# A New Technique for Reducing Size of a WPT System Using Two-Loop Strongly-Resonant Inductors

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## Abstract

**:**

## 1. Introduction

## 2. Theoretical Analysis of Strongly-Coupled WPT Systems

## 3. Numerical Analysis

#### 3.1. High Q Coil System Specifications

#### 3.2. Low Q Coil System Specifications without Additional Inductor

#### 3.3. Effect of the Additional Coil Resistance on the System Efficiency

## 4. Experimental Setup

#### 4.1. System Design and Measurements

#### 4.2. System Maximum Efficiency

## 5. Results

## 6. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

- Rabie, K.M.; Adebisi, B.; Rozman, M. Outage probability analysis of WPT systems with multiple-antenna access point. In Proceedings of the 2016 10th International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP), Prague, Czech Republic, 20–22 July 2016; pp. 1–5. [Google Scholar]
- Rabie, K.M.; Adebisi, B.; Alouini, M.S. Wireless power transfer in cooperative DF relaying networks with log-normal fading. In Proceedings of the 2016 IEEE Global Communications Conference (GLOBECOM), Washington, DC, USA, 4–8 December 2016; pp. 1–6. [Google Scholar]
- Lin, J.C. Wireless power transfer for mobile applications, and health effects [telecommunications health and safety]. IEEE Antennas Propag. Mag.
**2013**, 55, 250–253. [Google Scholar] [CrossRef] - Dickinson, R.M. Power in the sky: Requirements for microwave wireless power beamers for powering high-altitude platforms. IEEE Microw. Mag.
**2013**, 14, 36–47. [Google Scholar] [CrossRef] - Raible, D.E.; Dinca, D.; Nayfeh, T.H. Optical frequency optimization of a high intensity laser power beaming system utilizing VMJ photovoltaic cells. In Proceedings of the 2011 International Conference on Space Optical Systems and Applications (ICSOS), Santa Monica, CA, USA, 11–13 May 2011; pp. 232–238. [Google Scholar]
- Lu, Y.; Ma, D.B. Wireless power transfer system architectures for portable or implantable applications. Energies
**2016**, 9, 1087. [Google Scholar] [CrossRef] - Kim, S.; Ho, J.S.; Chen, L.Y.; Poon, A.S. Wireless power transfer to a cardiac implant. Appl. Phys. Lett.
**2012**, 101, 073701. [Google Scholar] [CrossRef] - Kilinc, E.G.; Conus, G.; Weber, C.; Kawkabani, B.; Maloberti, F.; Dehollain, C. A system for wireless power transfer of micro-systems in-vivo implantable in freely moving animals. IEEE Sens. J.
**2014**, 14, 522–531. [Google Scholar] [CrossRef] - Cook, N.P.; Sieber, L.; Widmer, H. Short Range Efficient Wireless Power Transfer. U.S. Patent 12,427,318, 21 April 2009. [Google Scholar]
- Xie, L.; Shi, Y.; Hou, Y.T.; Lou, A. Wireless power transfer and applications to sensor networks. IEEE Wirel. Commun.
**2013**, 20, 140–145. [Google Scholar] - Jawad, A.M.; Nordin, R.; Gharghan, S.K.; Jawad, H.M.; Ismail, M. Opportunities and challenges for near-field wireless power transfer: A review. Energies
**2017**, 10, 1022. [Google Scholar] [CrossRef] - Costanzo, A.; Dionigi, M.; Mastri, F.; Mongiardo, M.; Monti, G.; Russer, J.A.; Russer, P.; Tarricone, L. Conditions for a load-independent operating regime in resonant inductive WPT. IEEE Trans. Microw. Theory Tech.
**2017**, 65, 1066–1076. [Google Scholar] [CrossRef] - Fotopoulou, K.; Flynn, B.W. Wireless power transfer in loosely coupled links: Coil misalignment model. IEEE Trans. Magn.
**2011**, 47, 416–430. [Google Scholar] [CrossRef] - Li, Y.; Wang, Y.; Cheng, Y.; Li, X.; Xing, G. QiLoc: A Qi wireless charging based system for robust user-initiated indoor location services. In Proceedings of the 2015 12th Annual IEEE International Conference on Sensing, Communication, and Networking (SECON), Seattle, WA, USA, 22–25 June 2015; pp. 480–488. [Google Scholar]
- Liu, X. Qi standard wireless power transfer technology development toward spatial freedom. IEEE Circuits Syst. Mag.
**2015**, 15, 32–39. [Google Scholar] [CrossRef] - Van Wageningen, D.; Staring, T. The Qi wireless power standard. In Proceedings of the 14th International Power Electronics and Motion Control Conference EPE-PEMC 2010, Ohrid, Macedonia, 6–8 September 2010; pp. S15-25–S15-32. [Google Scholar]
- Kim, J.; Son, H.C.; Kim, D.H.; Park, Y.J. Optimal design of a wireless power transfer system with multiple self-resonators for an LED TV. IEEE Trans. Consum. Electron.
**2012**, 58. [Google Scholar] [CrossRef] - Geng, Y.; Li, B.; Yang, Z.; Lin, F.; Sun, H. A high efficiency charging strategy for a supercapacitor using a wireless power transfer system based on inductor/capacitor/capacitor (LCC) compensation topology. Energies
**2017**, 10, 135. [Google Scholar] [CrossRef] - Haldi, R.; Schenk, K.; Nam, I.; Santi, E. Finite-element-simulation-assisted optimized design of an asymmetrical high-power inductive coupler with a large air gap for EV charging. In Proceedings of the 2013 IEEE Energy Conversion Congress and Exposition, Denver, CO, USA, 15–19 September 2013; pp. 3635–3642. [Google Scholar]
- Wen, F.; Huang, X. Optimal magnetic field shielding method by metallic sheets in wireless power transfer system. Energies
