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Techniques to increase RF-to-DC conversion efficiency of rectenna
A rectifying circuit that gives large DC voltage and high efficiency even if incident power levels are very low is desired. RF-to-DC conversion efficiency is an important characterization of the rectifier in the energy harvesting system and this one is decided mostly by the behavior of the diode.
The diode normally modeled by the nonlinear characteristic in three main regions of operation: reverse biased while below the reverse breakdown voltage (region 1), off-state while between breakdown voltage and turn on voltage (region 2) and forward biased (region 3) while above turn-on voltage. However, with a very low power density of ambient RF energy, the input voltage of rectifier is very low and the diode can operate only in the transition between region 2 and region 3. The diode is only in “on-state” for a small fraction of the RF wave period [32].
On the other hand, circuit losses come from the impedance mismatch as harmonic generation and the device parasitic such as junction resistance and junction capacitance of diode can decrease significantly the circuit efficiency. Therefore, techniques to increase the power conversion efficiency concentrate on the behavior of the diode and various topologies concerning the diode.
Nonlinear device choices
The diode parameters have a crucial role to maximize power conversion efficiency of rectifying circuit. Fig. 1-2 shows the impact of the diode main parameters on the rectifying circuit efficiency [32] [33].
Fig. 1-2 Relationship between the efficiency and losses in rectifying circuit as a function of input power [32][33]
– The threshold voltage VT is the most important parameter especially in ambient RF energy harvesting because it limits efficiency at low input power (VT effect).
– The reverse breakdown voltage VBr also limits the diode efficiency as this allows energy to short the diode demonstrated by VBr effect. However, in the very low power system, input power is not high enough to exceed this voltage.
– The parasitic junction resistance RS will dissipate power in the semiconductor junction, therefore, limits diode efficiencies [33].
– The parasitic junction capacitance Cj and junction resistance RS constitute a low-pass filter, which limits the maximum operating frequency of the diode.
For these reasons, a careful choice of the diode with low threshold voltage, high reverse breakdown voltage, low series resistance and low junction capacitance can naturally increase the efficiency of the circuit. Nevertheless, due to the limitation in device material and fabrication, it is difficult to obtain all the optimal values simultaneously.
Fig. 1-3 State-of-the-art microwave rectifier circuits with various Schottky diodes [34]
The work of Hemour et al. [34] provides the state-of-the-art rectifiers reported in the literature with Schottky diodes. All the work reported in Fig. 1-3 is related to very different researches but the diode Schottky SMS 7630 had an important role in the map.
However, existing Schottky and CMOS devices are limited by the junction nonlinearity and the conversion efficiency is reduced to only a few percent at nW level. The magnetic tunnel junctions (MTJs) or spin diode was then analyzed in [34] with much smaller series resistance, junction capacitance as well as zero-bias junction resistance. This device, therefore, is a promising choice for low power energy harvesting apart from the traditional Schottky diode. In a companion work to this one, a specific rectenna is fabricated for intentional wireless energy transmission [36]. In fact, the received energy is not different in order of magnitude from ambient energy levels considered here. Most popular Schottky diodes have been analyzed and main parameters are reported in Table 1-5.
Circuit topologies choices
The microwave rectifying circuit may adopt different topologies depending on incident power and frequency and as a function of position and number of diodes in the circuit. Among the most common topologies are:
– Single series half-wave rectifier topology: this is the simplest topology and it can be used in the very low input power circuit. The drawback of this topology is the low output voltage.
– Single shunt half-wave rectifier topology: this topology is similar to the single series one and also adapted with very low power application.
– Voltage multipliers topologies: in order to boost the output voltage, a number of voltage multiplier topologies have been reported in the literature [37]:
o Voltage doubler
o Crockcroft Walton/Greinacher/Villard charge pump o Dickson charge pump
o Modified Cockcroft-Walton/Greinacher charge pump o Mandal-Sarpeshkar voltage multiplier
The voltage multipliers topologies help to increase the output voltage and in the high-power circuit, they can achieve fair efficiency values. However, with the very low incident power, the input voltage is very low compared to the voltage across the diode and the diode bridge-based rectification becomes very inefficient [37]. The more the diode is involved in the circuit, the more losses appear. Therefore, in the low-power circuit of ambient RF energy harvesting, the single series or shunt topologies are preferred.
