Wireless Power Transmitter Apparatus for Low-Voltage DC Power

Transmission Applications

C. Bambang Dwi Kuncoro

, Arvanida Feizal Permana

and Cornelia Adristi

Department of Refrigeration, Air Conditioning and Energy Engineering, National Chin-Yi University of Technology,

No. 57, Sec.2, Zhongshan Rd., Taiping Dist., Taichung City 41170, Taiwan

Department of Electrical Engineering, Faculty of Engineering, Universitas Indonesia,

Jalan. Kampus UI, Kukusan, Beji, Depok 16424, Indonesia

Keywords: Wireless Power, Low-Voltage, Magnetic Induction, Interoperability, Transmitter.

Abstract: Wireless power transfer (WPT) technology and applications have become popular in recent years. This is a

re-emerging technology that allows a convenient, simple, easy use of electrical energy delivery from a power

source to an electrical load within a medium distance in the absence of any electrical cord. This paper presents

a wireless power transmitter apparatus that has the capability to propagate wirelessly a low voltage to a load

coupled at a wireless power receiver output module. The voltage to be transmitted coming from 2.5 ‒ 3.5 V

at a maximum current of 2A DC power source. It differs from typical developed wireless power transmission

systems which usually transmit high voltage from either DC or AC power sources. The proposed apparatus

was developed applying the magnetic induction concept, working at the operating frequency of 110kHz at a

maximum transmission distance of about 10mm. Maximum power transfer is achieved at 5W with 5V

constant output voltages and the maximum output current is up to 1.4A. These specifications are suitable for

electronic device power charger applications. The experimental results show the developed apparatus has a

peak power transfer efficiency of around 70% and complies with the international standard requirement for

interoperability.

1 INTRODUCTION

Over years ago, wireless power transfer systems have

been developed by adopting electromagnetic

concepts either mutual induction or resonance

methods. Nowadays, this technology is re-emerging

and appealing to researchers and developers applying

it in many applications from electronic devices to

electrical vehicle systems.

The WPT systems enable an electrical load to

obtain electrical energy that propagates wirelessly

from energy sources over a distance (Costanzo et al.,

2014, Garnica et al., 2013). This concept confirmed

by Heinrich Hertz experiment in 1888

(wikipedia.org). His results were proof

experimentally the existence of electromagnetic

radiation for the first time. He used parabolic reflector

and induction coil to emit high-frequency electric

power generated by an oscillator over a tiny air gap.

Over years, this system has improved the overall

https://orcid.org/0000-0002-5054-2794

system efficiency, therefore various applications

adopted this technology to offer the user more

convenience and better experience.

Comparing with the conventional plug-in charger,

WPT offers a more compatible and attractive

alternative because it can recharge all electricity

operated devices using a single power source within

an average–sized room. Moreover, the contactless

feature of WPT also raises their freely and easily to

move, and reliability with low cost as it uses minimal

of insulation material and wire for the cables (Barman

et al., 2015). This technology has a prospect to be a

standard for wireless power transmission. The first

industry standard for mobile devices inductive

charging released by the Wireless Power Consortium

(WPC) (Johns, 2011), which is called “Qi”

(pronounce “chee”) (WPC, 2011).

In the literature, several wireless power transfer

works based on inductive electromagnetic coupling

have been developed and realized. In (Cao et al.,

Kuncoro, C., Permana, A. and Adristi, C.

Wireless Power Transmitter Apparatus for Low-Voltage DC Power Transmission Applications.

DOI: 10.5220/0010939500003260

In Proceedings of the 4th International Conference on Applied Science and Technology on Engineering Science (iCAST-ES 2021), pages 33-40

ISBN: 978-989-758-615-6; ISSN: 2975-8246

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

2014), the authors presented WPT system for portable

electronic device. The system is composed of

transmitter (Tx) and receiver (Rx) modules, and has

power efficiency of 65%. Tx and Rx coils were

developed using Fe-based soft magnetic composites

sheet to improve power transfer efficiency. The Tx

module is powered by 20V, 20W DC to provide more

than 5W at output power of Rx module.

