6. Rectification techniques

#### 6.1 Resonator design

than MHz frequencies with reduced ratio of f2 to f1. However, research work demonstrated in [29] reports on reduced PTE variance with higher frequency ratio by implementing other rectification techniques which will be discussed in subsequent sections. In general, it is observed that there is higher probability of concurrently capitalizing PTE for both frequencies by designing resonators with lowest

Multi-coil [31, 32, 39–42] and single-coil [34, 43, 44] are two predominant strategies in designing dual-band near-field coupled resonators for WET links. Each

> band RX resonators • Dual-band TX and singleband RX resonators

Possible Increased possibility

• Dual-band TX and dualband RX resonators • Dual-band TX and singleband RX resonators

Factors Single-coil Multi-coil Cross-coupling Reduced More Geometrical area for resonators Reduced More

Dual-band support configuration • Dual-band TX and dual-

Comparisons of design approaches for dual-band near-field WET resonator.

Summary of rectification techniques for dual-band near-field WET resonator.

possible ratio of f2 to f1.

Recent Wireless Power Transfer Technologies

5. Design approaches

Concurrent transfer efficiency capitalization for both frequencies

Table 3.

Figure 3.

54

There is a penchant in designing printed spiral coils which encompassed conductive trace etched on dielectric substrates over conventional solenoid coils owing to lightweight and compactness features apart from having privilege of freedom in geometrical optimization [45] and conformity capability on malleable substrates [46] as compared to its predecessor. Equivalent series resistance (ESR) and substrate's dielectric losses are commonly contemplated to boost Q-factor of loop resonator designed [47] since the performance metric indicator, kQ, product can be derived from impedance matrix components given by [48]:

$$kQ = |Z\_{21}| \text{ESR}^{-1} \tag{8}$$

where ESR = [(R11R22) � (R12R21)]0.5 for two-port system. Despite the fact that higher inductance could be acquired by increasing number of turns, ESR is not exempted. Direct current (DC) and alternate current (AC) resistances are components of ESR for a loop resonator. Hence, it is obligatory to exercise caution in geometrical layout alteration specifically number of turns, n, conductor width, wc, and spacing between conductor trace, sc. Figure 4 illustrates comparison between a conventional layout of spiral resonator with uniform distribution of wc and sc and nonuniform layout refinement between each loop turns.

By designing inconstant conductor width distributions and spatial distributions, Q-factor enhancement is attained in double-layer printed spiral resonator [49], printed circular resonator [50], and stacked multilayer printed spiral resonator [51]. This is made possible as losses induced by eddy current are hindered with designs that involve gradual decrement of conductive trace widths from the outermost loop to the innermost loop [52] unlike conventional constant width for all loops. It is worth keeping in mind that designing larger constant conductor width for all turns and smaller constant spacing between adjacent conductor trace will only reinforce the total resistance [53]. However, ensuring ratio between conductor width and spacing to be relatively small aids in the reduction of proximity effects [54].

Figure 4. Geometrical layout refinement: (i) uniform; (ii) nonuniform.

As such, geometrical layout refinement validated in performance enhancement for single-band resonator designs [55–58] can be adopted in resonators designed to operate at more than one frequency band. In [36], multi-coil approach with meticulous geometrical manipulation yields a nonuniform conductor widths and spacing between conductor trace of spiral resonators, collocating in a single-layer substrate. Combined with independent impedance transformation network, which will be covered in ensuing subsection, this leads to minimum transfer efficiency variation between both frequencies.

repeater entails more space allocation apart from frequency splitting manifestation of either one of the frequency bands owing to strong coupling. Moreover, designers ought to take into account that transfer efficiency is greatly affected by parasitic

Resonator configurations: (i) without intermediate element; (ii) with intermediate element (repeater).