A Rectifier Figure of Merit (RFoM) can be defined as the product between open-circuit DC output voltage and the efficiency when the rectifier reaches its maximum power point (optimal load) (1-1).
= × (1-1)
Authors in [6] have simulated four circuits to evaluate the RFoM of each topology. The results (Fig. 1-4) show that the single series diode topology offers a better trade-off in low power region below -5 dBm.
Harmonic termination techniques
Typically, the rectifier is optimized to have impedance matching with the antenna at the fundamental frequency. In a microwave rectifier, the nonlinear diode also generates current and voltage at the harmonics of the operating frequency [38]. As described in Fig. 1-2, as the incident voltage continues to increase, the energy lost in harmonics further increase. Therefore, the efficiency of the rectifier can be improved by terminating its harmonic component across the diode.
The choice of harmonic termination technique is based on series resistance RS and reverse breakdown voltage VBr. Table 1-6 shows most suitable rectification mode versus RS and VBr, but as the input RF decreases, each class begins to operate with approximately the same efficiency [39]. Furthermore, the most realistic diode is the diode with high RS and low VBr, which is most suitable for Class-F harmonic termination.
Class-F harmonic termination presents the diode with zero impedance at even harmonics and infinite impedance at odd harmonics, what will reshape the diode voltage and current waveforms. An ideal Class-F rectifier is presented in Fig. 1-5. This rectifier uses ideal RF choke, ideal DC block, lossless transmission line; the input impedance is a conjugated match with the source impedance (antenna impedance in case of RF energy harvesting) [40].
Authors in [41] proposed a compact Class-F RF-DC converter at 2.45 GHz. The harmonic power dissipations are eliminated by open and short terminations for the odd and the even harmonics, respectively. The proposed circuit consists of an impedance matching network and a third-harmonic rejection filter (Fig. 1-6). The results show that this circuit also realized short termination for the second and the fourth harmonic (Fig. 1-7). Authors in [42] proposed a similar work that has an inverse Class-F rectifier, which operates at short terminations for the odd harmonic and open for even harmonic.
Impedance matching
The choice of interface impedance between the antenna and the rectifier plays a crucial role in maximization the power conversion efficiency of rectenna systems. The interface between antenna and rectifier circuit is shown in Fig. 1-8 [43].
Typically, the induced voltage from antenna VA is very low due to the low power in the ambient and this input voltage amplitude is not greater than the threshold voltage Vth of the nonlinear component. For some applications, a very low start-up voltage of the rectifier is more important than the power conversion efficiency and in that condition, the impedance matching between the antenna and the rectifier in its cold-start is not desirable, hence the input RF filter, i.e. matching network, is not considered [44] [45].
However, when the antenna is not matched to the rectifier, the input voltage of rectifier is identical to the input voltage for the matched situation times a multiplicative factor. This factor is equal to 1 in the matched situation and smaller than 1 in other cases [46]. So in order to maximize the input voltage and power transfer, the interface should be impedance matched. Discussion about matching or not matching has been considered and it seems that there are more limitations than advantages to consider an unmatched scheme [47].
This does not implicate that 50- is the best reference impedance with respect to harvesting. Unfortunately, the majority of antenna interfaces are 50- due to the historical development of coaxial cable using in the experimental environment. A characteristic impedance of 50- is a compromise between power handling and signal loss in high power radio waves [47]. Therefore, great deals of reported rectenna in literature have been developed with 50 Ω interfaces.
On the other hand, the design consists of an antenna having an input impedance that is complex conjugate with the rectifier input impedance was proposed to increase the performance of the rectenna. This means the antenna has a low resistive part and high inductance, which make the antenna electrically small [48]. A comparison between the 50-Ω interface and complex conjugate interfaces were performed and presented in Fig. 1-9 and Fig. 1-10 [48]. The result shows that the compact antenna with conjugated matching has about 5% higher in efficiency and this difference due to the loss in the matching circuit.