A 36W WPT system for LED lighting

applications is presented by Chen et al. (Chen et al.,

2013). The Tx module powered with 34V DC to

provide 35.9W across a 30Ω load at Rx output

module. It has 82% power delivery efficiency with

the distance range of 5mm. A class E amplifier was

applied in Tx module.

Her et al. developed a WPT system that involve

the green energy concept. PIC16F1937

microcontroller was applied on Tx module to

generate 125kHz PWM (Her et al., 2012). The

developed system is used to charge a PRC Li-

Polymer 360mAh battery with power delivery

efficiency of 43.4%. Tx module is powered by 8V,

320mA DC to achieve output power of 3.73V,

298mA at Rx output module.

On chip Tx module for WPT system that comply

with Qi standard specification is presented by Berger

et al. (Berger et al., 2016). The module was proposed

to maximize the power efficiency. It is developed to

provide maximum output power at 5W for 3.3V

battery charger application. The developed chip is

characterized by three kind of peak receiver

efficiencies; 72% for passive mode rectifier control at

an output power of 1.4W, 82% for semi-active mode

rectifier control at an output power of 1.4W, and 88%

for fully active mode rectifier control at an output

power of 1.4W.

A full-bridge resonant inverter for WPT system is

presented by Galizzi et al. (Galizzi et al., 2013). The

bridge is coupled with Tx part and has various supply

in range of 2 ‒ 3 V to control total power transfer. It

has a peak efficiency of 70% and work with certified

WPT standard such as Qi standard.

Nataraj et al. present a DC‒DC WPTs simulation

model (Nataraj et al., 2017). The model was

developed using Multisim. The simulation worked

with the operating frequency range of 10Hz to 1kHz

and an Tx input signal of 5V DC. Their simulation

shows the output voltage is increased proportionally

with the load resistance.

A wireless powered sensor chip is introduced by

Yoo et al. (Yoo et al., 2010). The sensor consumes

12uW that is harvested energy surrounding using a

health monitoring chest band with 54.9% efficiency.

It works based on inductive coupling of WPT and an

Adaptive Threshold Rectifier (ATR).

A wireless power receiver with dual mode

transmission methods is presented by authors by

Satyamoorthy at el. (Satyamoorthy at el., 2014). The

receiver works in the 110 to 205 kHz (inductive

mode) and 6.78MHz (resonance mode) frequency

bands. It provides 5W output to comply with Qi

standard and achieves a peak efficiency at 84% in

inductive mode and 82% in resonant mode.

As mention in the literature above, some WPT

systems based on magnetic induction have been

developed for various applications. Some methods

also presented in order to improve and revise the

performance and functionality. In a couple of recent

years, many applications begin to change over to

WPT technology in order to transmit the power

because of some benefits offered this technology.

Although many WPT applications have been

developed, however, in some application cases, a low

voltage from DC power source needs to be transferred

wirelessly to the load, but the load needs a voltage

higher than the voltage provided by the power source.

This paper presents a wireless power transfer

apparatus which capable to transmit a low DC voltage

wirelessly to an electrical load that couples with

wireless receiver module. The voltage to be

transmitted coming from 2.5 ‒ 3.5 V at a maximum

current of 2A DC power source. Then, the receiver

can provide a stable output voltage at 5V and a

maximum current up to 1.4A. The developed

apparatus complies with the international standard

requirement for interoperability. The proposed

apparatus also differs from typical developed wireless

power transmission systems which usually transmit

high voltage from either DC or AC power source.

Some WPT applications in medical implantable also

supply a low voltage to be transmitted to the load but

the load also consumes voltage almost same as the

voltage transmitted by the transmitter.

