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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 25, NO. 9, SEPTEMBER 2015
Dual Resonance Frequency Selective Loop of Near-Field Wireless Charging and Communications Systems for Portable Device DukSoo Kwon, Tae-Dong Yeo, Kyoung-Sub Oh, Jong-Won Yu, Member, IEEE, and Wang-Sang Lee, Member, IEEE
Abstract—In this letter, a frequency selective loop isolator using dual resonance for a mobile wireless charging system is presented. Resonance of the proposed frequency selective loop isolates a transmitting loop from a receiving loop at one frequency whereas it transfers wireless power between them at the second harmonic frequency. Simple arrangement of lumped elements changes operation of the proposed frequency selective loop to a series resonator at 6.78 MHz and a parallel resonator at 13.56 MHz. Experimental results show the difference in the transmission coefficient between with/without the frequency selective loop is approximately 21 dB at 6.78 MHz whereas it is about 2 dB at 13.56 MHz, without degradation of efficiency. Moreover, this approach suggests how to transfer wireless power based on frequency selection. Index Terms—Antenna, filters, passive COMPONENTS/circuits.
Fig. 1. Structure of the WPT loop, the NFC loop, and the frequency selective loop. (a) Conventional structure. (b) Proposed structure with the frequency selective loop. (black pattern is top layer and gray pattern is bottom layer.).
I. INTRODUCTION
W
ITH the advance of wireless power technology, portable wireless devices are becoming an essential part of our daily life. They are widely utilized in charging portable devices and electronic payment systems. Recently, 6.78 MHz frequency has been used for wireless power transfer (WPT) under approval by A4WP [1], and Near Field Communication (NFC) utilizes 13.56 MHz for data transfer [2]. However, the transmitting power of WPT is higher than that of NFC. WPT is mainly used to transfer high power whereas NFC is for high data rates with low power despite that it uses twofold larger frequency than WPT. It is thus necessary to block NFC frequency data from high power WPT frequency to avoid interference. In this letter, we introduce a frequency selective loop located between a transmitting loop and a receiving loop. A WPT loop, a receiving loop matched at 6.78 MHz ( ), and a transmitting loop, referred to as a NFC loop, matched at 13.56 MHz ( ) are used to show the characteristics of the frequency selective loop. Adding lumped elements on frequency selective loop can change the characteristics of the resonance loop [3], [4]. A Manuscript received February 22, 2015; revised March 24, 2015; accepted June 07, 2015. Date of publication July 06, 2015; date of current version September 01, 2015. D. S. Kwon, T.-D. Yeo, and J.-W. Yu are with the Department of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, South Korea. K.-S. Oh is with Gamma Nu inc., Hwaseong-si, Gyeonggi-do, South Korea. W.-S. Lee is with the Department of Electronic Engineering/Engineering Research Institute (ERI), Gyeongsang National University, 501 Jinju-daero, Jinju, 660-701, South Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2015.2451352
Fig. 2. Four Configurations of the system with the proposed frequency selective loop. (a) Wireless power transmission between the two identical WPT loops. (b) NFC data communication between the two identical NFC loops. (c) and (d) Loop isolation between the NFC loop and the WPT loop.
method based on series and parallel resonance with lumped elements is proposed. II. DESIGN OF FREQUENCY SELECTIVE LOOP The frequency selective loop works as a band pass filter that isolates at whereas it passes the second harmonic frequency simultaneously. Fig. 1(a) and (b) show the structure of the implemented frequency selective loop with the WPT loop and the NFC loop. Fig. 2(a)–(d) show the configurations of the WPT loop, the NFC loop, and the frequency selective loop. We design the frequency selective loop to have dual resonance frequency by manipulating the current of the frequency selective loop. If the resonance of the LC lumped elements blocks the inductive
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KWON et al.: DUAL RESONANCE FREQUENCY SELECTIVE LOOP OF NEAR-FIELD WIRELESS CHARGING AND COMMUNICATIONS SYSTEMS
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Fig. 3. (a) Equivalent circuit of the Fig. 2(c) or (d) describing isolation between the NFC loop and the WPT loop with the proposed frequency selective loop. (6.78 MHz). (c) At (13.56 MHz). (The series resistance and are omitted to describe only (b) Resonance points of the frequency selective loop at resonance points.).
current from the WPT loop to NFC loop at , the frequency selective loop reduces the mutual inductance. On the other hand, if the resonance of the LC lumped elements allows current to pass that complies with the mutual inductance between the WPT loop and the NFC loop at , the frequency selective loop transfers wireless power. Fig. 3 shows the equivalent circuit model of Fig. 2(c) or (d). The self-inductance of the NFC loop is coupled to the selfinductance of the WPT loop . Mutual inductance between the transmitting loop and the receiving loop can hence be obtained. The frequency selective loop generates additional mutual inductance and from the frequency selective loop . to the WPT loop and the NFC loop with self-inductance The frequency selective loop has a series LC lumped elements and , where has series resistance representing loss, to have double resonance points. parallel with the capacitor The dual resonance circuit is designed to minimize the coupling between the transmitting loop and the receiving loop at frequency . At the same time it does not reduce the mutual inductance at frequency . Therefore, imaginary part is zero at and is the same as at (1) (2) is the voltage across the NFC loop, is the current where through the WPT loop, and is current through the NFC loop. We find the value of the inductance and the capacitances on the frequency selective loop by taking into account that the frequency selective loop resonates as a whole at and does not resonate at all at . Under this condition, the values of the lumped elements can be calculated by the following: (3) (4) (5) By choosing the variable ,
, we obtain , and
, . Note that
Fig. 4. Measurement setup on the distance ( ) 5 mm and the proposed prototype. (a) Transmission coefficient between the implemented prototype and the conventional structure as depicted in Fig. 6. (b) Top view and (c) Bottom view of the implemented prototype, respectively.