Techniques to increase the harvested power
Combining techniques
Harvested power from a single rectenna is mostly not sufficient in supplying energy for typical device operation. In order to build up the harvested power that can be suitable to supply low-power devices, techniques of combining several elements into one system are developed.
Combining single-frequency rectenna:
This direction concentrates in boosting the harvested power from RF sources at only one frequency. In the environment when only one mobile service is available, this technique is more productive. The most common practice is using multiple antennas arranged in a large array to produce a large amount of power. There are two most commonly used configurations [49]:
– RF-combiner: in which multiple antennas associated with an RF combiner and then connected with a rectifier. This array antenna works together as a single antenna with higher gain in order to increase the received power at the input of rectifier.
– DC-combiner: in which each antenna associated with its own rectifier and DC power then combined in parallel, series or hybrid. Besides, the DC-combiner can also be realized from two port of a single circular polarization antenna.
Combining multi-frequency rectenna:
This is an attractive approach since several RF sources exist in the ambiance from mobile services, especially in the urban and semi-urban environment. This trend has given rise to a lot of publications on the topic. However, due to the nonlinearities of the diode, the input impedance of the circuit varies as a function of the frequency, input power level, and load impedance. This creates a challenge for impedance matching of multi-frequency rectennas.
The multi-frequency solutions for RF energy harvesting include different approaches of rectenna (rectifier + antenna) architectures as described in Fig. 1-11.
– (I): Using different structures of single-band antenna and single-band rectifier side-by-side to form a multi-frequency rectenna
– (II): Using multiband or wideband antenna
o (II.1): Design multi-band antenna with different excitation points for different frequencies. The single-band rectifiers are used then to connect with each port of the multiband antenna.
o (II.2): Design consists of multi-band antenna connected to single-band rectifiers by adding multiplexers in between.
o (II.3): Design consists of reconfigurable frequency antennas, corresponding to each frequency band, and connected directly to multi-tone rectifiers.
o (II.4): Design consists of a multi-band antenna and a multi-tone rectifier
In the current literature, only the solutions I, II.2 and II.4 were proposed due to the complexity in the design of multiport antenna and reconfigurable frequency antenna.
Below are given some details about each technique and the results that have been reported.
Single-frequency: Multiple antennas in RF-combiner configuration
While using multiple antennas in an RF-combiner configuration, the phase of the incident wave is taken into account. This phased array of antenna offers to steer the beam in the desired direction. This topology has the advantage of harvesting more power near the main beam of the incident waves.
Mavaddat et al. proposed in [50] a 4×4 microstrip patch antenna array for millimeter-wave energy harvesting (Fig. 1-12). A step-impedance low-pass filter is used between the antenna and rectifier to suppress second-order harmonics generated by the diode. A GaAs Schottky diode MAE1317 was used in shunt topology as a half-wave rectifier. The antenna array has a gain of 19 dBi and the maximum RF-to-DC conversion efficiency was 67% at 35.7 GHz with an input power of 7mW.
Single-frequency: Multiple antennas in DC-combiner configuration
In the DC-combiner configuration, the rectifier received RF signal of each antenna prior to combining it at DC output. Each rectifier responds to the broader pattern of each antenna and therefore is less sensitive to the incidence angles.
Olgun et al. presented in [51] a 3×3 planar array of the simple Koch-type patch antenna and 3×3 rectifiers built-in directly in the lower layer of the substrate (Fig. 1-13). The rectifier is a single-stage full wave Greinacher rectifier and was optimized for the input power between -40 dBm to -20 dBm (Fig. 1-14).
The rectifier array was then combined by the series-connected array. This design has the advantage that the matched load is higher than the parallel topology and matches with the impedance of the energy management unit. The array setting also took into account the DC lines from each rectifier. In order to step-up and regulate the output voltage from the harvester circuit, the energy management units used the charge pump integrated circuit S882 and step-up DC-DC converter AS1310. When tested in the typical office environment, the device was able to supply a continuous current of 10 µA to the load. The maximum output current reached 50 µA. The total load varies between 85 and 105 kΩ, therefore the harvested power is about 10.5 µW.
