A Wireless Charging and Near-field Communication Combination Module

for Mobile Applications

Hiroki Shibuya, Tatsuaki Tsukuda, Hiroko Suzuki,

Tadashi Shimizu, Masahiro Dobashi, Shinji Nishizono, Mikio Baba, Hideki Sasaki and Katsushi Terajima

Package and Test Technology Division, Renesas Electronics Corporation

1753 Shimonumabe, Nakahara-ku, Kawasaki-shi, Kanagawa 211-8668, Japan

E-mail: hiroki.shibuya.ak@renesas.com

Abstract

This paper presents the first demonstration of an ultra-

miniature module combining with 13.56/6.78-MHz wireless

charging receiving functions and types-A/B/F near-field

communication (NFC) functions. In order to be able to embed

this module into mobile terminals, the electrical and thermal

designs are optimized and then the size is 14 x 26 x 1.86 mm.

A simulation bench for verifying the efficiency of the wireless

charging, performance of NFC by a wireless charging antenna,

thermal design for embedding the module into a mobile

terminal and EMI reduction of the wireless charging system

are described.

Keywords: wireless power transfer, wireless charging, near-

field communication, NFC, module

Introduction

Wireless power transfer or wireless charging is one of hot

topics in the technical fields of both RF and power electronics.

Existing wireless charging systems, such as Qi standardized by

Wireless Power Consortium (WPC), for mobile applications

uses inductive coupling with the frequency range from 100

kHz to 200 kHz. Since the frequency of the wireless power

transfer is lower than 200 kHz, the antenna is surely bigger

than the one for ex. wireless LAN and Bluetooth operated at

2.45 GHz. It generates a negative impact for downsizing

mobile terminals. Also, heat generation by several hundred-

kHz wireless charging in mobile terminals is typically larger

than heat generation of wire charging such as USB, because

the coil-type antenna generates heavy heat as well as power

management IC and the peripheral components generate heat.

The heat has the performance of Lithium ion batteries

degraded. Furthermore, strong magnetic field used by wireless

power transfer causes Electromagnetic Interference (EMI)

problems. EMI noise interferes with TV and broadcast and

decreases the performances of wireless communications in the

terminals.

In order to solve these problems, 13.56-MHz[1][2] or

6.78-MHz[3][4] wireless charging has been investigated. This

choice can downsize the antenna for wireless charging and can

decrease heat generation of the antenna. In addition, to realize

a safe wireless charging system, wireless communication

function suitable for wireless charging has been investigated.

This paper presents the first demonstration of an ultra-

miniature module combining with 13.56/6.78-MHz wireless

charging receiving functions and types-A/B/F near-field

communication (NFC) functions. The biggest advantage of the

system in the packaging point of view is to use the same

antenna for NFC and 13.56/6.78-MHz wireless power

transfer. This wireless communication can exchange

information on the temperature of the system for realizing a

safe system. This module includes a power management IC for

wireless charging and a microcontroller for NFC. The module

is also capable of 5-watt power transfer with small and thin

size (14 x 26 x 1.86 mm).

In order to be able to embed this module into mobile

terminals, a simulation bench for verifying the efficiency of

the wireless charging, performance of NFC by a wireless

charging antenna, thermal design for embedding the module

into a mobile terminal and EMI reduction of the wireless

charging system are described in this paper.

System block diagram and receiver module

Figure 1 shows a block diagram of a system combining

with wireless charging and NFC. The left side a transmitter

(Tx) module and the right side a receiver (Rx) one. The Tx

module is not necessary to miniaturize the size, because the

module with an antenna is implemented into a stand or box

putting a mobile terminal on. On the other hand, the Rx

module is strongly requested to miniaturize the size, because

the module with an antenna is embedded into a mobile

terminal.

Fig 1. A block diagram of a wireless charging system

Fig 2. NFC wireless charging receiver module

978-1-4799-2407-3/14/$31.00 ©2014 IEEE 763 2014 Electronic Components & Technology Conference

Figure 2 shows the prototype of wireless charging receiver

module we designed and fabricated. The module has both of

13.56/6.78-MHz wireless charging receiver function and

types-A/B/F NFC card mode function. The module is

composed of a power management IC, a choke coil, a rectifier,

ripple filters, a microcontroller, a crystal unit, passive

elements and some others. The size of the module is 14 x 26 x

1.86 mm. This size was designed for a mobile terminal such as

a smart phone. For example, the module can be embedded into

a back cover of a smart phone. This module can charge a

battery by connecting an antenna and a battery to the module.