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications

DOI: http://dx.doi.org/10.5772/intechopen.89218

Composition and construction of resonator is a complementary technique to geometrical layout modification as well as loop and shape design. In multi-coil approach, each resonator operating at f1 and f2 can be designed either sharing similar axis or surface as illustrated in Figure 6 (i) and (ii). The latter alleviates cross-coupling, but larger geometrical area is deemed necessary [39]. Nevertheless, this can be suppressed by designing a pair of asymmetrical coaxial resonators as shown in Figure 6 (iii) whereby inductance for transmitting resonator is higher than receiving resonator. It is also inferred that optimizing power transfer foremost reduces interference between power carrier and data telemetry since improved coupling that leads to higher transfer power efficiency counterbalances interference between power and data. Further miniaturization effort is described in [24] by

Resonator configurations: (i) coaxial; (ii) coplanar; (iii) asymmetrical configuration.

resistance originating from repeater.

Figure 5.

Figure 6.

57

The prevailing shapes for loop resonator design are circular and square. Nevertheless, there is an upsurge research trend in venturing into other types of resonators with diverse and irregular shapes such as defected ground structure (DGS). DGS resonator designs offer yet another miniaturization technique without forgoing WET performance especially high Q-factor such as circular DGS [23], interlaced DGS [24], C-shaped DGS [33], and bow-tie DGS [38]. However, the lowest frequency for all these designs hitherto is 300 MHz with ratio between frequency bands of not more than 3.

#### 6.2 Resonator configuration

Transfer range deficiency associated with conventional two elements of singleband WET system architecture steers toward the adoption of supplementary elements such as the inclusion of repeaters or relay resonators in between source and load as depicted in Figure 5. Hence, sequence of four-coil strongly coupled magnetic resonance (SCMR) configuration introduced in [59], namely, driver, primary, secondary, and load coils, demonstrates extended transfer distance of greater than threefold compared to the diameter of primary resonator. Generally, identical designs can be observed from the driver and load resonators aside from primary and secondary resonators for conventional SCMR configuration. Incorporating repeater as a third element rather than four-element configuration substantiates viability of transfer range extension in single-band WET system [60].

Likewise, incorporating intermediate elements such as repeater in a single-coil approach is a remedy to the predicament in achieving simultaneous high-energy transfer for dual-band as demonstrated in [28, 61]. It is worth noting that additional Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications DOI: http://dx.doi.org/10.5772/intechopen.89218

Figure 5. Resonator configurations: (i) without intermediate element; (ii) with intermediate element (repeater).

repeater entails more space allocation apart from frequency splitting manifestation of either one of the frequency bands owing to strong coupling. Moreover, designers ought to take into account that transfer efficiency is greatly affected by parasitic resistance originating from repeater.

Composition and construction of resonator is a complementary technique to geometrical layout modification as well as loop and shape design. In multi-coil approach, each resonator operating at f1 and f2 can be designed either sharing similar axis or surface as illustrated in Figure 6 (i) and (ii). The latter alleviates cross-coupling, but larger geometrical area is deemed necessary [39]. Nevertheless, this can be suppressed by designing a pair of asymmetrical coaxial resonators as shown in Figure 6 (iii) whereby inductance for transmitting resonator is higher than receiving resonator. It is also inferred that optimizing power transfer foremost reduces interference between power carrier and data telemetry since improved coupling that leads to higher transfer power efficiency counterbalances interference between power and data. Further miniaturization effort is described in [24] by

Figure 6. Resonator configurations: (i) coaxial; (ii) coplanar; (iii) asymmetrical configuration.

As such, geometrical layout refinement validated in performance enhancement for single-band resonator designs [55–58] can be adopted in resonators designed to operate at more than one frequency band. In [36], multi-coil approach with meticulous geometrical manipulation yields a nonuniform conductor widths and spacing between conductor trace of spiral resonators, collocating in a single-layer substrate. Combined with independent impedance transformation network, which will be covered in ensuing subsection, this leads to minimum transfer efficiency variation

The prevailing shapes for loop resonator design are circular and square. Nevertheless, there is an upsurge research trend in venturing into other types of resonators with diverse and irregular shapes such as defected ground structure (DGS). DGS resonator designs offer yet another miniaturization technique without forgoing WET performance especially high Q-factor such as circular DGS [23], interlaced DGS [24], C-shaped DGS [33], and bow-tie DGS [38]. However, the lowest frequency for all these designs hitherto is 300 MHz with ratio between frequency