Suffice to recall that a simple core microcontroller like STM32 by STMicroelectronics in its low-power configuration has current consumption under 1 µA that is equivalent to a resistor of 30 kΩ in standby mode [52]. Other way said, 1 µA is a non-negligible current with looking for digital computation.
Table of contents :
Introduction
State of the art
1.1 Solutions of energy harvesting
1.1.1 Ambient energy harvesting
1.1.2 Promising frequencies for ambient RF energy harvesting
Limitation standard
Available power densities
1.2 Rectenna structures for ambient RF energy harvesting
1.2.1 Techniques to increase RF-to-DC conversion efficiency of rectenna
Nonlinear device choices
Circuit topologies choices
Harmonic termination techniques
Impedance matching
1.2.2 Techniques to increase the harvested power
Combining techniques
Single-frequency: Multiple antennas in RF-combiner configuration
Single-frequency: Multiple antennas in DC-combiner configuration
Single-frequency: Multi-input of an antenna in DC-combiner configuration
Multi-frequency: Multiple single-band antennas and single-tone rectifiers
Multi-frequency: Wideband/Multiband antenna and single-tone rectifiers
Multi-frequency: Wideband/Multiband antenna and multi-tone rectifier
1.3 Flexible antenna in energy harvesting system
1.3.1 Antenna using flexible material
Textile material
Paper-based and polymer-based material
1.3.2 Techniques to increase realized gain of flexible antenna
1.3.3 Integration of flexible antenna in energy harvesting system
1.4 Conclusion
Antenna
2.1 Requirements of antenna for ambient energy harvesting
2.2 3D flexible multi-band antenna
2.2.1 Dipole-based antenna
Antenna Configuration
Simulation and measurement results
Applying for printed antenna on flexible substrate
2.2.2 Coplanar-based antenna
Antenna configuration
Bending and folding analysis
Fabrication and measurement
Conclusion
2.3 Technique to overcome high-loss substrate
2.3.1 Design and characterization of suspended patch antenna
Paper substrate characteristics
Antenna configuration
2.3.2 Simulation and measurement results
2.3.3 Applying for flexible multi-band antenna with suspended ground
Antenna configuration
Simulation and measurements
2.4 Adjustable frequency antenna using flexible substrate
2.4.1 Antenna configuration
2.4.2 Simulation and measurement results
2.4.3 Conclusion
2.5 High gain flexible antenna
2.5.1 Target of design
2.5.2 Antenna configuration
2.5.3 Fabrication and measurement
2.6 Conclusion
Rectifier
3.1 Diode modeling
3.1.1 Diode characterization
Parameters of a Schottky diode
Parameter extraction of equivalent circuit model
3.1.2 Measurements and results
Extraction of Is and n
Extraction of Rs
Extraction of Rj
Discussion
3.1.3 Proposed physical-based model
DC forward modeling
DC reverse modeling
C-V modeling
S-parameter modeling
Proposed model
3.2 Rectifier to combine different sources of energy
3.2.1 Energy addition at RF level using a diplexer
Band-stop filter design
Diplexer design:
Fabrication and measurement
Conclusion
3.2.2 Energy addition at DC level
Design of rectifier with 1-input at 2.45GHz
Design of rectifier with two inputs at 2.45 GHz
3.3 Conclusion
Global system and measurements
4.1 Co-design
4.1.1 Impedance matching in co-design rectenna
4.1.2 Choosing matching impedance
4.2 Operation of spatial diversity rectenna in realistic environment
4.2.1 Diversity antenna concept
4.2.2 Rectenna design
Antenna arrangement
Rectifier structure
4.2.3 Experimental setup
4.2.4 Measurement results
Antenna measurement
Rectifier measurement
Rectenna measurement in realistic environment
Conclusion
4.3 Flexible diversity rectenna
4.3.1 Flexible diversity antenna design
Antenna configuration
Technique to reduce mutual coupling and enhance the antenna gain
4.3.2 Interface between antenna and rectifier
On flexible-substrate rectifier
Separate rectifier
4.3.3 Fabrication and measurement
Fabrication
Antenna measurement
Diversity rectenna measurement in realistic environment
4.4 Conclusion
Conclusion and perspectives
5.1 Conclusion
5.2 Perspectives
References
