2.5 Size considerations

Considering that the fundamental voltage doubler topology that consists of two diodes and two capacitors is rather inflexible, the overall dimensions of any rectifier depend on the matching network. When a compact size is a priority, as for example in the case of implantable devices, the use of lumped components is preferred. Figure 12 shows how the overall size of a rectifier can be significantly reduced with the use of lumped components instead of distributed ones.

Using lumped inductors for the matching network is often preferred because they can also result in a more wideband design. In order to exploit the low losses of

Figure 12.

Size comparison (a) Rectifier A, (b) Rectifier I, (c) Rectifier E, and (d) Rectifier F.

Figure 13. Rectifier H and Rectifier I comparison; (a) Rectifier E |S11|, (b) Rectifier F |S11|, (c) |S11| at 10 dBm comparison, and (d) efficiency.

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

distributed printed inductors and further improve the bandwidth of rectifiers with distributed inductors, the use of multi-stage tapered microstrip lines was considered. Rectifier I was implemented on FR-4 and used a two-stage tapered microstrip line. The performance of Rectifier I is compared with the performance of Rectifier H, which uses conventional microstrip lines, in Figure 13. The bandwidth of Rectifier I is 40 MHz and the bandwidth of I is 36 MHz. The 10% bandwidth enhancement (Figure 13c) in addition to the improved efficiency, as can be seen in Figure 13d, implies that if there are not strict size constraints, the best efficiency rectifier should have the following characteristics: (a) should use Skyworks 7630 diodes, (b) should use tapered microstrip lines with distributed inductors for the matching circuit, and (c) should be fabricated on low-loss material. From the implemented rectifiers A to K, Rectifier A has the aforementioned characteristics.

#### 2.6 Proposed UHF rectifier – Rectifier A

conductivity affect the efficiency, as can be verified in Figure 10b. The importance of a low-loss substrate is evident in the measured efficiencies for the implemented rectifiers presented in Figure 11, where the lossy FR-4 (tanδ = 0.016) was used for Rectifier H, and the more expensive low loss (tanδ = 0.0009) Rogers 5880 was used for Rectifier G. Figure 11d verifies that the efficiency of rectifier G fabricated on the low-loss material is consistently higher than the efficiency of the similar design

Considering that the fundamental voltage doubler topology that consists of two diodes and two capacitors is rather inflexible, the overall dimensions of any rectifier depend on the matching network. When a compact size is a priority, as for example in the case of implantable devices, the use of lumped components is preferred. Figure 12 shows how the overall size of a rectifier can be significantly reduced with

Using lumped inductors for the matching network is often preferred because they can also result in a more wideband design. In order to exploit the low losses of

the use of lumped components instead of distributed ones.

Size comparison (a) Rectifier A, (b) Rectifier I, (c) Rectifier E, and (d) Rectifier F.

Rectifier H and Rectifier I comparison; (a) Rectifier E |S11|, (b) Rectifier F |S11|, (c) |S11| at 10 dBm

Rectifier H.

Figure 12.

Figure 13.

206

comparison, and (d) efficiency.

2.5 Size considerations

Recent Wireless Power Transfer Technologies

The detailed design parameters of the implemented Rectifier A are presented in Figure 14a and are summarized in the figure caption. The matching network consists of a shorted linear stub, a two-stage tapered microstrip line for bandwidth enhancement, and a printed inductor. The maximum measured efficiency is almost 60% at 3 dB. For input power higher than 3 dB, the voltage saturates close to 2 V and as a result the RF-to-DC efficiency degrades, since only the denominator RF power increases while the DC power remains saturated. The simulated results presented in Figure 14b indicate the non-linear dependence of the efficiency with the termination load. Although the peak efficiency per power level is shifted, the rectifier parameters were optimized for a termination load equal to 13.5 kΩ. The

#### Figure 14.

Rectifier A (a) schematic with design details, (b) simulated and measured efficiency vs. input power, (c) efficiency versus termination load, and (d) efficiency as a function of input power and the load resistance. All dimensions are in mm: L= 43, lt1 =7, lt2 =9.5, lt3 =6.85, lt4 =5.67, lt5 =7, lt6 =4.14, lrs =2, ls1 =2.2, ls2 =8.75, W= 20, wt1 = 2.4, wt2 = 1.6, wt3 = 5, wt4 = 4.35, wt5 = 0.315, θrs =60°, and θ<sup>s</sup> =90°.

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 input power and termination load are varied logarithmically.

The diodes and the lumped capacitors were modeled using .s2p files in Circuit

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

The integrated single-board directive rectenna can be seen in Figure 15c, and it can be used for far field wireless charging when the direction of arrival (DoA) is known and the directivity of the rectenna is aligned with it. The maximum implemented efficiency is around 43% and it occurs for 7 dBm input power.

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

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

Design studio for co-simulations of the integrated module.

4. Energy harvesting circuit

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

discussed.

Figure 16.

209

load for the rectifier design.

Schematic diagram of RF energy harvesting system.

## 3. Rectenna design

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 Section 4.

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.

#### Figure 15.

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 rectified voltage vs. input power [9].

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

The diodes and the lumped capacitors were modeled using .s2p files in Circuit Design studio for co-simulations of the integrated module.

The integrated single-board directive rectenna can be seen in Figure 15c, and it can be used for far field wireless charging when the direction of arrival (DoA) is known and the directivity of the rectenna is aligned with it. The maximum implemented efficiency is around 43% and it occurs for 7 dBm input power.