Transfer range deficiency associated with conventional two elements of singleband WET system architecture steers toward the adoption of supplementary elements such as the inclusion of repeaters or relay resonators in between source and load as depicted in Figure 5. Hence, sequence of four-coil strongly coupled magnetic resonance (SCMR) configuration introduced in [59], namely, driver, primary, secondary, and load coils, demonstrates extended transfer distance of greater than threefold compared to the diameter of primary resonator. Generally, identical designs can be observed from the driver and load resonators aside from primary and secondary resonators for conventional SCMR configuration. Incorporating repeater as a third element rather than four-element configuration substantiates viability of

Likewise, incorporating intermediate elements such as repeater in a single-coil approach is a remedy to the predicament in achieving simultaneous high-energy transfer for dual-band as demonstrated in [28, 61]. It is worth noting that additional

transfer range extension in single-band WET system [60].

between both frequencies.

Geometrical layout refinement: (i) uniform; (ii) nonuniform.

Recent Wireless Power Transfer Technologies

Figure 4.

bands of not more than 3.

56

6.2 Resonator configuration

designing a superimposed dual-band DGS instead of coplanar or coaxial configurations.

Furthermore, isolation techniques such as antiparallel loop structure [42] and frequency selective loop [40] structure functioning as band-pass filter have proven effective in rectifying interference caused by multi-coil mode. Band-stop filter in [31] filters' undesired parasitic eddy current from higher frequency induced across lower frequency coil path. The design is then revised in [32] with minimization of large spacing between low and high frequency coils and discrete inductor size along with total number of inverters. With careful combination of impedance values and coil-winding track, magnitude and phase of higher frequency voltage are fine-tuned to attain nil summation of total voltage across lower frequency path. As for displacement countermeasure, constructing more than one loop in an array structure reinforces tolerance toward detrimental consequences caused by imperfect orientations between transmitting and receiving resonators [62]. Robustness toward lateral displacement with wider coverage area is validated in [31] by incorporating asymmetrical configuration technique between a pair of coupled resonators.

parallel-series (P-S). Double capacitive CN connected either in series or parallel is also known as L-match network. WET system implemented with complex conjugate matching yields other combination of CN topologies apart from these topologies such as T-match network [19] as well as hybrid T-match network of inductor

Comprehensive analysis on impedance matching is discussed in [34], which

13.56 MHz thanks to two diverse impedance matching networks comprising of LC branch in series and LC tank in parallel. Unfortunately, this method is not applicable when variance between two target frequencies is considerably large particularly if one of the target frequencies originates from kHz band [31]. A way to overcome limitation of vast frequency separation is through specific coil arrangement by

As for multi-coil approach, designers do not have to concern much on the total target frequency separation since impedance matching can be executed independently. In [42], a high and a low Q impedance matching are performed separately at two different matching distances specifically closer distance for higher transfer efficiency aimed at wireless charging. Conversely, the low Q impedance match is completed at further distance as a trade-off for higher bandwidth intended for data

Eq. (9) is employed in [36] before performing simultaneous conjugate matching

On the other hand, simultaneous high-energy transfer is validated in [23] for both frequency bands using a compact two circular DGS resonators with independent coupling whereby series capacitance is implemented together with single stub for matching method. Correspondingly, comparable findings are also reported in [37] by adopting single-coil approach and single CN with dual-mode printed inductor at a minimized geometrical area requirement. In contrast, DGS resonator designs executed with impedance transformation network which consists of series capacitive lumped element and single stub matching in [24, 38] are unable to acquire minimum PTE variance between frequencies, but these designs excel in

7. Simulation of dual-band resonator design with multiple rectification

By applying single-coil approach, Figure 8 depicts an example of dual-band printed spiral resonator simulated with full-wave electromagnetic simulator, CST Microwave Studio separated at transfer distance of 15 mm. Three rectification techniques are implemented specifically resonator design, resonator configuration, and impedance transformation network. Since dimension constrictions are of paramount concern in near-field WET system, the overall dimension of transmitting resonator is limited to 75 mm by 82.5 mm. Other parameter properties are detailed in Table 4 after optimization of parametric studies via geometrical layout tuning.

zop\_ fn ¼ 0:3931do\_ fn (9)

using symmetrical double capacitive CN in order to acquire PTE equilibrium between both frequencies selected. Since the design is based on multi-coil mode, compromise should be made in selecting optimum separation distance between outermost side length of loop operating at f1 and f2. The relationship between optimal transfer distance, zop\_fn, and outermost side length of loop, do\_f1,f2, at maximum excited magnetic field derived in [58] for a square spiral resonator yields:

proposes single-coil mode supporting two resonant frequencies at 6.78 and

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications

positioning inner lower band coil and outer higher band coil [31].

and capacitors [63].