**2016**, 9, 733. [Google Scholar] [CrossRef] - RamRakhyani, A.K.; Mirabbasi, S.; Chiao, M. Design and optimization of resonance-based efficient wireless power delivery systems for biomedical implants. IEEE Trans. Biomed. Circuits Syst.
**2011**, 5, 48–63. [Google Scholar] [CrossRef] [PubMed] - Rozman, M.; Fernando, M.; Adebisi, B.; Rabie, K.M.; Kharel, R.; Ikpehai, A.; Gacanin, H. Combined conformal strongly-coupled magnetic resonance for efficient wireless power transfer. Energies
**2017**, 10, 498. [Google Scholar] [CrossRef] - Kurs, A.; Karalis, A.; Moffatt, R.; Joannopoulos, J.D.; Fisher, P.; Soljačić, M. Wireless power transfer via strongly coupled magnetic resonances. Science
**2007**, 317, 83–86. [Google Scholar] [CrossRef] [PubMed] - Somani, S.; Shaqfeh, E.S.; Prakash, J.R. Effect of solvent quality on the coil- stretch transition. Macromolecules
**2010**, 43, 10679–10691. [Google Scholar] [CrossRef] - Junaid, A.B.; Konoiko, A.; Zweiri, Y.; Sahinkaya, M.N.; Seneviratne, L. Autonomous wireless self-charging for multi-rotor unmanned aerial vehicles. Energies
**2017**, 10, 803. [Google Scholar] [CrossRef] - Dai, Z.; Wang, J.; Long, M.; Huang, H. A witricity-based high-power device for wireless charging of electric vehicles. Energies
**2017**, 10, 323. [Google Scholar] [CrossRef] - Hwang, K.; Cho, J.; Kim, D.; Park, J.; Kwon, J.H.; Kwak, S.I.; Park, H.H.; Ahn, S. An autonomous coil alignment system for the dynamic wireless charging of electric vehicles to minimize lateral misalignment. Energies
**2017**, 10, 315. [Google Scholar] [CrossRef] - Jiang, C.; Chau, K.T.; Liu, C.; Lee, C.H.T. An overview of resonant circuits for wireless power transfer. Energies
**2017**, 10, 894. [Google Scholar] [CrossRef] - Liu, D.; Hu, H.; Georgakopoulos, S.V. Misalignment sensitivity of strongly coupled wireless power transfer systems. IEEE Trans. Power Electron.
**2017**, 32, 5509–5519. [Google Scholar] [CrossRef] - Sample, A.P.; Meyer, D.T.; Smith, J.R. Analysis, experimental results, and range adaptation of magnetically coupled resonators for wireless power transfer. IEEE Trans. Ind. Electron.
**2011**, 58, 544–554. [Google Scholar] [CrossRef] - Vijayakumaran Nair, V.; Choi, J.R. An efficiency enhancement technique for a wireless power transmission system based on a multiple coil switching technique. Energies
**2016**, 9, 156. [Google Scholar] [CrossRef] - Kiani, M.; Jow, U.M.; Ghovanloo, M. Design and optimization of a 3-coil inductive link for efficient wireless power transmission. IEEE Trans. Biomed. Circuits Syst.
**2011**, 5, 579–591. [Google Scholar] [CrossRef] [PubMed] - Jow, U.M.; Ghovanloo, M. Design and optimization of printed spiral coils for efficient transcutaneous inductive power transmission. IEEE Trans. Biomed. Circuits Syst.
**2007**, 1, 193–202. [Google Scholar] [CrossRef] [PubMed] - Lee, H.; Kang, S.; Kim, Y.; Jung, C. Small-sized metallic and transparent film resonators for MR-WPT system. Electron. Lett.
**2016**, 52, 650–652. [Google Scholar] [CrossRef] - Hu, H.; Liu, D.; Georgakopoulos, S.V. Miniaturized strongly coupled magnetic resonant systems for wireless power transfer. In Proceedings of the 2016 IEEE International Symposium on Antennas and Propagation (APSURSI), Fajardo, Puerto Rico, 26 June–1 July 2016; pp. 155–156. [Google Scholar]
- Nguyen, V.T.; Kang, S.H.; Jung, C.W. Wireless power transfer for mobile devices with consideration of ground effect. In Proceedings of the 2015 IEEE Wireless Power Transfer Conference (WPTC), Boulder, CO, USA, 13–15 May 2015; pp. 1–4. [Google Scholar]
- Fujita, T.; Yasuda, T.; Akagi, H. A wireless power transfer system with a double-current rectifier for EVs. In Proceedings of the 2016 IEEE Energy Conversion Congress and Exposition (ECCE), Milwaukee, WI, USA, 18–22 September 2016; pp. 1–7. [Google Scholar]
- Mahdavifard, M.; Poorfakhraei, A.; Tahami, F. A novel method for reduction of coil weight and size in wireless power transfer. In Proceedings of the 2017 8th Power Electronics, Drive Systems Technologies Conference (PEDSTC), Mashhad, Iran, 14–16 February 2017; pp. 395–400. [Google Scholar]
- Theilmann, P.T.; Asbeck, P.M. An analytical model for inductively coupled implantable biomedical devices with ferrite rods. IEEE Trans. Biomed. Circuits Syst.
**2009**, 3, 43–52. [Google Scholar] [CrossRef] [PubMed] - Hu, H.; Georgakopoulos, S.V. Analysis and design of conformal SCMR WPT systems with multiple resonators. In Proceedings of the 2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Memphis, TN, USA, 6–11 July 2014; pp. 1347–1348. [Google Scholar]