Interface pads to a system board of a mobile terminal are

allocated backside of the module for mounting the module on

the system board as LGA.

Simulation bench for verifying power transfer efficiency

In order to design a high efficient wireless charging

system, at first, a simulation bench was set. Figure 3 shows a

rough sketch of a wireless charging system we investigated.

After extracting the impedance models of power transmitter

IC output and power receiver IC input, mainly, the simulation

models of driver circuit, Tx antenna, Rx antenna and rectifier

circuit were optimized for improving the efficiency of the

wireless power transfer.

Fig 3. A rough sketch of a wireless charging system

Fig 4. A simulation bench of a wireless charging system

Figure 4 shows a detailed simulation bench of the wireless

charging system we investigated. The simulation model of a

MOSFET is a behavior model. The balun model is a passive

element. The models of the filter, air-core coils, are consisted

of passive elements, but in order to improve the accuracy of

the models, the parameters of the models was extracted from S

parameters by using 3D electromagnetic simulator. The

models of Tx and Rx antenna were expressed as a black box

model of S parameters. The model was extracted by a 3D

electromagnetic field simulator. The rectifier model is

consisted of behavior models of diodes and a RC circuit

expressing to input impedance of the power receiver IC.

In order to realize a high efficient wireless charging

system, Tx and Rx antennas are optimized by using an

electromagnetic field simulator. The antennas were

constructed by three layers; an antenna board, a magnetic

sheet and a metal plate. The antenna board is a both-side

printed circuit board. The antennas are basically loop antennas

resonated at the frequency of wireless charging and NFC.

The magnetic sheet was utilized as the path of magnetic field

flux in between the loop antenna and the metal plate. The

metal plate was applied for stabilizing the antenna's Q (ωL/R)

value regardless of the surrounding environment. The Rx

antenna is a small one-coil antenna in order to be able to

embed this antenna into mobile terminal. On the other hand, a

Tx antennas have a larger size and a three-coil configuration.

One of them is a booster antenna for the resonance.

A higher Q antenna is preferable for a wireless charging.

However, it is not suitable for NFC. Table 1 shows

efficiencies of wireless charging in two cases. The size of Tx

antenna is 116 x 66 mm, and the one of Rx antenna is 44 x 31

mm. The Q factors of Tx antennas were the same, however,

the Q factors of the Rx antennas were different. The Q factor

of the Rx antenna in the case 1 was 39.1. It is an example

preferable for NFC, however, the efficiency was 36.0%. The

Q factor of the Rx antenna in the case 2 was 76.1 by

optimizing the shape of the antenna pattern. The efficiency

was 61.2%. It was reasonable performance for wireless

charging system. However, it was not good for NFC, because

the distance of the communication shortened.

Table 1. Q factors vs. power transfer efficiency@6.78MHz

Q of Tx

Antenna

Q of Rx

Antenna

Measured

Efficiency (%)

Case 1 161.2 39.6 36.0

Case 2 161.2 76.1 61.2

The accuracy of the simulation bench shown in Fig 4 was

confirmed by the correlation to the measurement results. In

this paper, the only data for 6.78-MHz wireless charging was

showed, because there are not clear differences from data for

13.56-MHz wireless charging. In the case 2, the simulated

efficiency was 64.7%, while the measured efficiency was

61.2%.

Performance of NFC with a wireless charging antenna

Figures 5, 6 and 7 show the measurement results of three

types of NFC, types A, B and F using the Rx antenna in the

case 1. The operational frequency of NFC is 13.56 MHz. The

Q factor of the Rx antenna at 13.56 MHz was approximately

58.3. The Q factor is approximately 5 times higher than that of

a typical NFC antenna. These measured results of Figures 5-7

shows that all types of NFC succeed in the distance from 0 to

24 mm. This distance is enough for general use cases of NFC.

