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Automatic Adaptation of Multi-Loop Wireless Power Transfer to Variable Coupling between Transmit and Receive Coils Kyeongmok Ryu and Jinho Jeong * Department of Electronic Engineering, Sogang University, Seoul 04107, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-705-8934 Received: 19 June 2018; Accepted: 5 July 2018; Published: 7 July 2018







Abstract: In the conventional wireless power transfer (WPT) using magnetic resonance coupling, power transfer efficiency (PTE) exhibits a peak only at a matched distance between transmitter (Tx) and receiver (Rx). That is, it rapidly degrades if the distance deviates from the matched distance. In order to achieve high PTE over a wide range of the distance, automatic range-adaptation technique is proposed in this work by using multi-loop technique and tunable matching circuit with digital capacitors. For automatic range adaptation, the microcontroller unit (MCU) in Rx runs an algorithm to find optimum loop and capacitance for best PTE based on the received power. Tx and Rx are synchronized by using low power Bluetooth wireless communications. Instead of the conventional relays, microelectromechanical system (MEMS) switches with low loss and high isolation are employed to minimize the power dissipation. The entire WPT system automatically maximize PTE with the distance, achieving high PTE of 80.5% at 30 cm and 29.7% at 100 cm. Keywords: wireless power transfer (WPT); magnetic resonance; tunable impedance matching; range adaptation

1. Introduction Magnetic resonant coupling technique allows an efficient wireless power transfer (WPT) up to a few meters, which can overcome the problem of the limited operating range of the magnetic induction technique. Therefore, it finds a lot of applications in the mid-range wireless charging of home appliances and electric vehicles. However, the efficiency peaks only at a specific distance between transmitter (Tx) and receiver (Rx), and it rapidly drops as the distance changes. Therefore, there are intensive researches to achieve the high efficiency even though the distance changes, what is called, range-adaptive WPTs, using frequency and impedance tuning techniques [1–8]. In the frequency tuning method, operating frequency is adjusted to provide low reflection and thus high efficiency depending on the distance or to overcome the frequency splitting effect at very near distance [1–4]. However, it needs a wide bandwidth of operation and frequency-tunable signal source. Impedance tuning technique is commonly used for range-adaptive WPTs. It is based on the fact that input impedance of magnetic resonant WPT is matched to the system impedance only at a specific distance and varies according to the distance. Therefore, the efficiency can be recovered if tunable matching circuit is utilized to match input impedance to system impedance depending on the distance. In [9–12], tunable capacitors, switching capacitors, and switches were used as tunable matching circuit (TMC) for the range-adaptive WPTs. However, the input impedance of magnetic resonant WPTs exhibits very wide variation according to the distance. Therefore, the impedance tuning method alone can allow the very limited performance in the range adaption. In order to extend the adaptation range, the coupling between loop and coil can

Energies 2018, 11, 1789; doi:10.3390/en11071789

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be adjusted according to the distance by changing the orientation or the distance between loop and coil [5]. However, this method requires a mechanical tuning using motors for automatic adaptation which leads to the increase of DC power consumption and system complexity. Recently, multi-loop WPT has been introduced to improve the range adaptation [13] by the authors’ group of this paper, where four loops with different size were employed, and only one loop was selected according to the distance. It adjusts the coupling factor between the coil and loop such that the variation of input impedance with distance is dramatically reduced. It allows the WPT to adapt and maintain high efficiency over wider range of the distance using simple TMC. However, the multi-loop WPT in [13] employed three relays for the loop switching among four loops at each Tx and Rx. Even though relays provide very low loss, they have several drawbacks such as high power dissipation and low switching speed. In addition, input impedance was measured using directional coupler and rectifier which are also bulky and expensive. Moreover, a personal computer was used and connected to both Tx and Rx to calculate input impedance and to control the loop-switching and TMC. Thus, the WPT in [13] was not practical for real applications. In this work, we propose an automatic range-adaptive WPT which is compact and low-power consuming, using microcontroller unit (MCU), MEMS switch, digital capacitors, and Bluetooth module. Therefore, it is well-suited for the practical applications. In the proposed WPT, MCU measures the received power at Rx and runs the algorithm to find the optimum loop and capacitances for impedance matching, so that the complex impedance measurement circuits and personal computer can be removed. The same loop and capacitance as those in Rx are selected in Tx via Bluetooth Low Energy (BLE) communications. In addition, MEMS SP4T switch allows low-loss and fast loop switching with almost no power consumption, compared with conventional relay switches. The varactor in the TMC is replaced with digital capacitors which can be easily controlled by digital signals of MCU. In Section 2, the details of the proposed system are explained together with the design considerations. The measured performance is presented in Section 3. 2. Proposed WPT System 2.1. System Operation Figure 1 shows the block diagram of the proposed WPT system in this work, where four loops and single coil were used at the respective Tx and Rx. The operation frequency f 0 was 13.56 MHz. The loops had different diameters, and thus different mutual inductance with coil. During the operation, one of four loops was connected by the SP4T switch, and the impedance was matched by TMC, to provide the best efficiency with the distance between Tx and Rx. All of these procedures were automatically controlled by the MCUs, which are placed in both Tx and Rx. The MCU in Rx sets up wireless connection between the central and peripheral BLE modules to communicate the information on control voltages of the switches and TMCs. Then, both MCUs connect one of the loops (the same loop in Rx and Tx) and tune the TMC (the same value in Rx and Tx), and the Rx MCU measures the rectified voltage and computes efficiency. Finally, the Rx MCU finds and sets the optimum loop and TMC that generate maximum efficiency.