DOI: http://dx.doi.org/10.5772/intechopen.89218

transfer applications.

terms of maximizing FOM.

techniques

59

#### 6.3 Impedance transformation network

Impedance transformation network functions as alleviative measures of mutual inductance disparities triggered by spatial distance or load fluctuations between near-field coupled resonators. Resonance tuning and impedance matching, also referred to as compensation network, are commonly applied as front-end resonator design before AC-DC rectification. Implementing appropriate reactive compensation is necessary toward realizing maximum power transfer efficiency at preferred resonance frequency. Figure 7 illustrates capacitive compensation topologies which can be generally categorized into symmetrical and asymmetrical compensation network (CN) for single-band near-field WET system. Single capacitive compensation commonly employed each at transmitting and receiving resonator sides encompasses series-series (S-S), parallel-parallel (P–P), series-parallel (S-P), and

#### Figure 7.

Symmetrical compensation network topologies: (i) series-series (S-S); (ii) parallel-parallel (P–P); (iii) series parallel-series parallel (SP-SP); (iv) parallel series-parallel series (PS-PS). Asymmetrical compensation network topologies: (v) parallel-series (P-S); (vi) series-parallel (S-P); (vii) series parallel-parallel series (SP-PS); (viii) parallel series-series parallel (PS-SP).

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications DOI: http://dx.doi.org/10.5772/intechopen.89218

parallel-series (P-S). Double capacitive CN connected either in series or parallel is also known as L-match network. WET system implemented with complex conjugate matching yields other combination of CN topologies apart from these topologies such as T-match network [19] as well as hybrid T-match network of inductor and capacitors [63].

Comprehensive analysis on impedance matching is discussed in [34], which proposes single-coil mode supporting two resonant frequencies at 6.78 and 13.56 MHz thanks to two diverse impedance matching networks comprising of LC branch in series and LC tank in parallel. Unfortunately, this method is not applicable when variance between two target frequencies is considerably large particularly if one of the target frequencies originates from kHz band [31]. A way to overcome limitation of vast frequency separation is through specific coil arrangement by positioning inner lower band coil and outer higher band coil [31].

As for multi-coil approach, designers do not have to concern much on the total target frequency separation since impedance matching can be executed independently. In [42], a high and a low Q impedance matching are performed separately at two different matching distances specifically closer distance for higher transfer efficiency aimed at wireless charging. Conversely, the low Q impedance match is completed at further distance as a trade-off for higher bandwidth intended for data transfer applications.

Eq. (9) is employed in [36] before performing simultaneous conjugate matching using symmetrical double capacitive CN in order to acquire PTE equilibrium between both frequencies selected. Since the design is based on multi-coil mode, compromise should be made in selecting optimum separation distance between outermost side length of loop operating at f1 and f2. The relationship between optimal transfer distance, zop\_fn, and outermost side length of loop, do\_f1,f2, at maximum excited magnetic field derived in [58] for a square spiral resonator yields:

$$z\_{op\\_fn} = \mathbf{0}.\mathbf{\mathbf{3}}\mathbf{\mathbf{3}}\mathbf{1}d\_{o\\_fn} \tag{9}$$

On the other hand, simultaneous high-energy transfer is validated in [23] for both frequency bands using a compact two circular DGS resonators with independent coupling whereby series capacitance is implemented together with single stub for matching method. Correspondingly, comparable findings are also reported in [37] by adopting single-coil approach and single CN with dual-mode printed inductor at a minimized geometrical area requirement. In contrast, DGS resonator designs executed with impedance transformation network which consists of series capacitive lumped element and single stub matching in [24, 38] are unable to acquire minimum PTE variance between frequencies, but these designs excel in terms of maximizing FOM.