2 MATERIALS AND METHODS

2.1 Basic Design Concept

The proposed apparatus is design based on

electromagnetic induction principle. The concept is

described in Maxwell’s equations is composed of

Ampere's circuital and Faraday's induction laws (Ban

et al., 2013). Ampere's circuital law introduces the

relation between magnetic fields and the electric

current, while the Kelvin-Stokes theorem is

iCAST-ES 2021 - International Conference on Applied Science and Technology on Engineering Science

equivalent to Ampere's circuital law and written in

either integral form or differential form. Ampere's

circuital law is a line integral of magnetic field around

an arbitrary closed curve c. The electric current passes

through a surface s which in turn bounded and

enclosed by curve c. The total current i passes through

the surface s enclosed by c is proportional to the

magnetic B field around a closed curve c (Sadiku,

2015):

𝐁.d𝐥=µ





 𝐉.d𝐬= µ



i



(1)

where µ

is the magnetic constant, and J is the free

current density.

Due to symmetry, the magnetic field lines

perform concentric circles in planes perpendicular to

the wire. The direction is the right-hand curl fingers

if the wire is wrapped by them with the thumb in

current direction. Suppose that the closed curve c is a

circle of radius r centered on straight wire. The

magnetic flux density can be expressed in line

integral as follow (Sadiku, 2015):

𝐁.d𝐥= 2πrB



= µ



𝑖

(2)



2πr

(3)

Actually, Faraday’s law describes the

phenomenon is called electromagnetic induction. An

electromotive force (emf) is induced in any closed

circuit using a time-varying magnetic flux through the

circuit. Faraday’s law of induction uses the magnetic

flux Ф through a hypothetical surface s whose

boundary is a wire loop l. The magnetic flux is

defined by a surface integral (Hayt et al., 2012):

Ф= 𝐁.d𝐬

𝒔

(4)

The induced emf ε is proportional to the rate of

change of the magnetic flux (Hayt et al., 2012):

ε= −

dФ

(5)

Lenz’s law expresses the direction of the induced

current caused by the emf, which indicates that the

magnetic field produced by the induced current

opposes the original change in magnetic flux.

The magnetic field generated by one coil is

coupled with other coils. The first coil (coil

) has N

turns which carries a current i

and emits a magnetic

field vector B

. The magnetic field lines pass through

the coil

will then also pass through the coil

. The

magnetic flux through coil

due to i

is expressed by

. Thus, there will be an induced emf associated

with the changing magnetic flux in the second coil by

varying i

with time (Hayt et al., 2012):

𝜀



=−N



dФ



=−

𝐁





.d𝐀



(6)

2.2 Wireless Power Consortium (WPC)

Standard

There are three essential points which is defined by

the WPC regarding standard of inductive charging of

mobile devices; the power transmitter, the power

receiver, and the communications protocol between

the two devices (Wei et al., 2009). Low power for

wireless power transfer is defined transmitting the

power in the range of 0 to 5 W, and the distance

between the two coils typically is 5mm (Johns, 2011).

The power transmission uses two planar coils to emit

electric energy from the power transmitter to the

power receiver based on the inductive coupling

method.

The WPC was founded in 2009, and at least 80

international companies joined as members at that

time. The year the consortium was established the

first international standard on wireless power for

mobile devices of to 5W released. According the

wireless power consortium, the key low-frequency

specification for wireless power transmission is

(WPC, 2011):

• Architecture: magnetic induction

• Antenna structure: planar

• Coupling: tightly coupled

• Operating frequency: 110 – 205 kHz.

2.3 Proposed Design

The proposed WPT transmitter apparatus was

designed to transmit wirelessly 2.5‒3.5 V DC to an

electrical load that couples with a wireless receiver

module. The maximum current source is 2A provided

by a DC power source. In general, a WPT transmitter

apparatus is used to propagate high voltage from

either DC or AC power source to an electric load.

Therefore, the proposed apparatus differs from the

typical WPT application.

Wireless Power Transmitter Apparatus for Low-Voltage DC Power Transmission Applications

Overall, the proposed WPT system is shown in

Figure 1. It is composed of Tx apparatus (Figure 1(a))

and Rx apparatus (Figure 1(b)). The proposed WPT

system adopts an inductive coupling method and is

compatible with the standard wireless power

consortium. The proposed Tx apparatus was equipped

with a front-end DC‒DC and boost regulator

converter circuit. This circuit provides suitable

voltage for a frequency and oscillator circuit. A power

amplifier circuit is excited by a signal from the

frequency and oscillator circuit to generate a stable

high-power electromagnetic wave. Finally, this

electromagnetic wave is emitted by the Tx coil to the

Rx apparatus.