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Tx Tx

MCU MCU 4Ch 4Ch

2Ch 2Ch

,,

13.56MHz 13.56MHz Tunable Tunable Matching Matching Circuit Circuit

Signal Signal generator generator

SP4T SP4T Switch Switch

Bluetooth Bluetooth (peripheral) (peripheral)

Coil Coil

Loop1-4 Loop1-4

Control Control

Power Power Rx Rx

Tunable Tunable Matching Matching Circuit Circuit

Rectifier Rectifier

2Ch 2Ch

SP4T SP4T Switch Switch

Bluetooth Bluetooth (central) (central)

Coil Coil

Loop1-4 Loop1-4

,, 4Ch 4Ch

MCU MCU

81 81 82 82

Figure Figure 1. 1. Block Block diagram diagram of of the the proposed proposed wireless wireless power power transfer transfer (WPT) (WPT) system. system. Figure 1. Block diagram of the proposed wireless power transfer (WPT) system.

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Figure Figure 22 shows shows the the analog analog circuit circuit part part of of the the proposed proposed WPT WPT system system which which consists consists of of aa coil coil and and Figure 2 shows the analog circuit part of the proposed WPT system which consists of a coil and four for four loops loops at at Tx Tx and and Rx, Rx, respectively. respectively. As As shown shown in in this this figure, figure, Tx Tx and and Rx Rx were were symmetric symmetric except exceptfour for loops at Tx and shown in this figure, Tx capacitors and Rx were(𝐶 except for the rectifier the in Rx. That coils, loops, series loop ), SP4T switches, and TMCs the rectifier rectifier in Rx, Rx.respectively. That is, is, the theAs coils, loops, series loop capacitors (𝐶symmetric ), SP4T switches, and TMCs 𝑙,𝑖 𝑙,𝑖 in Rx. That is, the coils, loops, series loop capacitors SP4T that switches, and TMCs capacitor l,i ), Note (digital capacitor banks) were identical in Rx. the four loops only (digital capacitor banks) were identical in Tx Tx and and(C Rx. Note that one one of of the four(digital loops was was only banks) were identical in Tx and Rx. Note that one of the four loops was only connected by an SP4T connected shunt connected by by an an SP4T SP4T switch switch during during the the operation. operation. TMC TMC was was simply simply designed designed using using aa switch shunt during the operation. TMC was simply using a shunt capacitance (digital capacitorvalues. bank) by capacitance (digital bank) by modifying the capacitances from the It capacitance (digital capacitor capacitor bank) by designed modifying the loop loop capacitances from the resonant resonant values. It modifying the loop capacitances from the resonant values. It will be explained later in more detail. will will be be explained explained later later in in more more detail. detail. 𝑘𝑘12 12

Tx Tx

𝑘𝑘23 23

Rx Rx

SP4T SP4T switch switch

𝑐𝑙𝑘 𝑐𝑙𝑘 𝑎𝑎 𝑎𝑎

𝐶𝐶00

𝐶𝐶11 … …

Identical Identical circuit circuit to to Tx Tx side side ((𝐶𝐶𝑙,𝑖 , SP4T , SP4T 𝑙,𝑖 switch, switch, digital digital capacitor capacitor bank) bank)

𝐶𝐶10 10

𝐶𝐶𝑙,𝑖𝑙,𝑖

Schottky Schottky diode diode

𝑅𝑅𝐿𝐿 𝐶𝐶

𝑅𝑅

Rectifier Rectifier

𝑠𝑠 ,𝑖,𝑖

Digital Digital capacitor capacitor bank bank

𝑃𝑃𝐿𝐿

𝑠𝑠 ,𝑖,𝑖

(digital) (digital)

Four Four loops loops Tx Tx coil coil

90 90

12 12

𝑐𝑙𝑘 𝑐𝑙𝑘

𝑎𝑎 𝑎𝑎

Rx Rx coil coil 23 23

33

= =

12 12

91 91

Figure Figure 2. 2. Analog Analog circuit circuit part part of of the the proposed proposed WPT WPT system. system. Figure 2. Analog circuit part of the proposed WPT system.