increases, increases, and decreases as the variable decreases. The behavior of the frequency selective loops changes depending on frequency. It can be used to have series resonance to reflect wireless power at or to have parallel resonance to pass wireless power. Fig. 3(b) and (c) show equivalent circuits of the frequency selective loop at the two resonance frequencies. In Fig. 3(b), the WPT frequency, , is lower than the resonance and . The series LC is seen as frequency of series LC with capacitance, because is dominant. It forms parallel LC with and , allowing maximum current flow on . That is, the effective area of the frequency selective loop is the same as that of the entire loop, and the frequency selective loop blocks mutual inductance between the WPT loop and the NFC loop. On the other hand, is higher than the resonance frequency of series LC with and , because becomes dominant. The composite inductance of and resonates with . If , inductive current is confined to and the is lower than effective area of the frequency selective loop is also confined to the area of lumped elements , , and as described in Fig. 3(c). The reduced effective area does not disturb the mutual inductance from the WPT loop to the NFC loop.
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 25, NO. 9, SEPTEMBER 2015
Fig. 6. Isolation between the NFC loop and the WPT loop on 5 mm with/ without frequency selective loop. (Dot gray line is simulated result and red line is measured result.).
Fig. 5. Transmission coefficient between the identical loops with the frequency selective loop depending on the distance ( ) and frequency. (a) Between the two WPT loops. (b) Between the two NFC loops. (Dot gray line is simulated result and red line is measured result.).
III. MEASUREMENT RESULTS Implemented prototype with the WPT loop, the NFC loop, and the frequency selective loop used as the transmitting loop, and the transmitting loop is aligned to the conventional structure as the receiving loop. The WPT loop, the NFC loop, and the frequency selective loop are fabricated on a FR4 dielectric substrate of 1 mm thickness with relative permittivity of 4.6. The size of the WPT loop is 45 mm 61 mm with 0.5 mm width and 5 turns, and that of the NFC loop is 33 mm 45 mm with 0.4 mm width and 4 turns. The implemented prototype with its measurement is shown in Fig. 4. 50 port impedance is used as an output impedance. The frequency selective loop is put on the bottom layer of the prototype. The frequency selective loop has the size of 33 mm 49 mm with the width of 7 mm. The frequency selective loop is composed of lumped elements with , , and . In this case, a 47 nH Coilcraft ML376RAA chip inductor is chosen as ; it has -factor above 10 at and self-resonant frequency greater than . The WPT loops are designed to transfer at as shown in Fig. 5(a) with port 1 and 4 opened. Fig. 5(b) indicated that the two NFC loops show maximum transmission at with
port 2 and 3 opened using the Rx loading effect [5], [6] . The mutual inductance between the WPT loop and the NFC loop varies depending on the frequency with the frequency selective loop. In Fig. 6, the difference between measured isolation ( ) with/without the frequency selective loop is approximately 21 dB on 5 mm distance at the WPT frequency, . is without the frequency selective loop on similar to the case of the Tx NFC loop. Due to the frequency selective loop, the difference of measured isolation between the WPT loop and the NFC loop on the same layer with/without the frequency selective loop is approximately 10 dB. IV. CONCLUSION In this letter, a frequency selective loop isolator using the dual resonance circuit for a mobile wireless charging system has been designed and demonstrated. In order to control mutual inductance, we inserted the frequency selective loop between a transmitting loop and a receiving loop. The proposed dual resonance circuit is a simply implementable filter with no connection for portable mobile devices. REFERENCES [1] R. Tseng, B. von Novak, S. Shevde, and K. A. Grajski, “Introduction to the alliance for wireless power loosely-coupled wireless power transfer system specification version 1.0,” in Proc. IEEE Wireless Power Transfer (WPT), Perugia, Italy, 2013, pp. 79–83. [2] V. Coskun, K. Ok, and B. Ozdenizci, Near Field Communication (NFC): From Theory to Practice. New York: Wiley, 2011. [3] K. Sasaki, S. Sugiura, and H. Iizuka, “Distance adaptation method for magnetic resonance coupling between variable capacitor-loaded parallel-wire coils,” IEEE Trans. Microw. Theory Tech., vol. 62, no. 4, pp. 1–9, Apr. 2014. [4] K.-S. Oh, W.-S. Lee, W.-S. Lee, and J.-W. Yu, “A capacitor-loaded cylindrical resonant coil with parallel connection,” Appl. Phys. Lett., vol. 101, no. 6, Aug. 2012. [5] M. Roland, H. Witschnig, E. Merlin, and C. Saminger, “Automatic impedance matching for 13.56 MHz NFC antennas,” in Proc. 6th Int. Symp. Commun. Syst., Netw. Dig. Signal Processing (CNSDSP), Graz, Austria, 2008, pp. 288–291. [6] M. Gebhart, T. Baier, and M. Facchini, “Automated antenna impedance adjustment for Near Field Communication (NFC),” in Proc. 12th Int. Conf. Telecommun. (ConTEL), Zagreb, Croatia, 2013, pp. 235–242.