**Figure 1.**Electromagnetic field between the transmitter and receiver loops, which occurs due to changing polarity of the current flow.

**Figure 2.**(

**a**) A comparison between the conventional WPT system with a single set of coils ; and (

**b**) the proposed technique with an additional coil.

**Figure 3.**The circuit diagram of the proposed two-loop resonant WPT system with additional inductor. ${I}_{IN}$, ${I}_{2}$, ${I}_{3}$, ${I}_{4}$, ${I}_{5}$ and ${I}_{6}$ represent the currents in each branch, while the ${Z}_{T1}$, ${Z}_{T2}$, ${Z}_{R}$ and ${Z}_{R2}$ represent impedance of the inductors.

**Figure 4.**Calculated efficiency of the system with high Q of the transmitter and receiver coils, built based on the value of the elements from Table 1.

**Figure 5.**Efficiency of the system with low Q factor of the coil without additional inductor, calculated with advanced design system (ADS) software. A coupling factor at which maximum efficiency appears is much higher than that for the system with high Q.

**Figure 6.**Comparison between the coupling factor needed for the maximum efficiency to appear for the system with low Q and high Q of the coils.

**Figure 7.**The effect of the different values of additional inductors on the coupling factor needed to reach maximum efficiency. The coupling factor needed to reach maximum efficiency drops with the decrease of the value of additional inductor.

**Figure 8.**Comparison of the maximum distance between the ${T}_{X}$ and ${R}_{X}$ loops of the system with low Q design, with and without additional inductors and a system with high Q coil design.

**Figure 9.**Calculated efficiency of the proposed system with low Q of the coils with two additional inductors. The coupling factor at which maximum efficiency is much smaller than that for the low Q design without additional coils.

**Figure 10.**Effect of different resistance of added coils on the systems efficiency. The smaller the resistance, the higher the efficiency and vice-versa.

0.48 | 0.48 |

(a) b | (b) b |

**Figure 12.**Comparison between the measured and calculated maximum efficiency and frequency pattern for the modified circuit with additional coils. The results show close similarity of calculated and measured results.

**Figure 13.**Comparison between the measured and calculated maximum efficiency and frequency pattern of the conventional system with a high Q of the coil. A designed system shows close similarities with the calculations.