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Prior Prior to to discussing discussing multi-loop multi-loop technique, technique, aa conventional conventional wireless wireless power power transfer transfer system system using using aa Prior discussing multi-loop conventional power system using single loop analyzed using equivalent given Figure Coils are with single looptowas was analyzed using an an technique, equivalentacircuit circuit given in inwireless Figure 3. 3. Coilstransfer are represented represented with a single loop was analyzed using an equivalent circuit given in Figure 3. Coils are represented with self-inductance self-inductance 𝐿𝐿𝑐𝑐,, self-capacitance self-capacitance 𝐶𝐶𝑐𝑐,, and and resistance resistance 𝑅 𝑅𝑐𝑐.. They They were were designed designed to to resonate resonate at at 𝑓𝑓00 and and self-inductance Lc , self-capacitance designed f 0 and c , and resistance c . They were to (Q) single-turn loop represented with 𝐿𝐿𝑙𝑙 and .𝑙 . 𝐶𝐶𝑙𝑙 is to provide provide high high quality quality (Q) factor. factor. A AC single-turn loop is is R represented with andto𝑅 𝑅𝑙resonate is an anatexternal external to provideincluding high quality (Q) factor. A single-turn is represented with Ll and Rl .𝐿𝐿Clat is 𝑓 an.. external capacitor aa self-capacitance of and to with capacitor including self-capacitance of the the loop looploop and determined determined to resonate resonate with 𝑘12 and 𝑙𝑙 at 𝑓 00 𝑘 12 and capacitor including a self-capacitance of the loop and determined to resonate with L at f . k and 0 12 l 𝑘𝑘23 are the the coupling coupling coefficients coefficients between between coil coil and and loop loop and and between between Tx Tx and and Rx Rx coils, coils, respectively. respectively. 23 are kPower the coupling coefficients and loop(𝑍 between Tx and Rx coils, respectively. 23 are transfer Power efficiency (PTE) and impedance ) of the magnetic resonant WPT at transfer efficiency (PTE) ηηbetween and input inputcoil impedance (𝑍and ) of the magnetic resonant WPT at 𝑓𝑓00 are are 𝑖𝑛 𝑖𝑛 Power transfer efficiency (PTE) η and input impedance (Z ) of the magnetic resonant WPT at f are 0 in given by (1) and (2), respectively [5]: given by (1) and (2), respectively [5]: given by (1) and (2), respectively [5]: 𝑃𝑃

2𝑘 2𝑘 𝑘𝑘22 𝑄 𝑄𝑄 𝑄22

22

23 23 12 12 11 22 𝜂𝜂 = ]] ,, #2 = 𝑃 𝐿𝐿" = = [[ 22 𝑄 𝑄 )22+𝑘 22 𝑄 22 𝑃𝑎𝑣𝑠 (1+𝑘 𝑎𝑣𝑠 (1+𝑘12 𝑄112 𝑄22 ) +𝑘23 12 2k23 k12 Q1 Q2322𝑄22 PL η= = ,
222Q Q 2 2+ 2 Q2 Pavs 2 + 111+ k 𝑄 𝑄 (𝑄 𝑄 𝑘 1) 𝑄 𝑄22k𝑘𝑘1212 (𝑄 𝑄 𝑘 + 1) 2 1 12 11 22 12 12 23 2 𝑍𝑍𝑖𝑛 =𝑅 𝑅00 22 22 𝑖𝑛 = 22 𝑄 + 𝑄 𝑄 𝑘 𝑄22𝑘𝑘23 + 𝑄 𝑄 𝑘 + 11 1 2 1 2 12 23 12 +

100 100 101 101

(1) (1)

(1) (2) (2)

where where 𝑄 𝑄11 and and 𝑄 𝑄22 are are Q-factors Q-factors of of loop loop and and coil coil at at 𝑓𝑓00,, respectively. respectively. 𝑃𝑃𝑎𝑣𝑠 and 𝑃𝑃𝐿𝐿 are are the the available available 𝑎𝑣𝑠 and power power from from Tx Tx signal signal generator generator with with source source resistance resistance 𝑅 𝑅00 (50 (50 Ω), Ω), and and the the power power delivered delivered to to the the

