A Study on Wireless Power Transfer of Small

Device using Multi-Layer Coil

Juwan Kim, Wonshil Kang, Hyunchul Ku

Electric & Communication Engineering, Konkuk University, 120 Neung-doong ro, Gwangjin-Gu, Seoul, Republic of Korea

Abstract - In this paper, we propose a multi-layer coil (MLC)

for wireless charging for small electronic devices(SED) such as implantable medical devices(IMD) and wearable devices.

Various types of coils are being studied to increase the efficiency of wireless power transfer (WPT). However, the sizes and shapes of WPT coils for SED are limited, and also it is

difficult to apply the conventional spiral and helical coils to SED. Therefore, in this paper, we propose a multi-layer coil and equivalent model with combination of spiral and helical coil. In

order to verify the effectiveness of MLC, the simulation results of the proposed equivalent model are compared with the experimental results. As a result, the average power transfer

efficiency (PTE) of SC (Spiral Coil) and HC (Helical Coil) was 6.1% and 7.6% in 10mm~25mm transfer distance, respectively, and on the other hand, the average PTE of MLC was 48.4%.

Index Terms — small electronic device (SED), wireless power transfer(WPT), multi-layer coil

1. Introduction

Recently, wireless power transfer (WPT) has emerged as a

method of supplying energy to small electronic devices

(SED) such as an implantable medical device (IMD), smart

watch and so on. Since the size of the coil for small devices

WPT is limited and need to small, the coil has been

developed through the structural change of self-inductance of

coil. [1], [2]

Therefore, in this paper, we design and analyze mm-sized

multi-layer magnetic resonant coil for WPT to increase self-

inductance, and propose MLC that can increase transfer

efficiency and distance in limited space. To verify, we

propose an equivalent model of MLC and compare the

simulation results of model with experimental results.

2. Multi-Layer Coil Modeling

(1) Modeling of Multi-Layer Coil

The example model of MLC and equivalent model of

MLC can be represented as shown in Fig. 2, 3 and Fig. 4,

respectively. The SC of each layer are influenced by the

mutual inductance M. To find out the self-inductance of a

MLC, we need to first determine the self-inductance iL of a

single layer coil. Since, each layer of MLC is a SC, its L can

be defined equation (1). Mutual inductance of MLC depends

on the distance between the coils. For 2-layer MLC, 1,2M

has 116nH. In case of 3-layer MLC, 1,2M and 2,3M have the

same value of 170nH.

Fig. 2. Example model of MLC

In Fig.3, piR is the parasitic resistance and layerR is the

parasitic resistance between the layers. ,i jC consists of the

area between the layers. The capacitance is inversely

proportional to the distance O and is proportional to the area.

,i jC can be obtained using equation (2). The proposed MLC

can be represented as , 1, 1i j i jC C because the

characteristics of each layer are the same. piC , on the other

hand, is the parasitic capacitance of the spiral coil of each

layer, and piC can be ignored because the value is very small

( 1,2,3i and 2,3j ).

In equivalent model of Fig.3, the total impedance MLCZ is

calculated. In a simplified model with neglected values

( 1,3, layer piR C and C ) removed, where V is the input voltage,

and 1 2 V and V is the node voltages. If KCL is applied to the

equivalent model, the node voltages 1 2 V and V can be

expressed by equation (4), where RL pi iZ R j L

1,2 2,3C C C .and 1/CZ j C .

Fig.3. Equivalent circuit of MLC in 3-layer MLC

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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2

1

2 22

2 2

( 3 )( ) 2

RL C C

RL C RL C RL RL C C

Z Z ZV V

V Z Z Z Z Z Z Z Z

(1)

The input current is I can be represented as:

1 2( ) ( )

RL C

V V V VI

Z Z

(2)

Since the total impedance of circuit is /SZ V I , the real

part of SZ means the sum of parasitic resistance and the

imaginary part means the sum of total inductance and the

parasitic capacitance of MLC.

3. Simulation and Measurement

Fig.4 shows WPT circuit diagram and Table 1 sum-

marizes the overall system information and model

parameters.

Fig.4. Circuit of WPT; TX is series resonance, RX is parallel

In Fig.4, rt rrC and C is resonance capacitance of trans-

mitter coil and receiver coil. ps psR and C are sum of

parasitic resistance and capacitance, respectively.

The simulations use HFSS software, and the designed

multi-layer is shown in Fig.5. In Table1, equation (1) is used

for the simulation data of inductance L, data for R is used for

equation (4) and (5), and the efficiency of series-parallel

resonance of MLC is used for (6), (7) and (8). The operating

frequency for the experiments is 13.56MHz. Furthermore, in

the circuit shown in Fig.4, when PTE equation can be

obtained by applying the parameters (self-inductance,

parasitic resistance and resonant capacitance) of the

equivalent model of the proposed coil. I2 shows secondary

current (RX) for WPT.

TABLE 1

OVERALL SYSTEM INFORMATION & MODEL PARAMETERS

@13.56MHz Parameters

L(nH)

Diameter 10mm

Turns 4

SL 1-layer 2-layer 3-layer

MLC sim 230 680 998

mea 229 686 1003

SC mea 220

HC mea 989

psR ( ) MLC sim 0.1592 0.6862 1.3782

mea 0.1678 0.6993 1.3727

psC (pF) MLC sim 0.35

mea 0.33

Fig.5.Example of designed coils and measurement set-up

Fig.6 shows the PTE of MLC, SC and HC with distance

variation. The simulation results of the proposed MLC

equivalent model are compared with experimental results

Fig. 6. PTE according to changing distance

4. Conclusion

In this paper, we propose a magnetic resonance MLC for

WPT of SED. We verified that the proposed 10mm sized

MLC equivalent model consists of SC and HC and

experimented and verified the effectiveness of MLC in

limited space.

As a result, when the PTE simulation and the experimental

results of the proposed MLC equivalent model are compared,

the MLC results show that PTE and transfer distance are

increased compared to SC and the average PTE of HC. SC

and HC were 6.1% and 7.6% in 10mm~25mm, respectively,

but the MLC was 48.4%.

ACKNOWLEDGEMENT

This research was supported by Intelligent semi-conductor

specialist training program funded by Korea Semiconductor

Industry Association (N0001883)

This research was supported by Basic Science Research

Program through the National Research foundation of Korea

(NRF) funded by the Ministry of Education (NRF-

2017R1A5A1015596)

References

[1] Chin-Lung Yang and Lih-Yih Chiou, “Efficient Four-Coil Wireless Power Transfer for Deep Brain Stimulation”, IEEE Transactions on Microwave Theory and Techniques, vol.65, pp.2496-2507, July.2017

[2] C.Akyel, S.Babic, and S.Kincic, “New and fast procedures for calculating the mutual inductance of coaxial circular coils (circular coil-disk coil)”, IEEE Transactions on Magnetics, vol 38, pp.2367–2369, Dec.2002.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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