As mentioned earlier, the RF-to-DC efficiency depends on the termination load which in this case it is equal to the input impedance of the subsequent booster device. Although the input impedance of the power booster depends on its operation conditions, it is approximated to be 5.1 KΩ, and this is the assumed termination load for the rectifier design.

Figure 16. Schematic diagram of RF energy harvesting system.

5. Conclusion

DOI: http://dx.doi.org/10.5772/intechopen.89271

Acknowledgements

Conflict of interest

Author details

Abdul Quddious<sup>1</sup>

Nicosia, Cyprus

211

and Symeon Nikolaou<sup>2</sup>

For many WPT applications, the voltage doubler is the preferred rectifier topology. This book chapter has outlined the operating principles of voltage-doubler rectifiers and has presented the most important design considerations for the implementation of such rectifying devices as standalone circuits. It has also presented examples of the use of voltage-doubler rectifiers as part of a directive rectenna system and as part of an energy harvesting system where a voltagedoubler rectifier was cascaded with an active DC-to-DC booster, demonstrating

Voltage-Doubler RF-to-DC Rectifiers for Ambient RF Energy Harvesting and Wireless Power…

the successful implementation of wireless power transfer systems using

This research was partially funded from the Cyprus RPF, under the

Babar Ali Abbasi from the Centre of Wireless Innovation (CWI) at Queen's

authors. The authors declare that there is no conflict of interest, financial or

1 Department of Electrical and Computer Engineering, University of Cyprus,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 Frederick University and Frederick Research Center, Nicosia, Cyprus

RESTART 2016-2020 projects: SWITCH-EXCELLENCE/1216/376 and RF-META-INFRASTRUCTURES/1216/0042. The authors would like to thank Dr. Muhammad

All authors listed have contributed sufficiently to the project to be included as

, Photos Vryonides<sup>2</sup>

voltage-doubler circuits as the preferred rectifier topologies.

University Belfast UK for his assistance and contribution.

other regarding the publication of this book chapter.

\*, Marco A. Antoniades<sup>1</sup>

\*Address all correspondence to: quddious.abdul@ucy.ac.cy

provided the original work is properly cited.

Figure 17.

1.6 GHz energy harvesting system (a) simulated and measured |S11| of PIFA antennae, (b) measured |S11|of 1.6 GHz rectifier, (c) 1.6 GHz rectenna system on Roger RO4003C, and (d) simulated and measured efficiency and rectified voltage vs. input power [10].

#### Figure 18.

Power management unit (a) schematic of DC-to-DC power booster and (b) measured voltage across DC-to-DC power booster and rectifier output with variable distance between transmitter and rectenna at 15 dBm transmitted power from the signal generator [10].

As can be seen in the IC schematic, the enabling terminal (EN) must be connected with the output of the voltage doubler. When the rectified voltage and thus the VEN voltage gets higher than 0.43 V, the DC voltage on the output terminal becomes equal to the Vcc biasing voltage, which can be anywhere between 0.9 and 1.8 V. This way the DC-to-DC booster output voltage can be elevated up to 1.8 V assuming that the rectified voltage is higher than 0.43 V. Figure 18b indicates how the output voltage of the booster (VBooster) goes from 0 to 1.8 V depending on the continuous variation of the rectified voltage at the output of the voltage-doubler rectifier (VRectified) and whether this is larger or smaller than the minimum VEN voltage, which is 0.43 V.

Voltage-Doubler RF-to-DC Rectifiers for Ambient RF Energy Harvesting and Wireless Power… DOI: http://dx.doi.org/10.5772/intechopen.89271