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Q1 Q2 k2 Q1 Q2 k212 + 1 Zin = R0 2 2 12 Q2 k23 + Q1 Q2 k212 + 1


(2)

where Q1 and Q2 are Q-factors of loop and coil at f 0 , respectively. Pavs and PL are the available power 2018, 11, x FOR PEERwith REVIEW 12 fromEnergies Tx signal generator source resistance R0 (50 Ω), and the power delivered to the4 of matched load (R0 ) in Rx, respectively. Therefore, the PTE η in (1) is equal to transducer power gain (|S21 |2 ) of 102 matched load (𝑅 ) in Rx, respectively. Therefore, the PTE η in (1) is equal to transducer power gain the 2-port2 network 0shown in Figure 3a with reference impedance R0 [13,14]. In this paper, the term 103 (|𝑆 | ) of the 2-port network shown in Figure 3a with reference impedance 𝑅 [13,14]. In this paper, PTE or21efficiency is used to evaluate the performance of the wireless power0transfer system. Note that 104 the term PTE or efficiency is used to evaluate the performance of the wireless power transfer system. k is proportional to 1/d323 , where d23 3is the distance between the coils in Tx and Rx [14,15]. From (1), 105 23 Note that 𝑘23 is proportional to 1/ 23 , where 23 is the distance between the coils in Tx and Rx be found strongthat function the distance, , and there exists an optimum k23,opt or 23the 106it can[14,15]. Fromthat (1), PTE it canisbea found PTE is of a strong functiondof distance, 23 , and there exists an given by (3), providing a maximum PTE, at which the input impedance (Zin ) isimpedance matched to R0 107d23,opt optimum 𝑘23, 𝑝 or 23, 𝑝 given by (3), providing a maximum PTE, at which the input 108(system (𝑍𝑖𝑛 )reference is matchedimpedance): to 𝑅0 (system reference impedance): q k23,opt = 𝑘23,

𝑝

k4 Q2 − 1/Q22

12 21 = √𝑘12 𝑄1 − 1/𝑄22

(3)

(3)

Figure 3b shows the simulated PTE using the parameters of the loop and coil fabricated in this 109work which Figure shows simulated using the1parameters of the and coil with fabricated in this are3b given inthe Tables 1 andPTE 2. The loop was used in thisloop simulation a capacitance Cl 110 work which are given in Tables 1 and 2. The loop 1 was used in this simulation with a capacitance 𝐶𝑙 of 120.7 pF resonating with Ll = 1.141 µH at f 0 = 13.56 MHz. As shown in this figure, the conventional 111 of 120.7 pF resonating with 𝐿𝑙 = 1.141 μH at 𝑓0 = 13.56 MHz. As shown in this figure, the WPT system with a single loop (loop 1) showed a very high PTE of 87.0% at d = 32.5 cm which was an 112 conventional WPT system with a single loop (loop 1) showed a very high PTE 23 of 87.0% at 23 = 32.5 distance (d23,opt ) in this case. (However, PTE rapidly decreased if d23 deviated from 32.5 cm. 113optimum cm which was an optimum distance 23, 𝑝 ) in this case. However, PTE rapidly decreased if 23 Figure 4from shows the simulated Re {Zin } and Im {Zin } as a function of the distance d23 of 114 deviated 32.5 cm. WPTthe system (a). ReIt {𝑍is𝑖𝑛 }shown this thatofZthe matched to R at 115the conventional Figure 4 shows simulated and Imin {𝑍𝑖𝑛 } as figure a function distance in was 23 of the 0 d23,opt = 32.5 cm,system and varied with the in distance, as that also 𝑍implied by (1) and (2). 116d23 =conventional WPT (a). It is shown this figure to 𝑅 𝑖𝑛 was matched 0 atTherefore, 23 = 23, in 𝑝 order 117to achieve = 32.5 cm, andPTE, varied withcan the be distance, as also implied and (2).be Therefore, order to achieve high TMC designed such that by Zin(1)should matchedinto R0 as d23 changes. 118However, high PTE, can be designed such that 𝑍𝑖𝑛showed should very be matched to 𝑅0 as in However, 23 Zchanges. the TMC conventional single-loop WPT wide variation in as d23 changed from 11910 tothe conventional single-loop WPT showed very wide variation in 𝑍 as changed from 10 to 100 𝑖𝑛 from 23 0.0 to −j85.6 Ω. 100 cm; real part from 4.5 to 760.4 Ω, and the imaginary part Therefore, 120 cm; real part from 4.5 to 760.4 Ω, and the imaginary part from 0.0 to −j85.6 Ω. Therefore, complex complex TMC is required to transform widely-varying Zin to R0 [9–11]. 121 TMC is required to transform widely-varying 𝑍𝑖𝑛 to 𝑅0 [9–11].