**Figure 14.**A comparison between the measured and calculated maximum distances between the low Q design coil with and without added inductor at frequency of 9.23 MHz. The system with an additional coils shows an increase in the distance between ${T}_{X}$ and ${R}_{X}$ at which mximum efficiency occurs.

**Figure 15.**Comparison between measured and calculated maximum distances between the ${T}_{X}$ and ${R}_{X}$ coil between the system with high Q coils and the system with added inductor. The results show a similar distance of transmission that can be achieved with both systems.

**Table 1.**Calculated elements of two-loop strongly coupled WPT system with high Q of coils, used to build a practical system.

Capacitance | Inductance | Resistance | Resistance |
---|---|---|---|

${C}_{1}$ = 1.6 pF | ${L}_{1}$ = 186.6 $\mathsf{\mu}$H | ${R}_{L1}$ = 0.15Ω | ${R}_{S}$ = 50Ω |

${C}_{2}$ = 1.3 pF | ${L}_{2}$ = 229.6 $\mathsf{\mu}$H | ${R}_{L1}$ = 0.15Ω | ${R}_{L}$ = 50Ω |

**Table 2.**Calculated base model of two-loop loosely coupled WPT system. The specification will be later used to built a practical model.

Capacitance | Inductance | Resistance | Resistance |
---|---|---|---|

${C}_{1}$ = 18 pF | ${L}_{1}$ = 16.37 $\mathsf{\mu}$H | ${R}_{L1}$ = 0.15Ω | ${R}_{S}$ = 50Ω |

${C}_{2}$ = 15 pF | ${L}_{2}$ = 19.65 $\mathsf{\mu}$H | ${R}_{L1}$ = 0.15Ω | ${R}_{L}$ = 50Ω |

**Table 3.**Calculated values of the elements used in the proposed model with two loop loosely coupled WPT system with a low Q of the coils and two additional inductors.

Capacitance | Inductance | Resistance | Resistance |
---|---|---|---|

${C}_{TX}$ = 18 pF | ${L}_{T2}$ = 16.37 $\mathsf{\mu}$H | ${R}_{L2}$ = 0.15Ω | ${R}_{L}$ = 50Ω |

${C}_{RX}$ = 15 pF | ${L}_{T1}$ = 0.2105 $\mathsf{\mu}$H | ${R}_{T1}$ = 0.15Ω | ${R}_{L}$ = 50Ω |

${L}_{R1}$ = 19.65 $\mathsf{\mu}$H | ${R}_{R1}$ = 0.15Ω | ||

${L}_{R2}$ = 0.2359 $\mathsf{\mu}$H | ${R}_{R2}$ = 0.15Ω |

**Table 4.**Comparison of the physical parameters of the conventional and modified systems. Proposed system shows a great improvement towards the conventional design.

Conv. ${\mathit{L}}_{\mathbf{TX}}$ | Mod. ${\mathit{L}}_{\mathbf{TX}}$ | Diff. % | Conv. ${\mathit{L}}_{\mathbf{RX}}$ | Mod. ${\mathit{L}}_{\mathbf{RX}}$ | Diff. % | |
---|---|---|---|---|---|---|

Length | 15 mm | 3 mm | 80% | 20 mm | 4 mm | 80% |

Weight | 7.37 g | 1.58 g | 78.6% | 8.63 g | 1.78 g | 79.4% |

Turns | 41 | 8 | 80.5% | 48 | 48 | 81% |

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**MDPI and ACS Style**

Rozman, M.; Fernando, M.; Adebisi, B.; Rabie, K.M.; Collins, T.; Kharel, R.; Ikpehai, A. A New Technique for Reducing Size of a WPT System Using Two-Loop Strongly-Resonant Inductors. *Energies* **2017**, *10*, 1614.
https://doi.org/10.3390/en10101614

**AMA Style**

Rozman M, Fernando M, Adebisi B, Rabie KM, Collins T, Kharel R, Ikpehai A. A New Technique for Reducing Size of a WPT System Using Two-Loop Strongly-Resonant Inductors. *Energies*. 2017; 10(10):1614.
https://doi.org/10.3390/en10101614

**Chicago/Turabian Style**

Rozman, Matjaz, Michael Fernando, Bamidele Adebisi, Khaled M. Rabie, Tim Collins, Rupak Kharel, and Augustine Ikpehai. 2017. "A New Technique for Reducing Size of a WPT System Using Two-Loop Strongly-Resonant Inductors" *Energies* 10, no. 10: 1614.
https://doi.org/10.3390/en10101614