Therefore, the results show that NFC functions of this module

can be achieved by using a wireless charging antenna.

There are no data of the measurement results using the Rx

antenna of the case 2. The Q factor of the antenna is

approximately two times higher than that of the antenna in

case 1. Through our experience, the NFC performance using

the antenna of the case 2 is predicted to degrade. The

remaining work is to confirm the trade-off between

performance of NFC and efficiency of wireless charging and

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to clarify the design rule of the antenna suitable for both of

wireless charging and NFC.

Fig 5. NFC performance for Type-A

Fig 6. NFC performance for Type-B

Fig 7. NFC performance for Type-F

Thermal design for embedding the module into a mobile

terminal

In order to reduce the module dimentions, in general, small

and thin components are densely mounted on the module.

Smaller and thiner components however generate larger heat

due to increase of the thermal resistance. In addition, the

smaller module has the smaller diffusion area of the heat.

Therefore, the reduction of the demensions raises the module

temperature. On the other hand, almost all mobile terminals

have a battery. A battery hates high temperature because high

temprature causes degradation of the battery performance. In

addition, the surface of a housing covering a mobile terminal

should keep lower temprature because of avoiding skin burn.

Thus, it is important to suppress increase of the temperature

caused by downsizing of the module. In this demonstration,

the target temperatures of the prototype module were lower

than 60 degrees Celsius at the surface of components and

around 45 degrees Celsius at the surface of the housing.

Fig 8. Thermal simulation model of the module embedded

into a mobile terminal

Fig 9. A cross section of the thermal simulation model

Figures 8 and 9 show thermal simulation models of the

module embedded into a mobile terminal. The module was

mounted on a mother board in the housing made of plastic. To

control the temperature on the surface of plastic housing

cover, the air gap between heating components and the plastic

cover was changed. The heating components of this simulation

were a power receiver IC and an inductor of the DC-DC

converter. The height of the inductor is higher than that of a

power receiver IC. Then, the definition of the air gap is the

distance d between the top of the inductor and the bottom of

the plastic cover. In figure 9, the left component the inductor

and the right the power receiver IC.

Detailed parameters of the thermal simulation model are

followings. The module substrate has six metal layers. The

dimension is 14 x 26 x 0.46 mm. The mother board has six

metal layers. The dimention is 28 x 54 x 0.40 mm. Ambient

temperature is 25 degree Celsius. Thermal conductivity of the

plastic is 0.3 W/m･K. The estimated charging power is 5

watts, however, it is not the total power to provide the heating

components. The total power consumed at the heating

components was estimated by correlation with masurement

data.

Figure 10 shows a thermal simulation result for the surface

of the heating components. Heat from the inductor and the IC

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spreads into the mother board and the body of the mobile

terminal through the module. The maximum temperature in

this situation was 73.7 degree Celsius at the surface of the

inductor. On the other hand, Figure 11 shows a thermal

simulation result for the surface of the plastic housing cover.

The maximum temperature on the surface of the housing cover

was 45.2 degree Celsius. It almost accepted the target

temperature of 45 degree Celsius.

Fig 10. Thermal simulation result on the components

Fig 11. Thermal simulation result on the housing

In order to confirm the simulation results, the temperature

of the heating components was measured. Figure 12 shows a

picture of the module mounting on the board assumed as a

mother board. The temperature was measured with a

thermistor by mounting it on the heating components.

Fig 12. The module for thermal measurement

Table 2 Temperature on the surface of the inductor

Conditions Temperature (degree C)

Simulation 73.7

Measurement 70.6

As mentioned before, in this investigation, the inductor

generated the highest heat. Table 2 shows the temperatures of

simulation and measurement on the surface of the inductor.

The measurement result, 70.6 degree C, was lower than

simulation result, 73.7 degree C. This comparison data shows

that the thermal simulation provides reasonable data for

thermal design.

Fig 13. Thermal simulation results of the cover temp.