(a)

(b) 122 123

Figure 3. A conventional single-loop WPT system: (a) Equivalent circuit; (b) Simulated efficiency with

Figure 3. A conventional single-loop WPT system: (a) Equivalent circuit; (b) Simulated efficiency with the distance ( 23 ). the distance (d23 ).

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(b)

124 125

Figure 4. Simulated input impedance 𝑍𝑖𝑛 of WPT system with the distance: (a) real part; (b) Figure 4. Simulated input impedance Zin of WPT system with the distance: (a) real part; imaginary part. (b) imaginary part.

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In order to solve this problem, the multi-loop WPT can be used based on the fact that 𝑘23, 𝑝 or In order to solve this problem, the multi-loop WPT can be used based on the fact that k23,optbeor 23, 𝑝 is a function of 𝑘12 from (3) [13]. The coupling factor 𝑘12 between loop and coil can d of kthe (3) [13].12 The between loop and coil can be adjusted by 23,opt is a 12 from adjusted byfunction changing distance orcoupling the sizefactor of thek12loop. In the multi-loop WPT, 𝑘12 was changingby theconnecting distance d12a or the size size of theloop loop.depending In the multi-loop k12 was adjusted connecting adjusted different on 23WPT, , so that there could by exist several a different size loop depending on d , so that there could exist several optimum distances providing 23 optimum distances providing impedance matches. In addition, connecting one of four loops impedanceon matches. In addition, connecting of fourasloops depending the distance depending the distance greatly reduced 𝑍𝑖𝑛 one variation shown in Figureon 4, where Re {𝑍𝑖𝑛greatly } and reduced Zin variation in Figure 4, where {Zin } varied from 4.5 to 67.5 Ω, in } and Im while Im {𝑍𝑖𝑛 } varied from 4.5 as to shown 67.5 Ω, and from 0.2 to −1.9Re Ω, {Z respectively, changed from 10 to 23 and from 0.2 to − 1.9 Ω, respectively, while d changed from 10 to 100 cm. Therefore, the combination 23 100 cm. Therefore, the combination of the multi-loop technique and tunable matching can maintain of the multi-loop technique tunable matching canvaries maintain high PTEimpedance over a wide range ofThe the high PTE over a wide range and of the distance. 𝑍𝑖𝑛 also as the load changes. distance. Zshows varies as variation the load impedance The simulation shows thatbythis variation due in also that simulation this due to the changes. load impedance can be mitigated using multiple to the load impedance can be mitigated by using multiple loops instead of a single loop. loops instead of a single loop.

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146 147

2.2. Tunable Matching Circuit 2.2. Tunable Matching Circuit As stated earlier, the multi-loop technique can greatly reduce the Z variation with respect to As stated earlier, the multi-loop technique can greatly reduce the 𝑍𝑖𝑛invariation with respect to d23 , which simplifies TMC design. In the conventional single-loop WPT system, the loop capacitance 23 , which simplifies TMC design. In the conventional single-loop WPT system, the loop capacitance C was determined to resonate at f with the loop inductance L . In this work, we used higher loop 𝐶𝑙 l was determined to resonate at 𝑓00 with the loop inductance 𝐿𝑙l. In this work, we used higher loop capacitance(𝐶 (Cl,i) )(as (aslisted listedin inTable Table 3) 3) so so that that input input impedance impedance became capacitance became inductive, inductive,as asshown shownin inFigure Figure5, 𝑙,𝑖 and thus impedance match could be simply fulfilled by using a single shunt capacitor only. Figure 5 5, and thus impedance match could be simply fulfilled by using a single shunt capacitor only. Figure system on on aa Smith Smith chart chart at atdistances distances40, 40,45, 45,and and50 50cm cmwhen when 5shows showsthe the input input impedance impedance of of the the WPT WPT system the loop 2 is selected. As illustrated, a simple shunt capacitor (173, 120, and 55 pF, respectively) is the loop 2 is selected. As illustrated, a simple shunt capacitor (173, 120, and 55 pF, respectively) is enoughtototransform transforminput inputimpedances impedancestoto 𝑅R0.. enough 0 A tunable capacitor can be implemented by varactor or digital capacitor bank. The varactor exhibits a relatively small capacitance variation ratio (