Figure 13 shows thermal simulation results of the cover

temperature. In order to realize that the temperature of the

cover surface is 45 degree C and less, these simulations were

achieved. The estimated receiving powers by wireless

charging were 5, 3 and 1 watt. In the case of 1 watt, it was

unnecessary to keep an air gap between the top of the heating

components and the bottom of the plastic cover. In the cases

of 3 and 5 watts, however, it was necessary to keep the air

gap. 0.75-mm air gap was enough for 3-watt wireless

charging, however, it was not enough for 5-watt wireless

charging. In the case of 5-watt wireless charging, 1.26-mm air

gap was necessary for realizing that the surface temperature of

the cover was 45 degree C as shown in Fig 11.

Table 3 Meausred temperatures depending on module

board condition @5-watt wireless charging

# of Layer # of Via Max. Temp.(degree C).

Case 1 6 Typical 70.6

Case 2 8 Rich 52.6

Table 3 shows measured maximum temperatures of the

heating components depending on the module board

conditions. The case 1 is the module board condition

described before. On the other hand, the case 2 is the different

board condition for aggressively decreasing the temperature.

Although the thickness of the module boards are almost the

same, the numbers of the layer and the via in the module

boards are different from each other. Larger numbers of the

layer and the via decreases the maximum temperature of the

heating components mounting on the module board. This

result shows improvement of heat spread for the board module

is one of good options for thermal design.

EMI reduction of the wireless charging system

Wireless charging systems necessarily generate strong

electromagnetic radiation and cause EMI (Electro-Magnetic

Interference) problem with other wireless communications,

because power transfer from a charger to a terminal utilizes

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electromagnetic coupling with Tx and Rx coil antennas. In

order to reduce radiated emissions, sources and mechanism of

EMI on wireless charging system we designed were

investigated.

In order to find the source of EMI, near magnetic field

distributions were measured by using a small magnetic field

probe. Figure 14 shows measured results of near magnetic

field distribution over module boards of the wireless charging

receiver. These modules are not exactly the same as the

module shown in Figure 2. The basic circuit of the wireless

charging receiver module is the same as that of the previous

module. The sizes of the modules are however larger than the

previous one for inserting several EMI filters into the circuit.

The measured frequency of near field is 142.38 MHz. It is

twenty first harmonics of 6.78 MHz. In the frequency, radiated

emission from the system was larger than the other

frequencies.

Figure 14 (a) shows measured near magnetic field

distribution of the module without EMI filters and Figure 14

(b) shows measured near field distribution with EMI filters.

The arrows show direction of the magnetic field. The EMI

filters inserted between the antenna pad and the rectifier

shown in Fig 14(b). Strength of the magnetic field above the

module was reduced. This experiment demonstrates that a

rectifier is one of large EMI sources in a wireless charging

system.

(a) without EMI filters

(b) with EMI filters

Fig 14. Measured near field distributions@142.38 MHz

Figure 15 shows measured results of far-field emissions

radiated from the wireless charging system we designed. The

distance of the measurement is 10 m accorrding to CISPR 32.

At approximetely 150 MHz of the horizontal field, the

emission slightly exceeded the limitation of CISPR32.

However, at the wide range of both holizontal and vertical

fields, the emissions were lower than the limitations. This

measurement demonstrates that the wireless charging system

we designed has highly potential to pass EMI regulation of

CISPR32.

(a) Horizontal far field

(b) Vertical far field

Fig. 15 measured results of 10-m radiated emissions

Conclusions

This paper presented an ultra-miniature module combining

with 13.56/6.78-MHz wireless charging receiving functions

and types-A/B/F near-field communication (NFC) functions.

In order to be able to embed this module into mobile

terminals, the electrical and thermal designs were optimized

and then the size was 14 x 26 x 1.86 mm. The simulation

bench we set could verify the efficiency of a wireless

charging. The antenna we designed for wireless charging

could function as NFC antenna. Thermal simulations and

measurements showed that the temperature of surface on a

mobile terminal could control by designing an air gap between

the cover and components. Finally, measurement results of

near fields and far fields demonstrated that the wireless

charging system we designed had a potential to pass EMI

regulation of CISPR32.

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138.

2. M. Fu, et al., "A 13.56 MHz Wireless Power Transfer

System Without Impedance Matching Networks," in Proc.

IEEE Wireless Power Transfer Conf. (WPTC), Perugia,

Italy, May 15-16, 2013, pp.222-225.

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Coupling for Wireless Power Transfer to Multiple

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