(a) (b)

Figure 1: Block diagram of the proposed apparatus:(a)

transmitter; (b) receiver.

A wireless Rx apparatus is also designed to

conduct a performance test of the proposed wireless

Tx apparatus. The wireless Rx apparatus main circuit

includes a rectifier, filter, and voltage regulator as

shown in Figure 1(b). It can provide a stable 5V

output voltage at a maximum output current of up to

1.4A.

3 DESIGN AND RESULTS

The apparatus prototype has been developed

according to the block diagram shown in Figure 1. It

consists of two parts; the coil and electronic circuitry.

3.1 Coil Design

In coil design of WPT system, the magnetic field is

essential because it has key role to create inductively

coupling between Tx and Rx modules. The magnetic

fields generate self-inductance of each coil and

mutual-inductance between the two coils. Several coil

parameters such as geometry, type and size of wire,

number of turns, self-inductance and mutual-

inductance are necessary to consider in coil design.

Those coil parameters can maximize coupling

coefficient between two coils.

The proposed apparatus utilizes a flat spiral air

core coil. It was chosen to maximize the Q-factor,

efficiency, power handling, and the related magnetic

field generated by the transmitter coil (Mitcheson et

al., 2004). The flat spiral air core coil self-inductance

can be described by Harold A. Wheeler’s

approximations formula (Mohan et al., 1999,

Wheeler, 1928):



30.A −11.D



(7)



+N.(W+S)

(8)

where D

is the coil inner diameter (inch), W is wire

diameter (inch), N is the turn total number, S is the

turn spacing (inch) and L is the inductance

(microHenries (µH)).

Tx coil was designed having N= 17, number of

turns is 1, inner diameter (D

) is 25mm, turn spacing

(S) is 0mm, wire diameter (W) is 0.64mm, outer

diameter (D

) is 49.89mm and the wire length (Wl) is

191.97cm. The coil inductance is 13.9µH that was

calculated using Equations 7 and 8. Due to manual

manufacturing, Tx coil is characterized by 13.7µH of

inductance (L) and an outer diameter (D

) of

49.36mm. It was built using AWG22 wire that has

small diameter (0.64mm diameter and 0.0646mm

insulating layer) and can be used for carrying a

maximum current of 7A. Figure 2(a) shows Tx coil

geometry.

(a) (b)

Figure 2: Coil: (a) transmitter; (b) receiver.

Rx coil was characterized by 14.02µH of

inductance (L), an outer diameter (D

) of 49.50mm,

and 192.5cm in wire length (Wl). The coil parameter

and diameter were designed same as Tx coil to

maximize coupling efficiency. Figure 2(b) shows Rx

coil geometry.

3.2 Wireless Power Transmitter

Apparatus

Figure 3 shows the integrated Tx coil and wireless

power transmitter PCB (80 × 17 mm) prototype. Part

A is a DC‒DC and boost regulator converter was

applied using a high efficiency step up converter chip

(MT3608), input and output capacitor filters (22µF),

iCAST-ES 2021 - International Conference on Applied Science and Technology on Engineering Science

an inductor (22µH) to avoid saturation current, and a

diode (SS34) to limit peak current.

The output voltage is given by Equation (9):



= 𝑉





𝑥(1 +





)

(9)

where V

Ref

is the internal reference voltage in Volt,

R1 and R2 are the resistances in Ohm (Ω), and Vout

is the output voltage in Volt. Typical V

Ref

is 0.6V.

With R

=23,46kΩ (adjusting by potentiometer) and

=2.2kΩ, the boost regulator converter provides

output voltage of 7V at maximum current 2.5A from

input voltage source (Vs) of 2.5 ‒ 3.0 V.

A radio frequency power supply chip (XKT‒412),

resistors of 8.2kΩ and 47kΩ, and a capacitor of 1nF

built a high frequency generator (part B). Square

waves with frequency of around 70kHz was

generated by the radio chip. The signals have voltage

of 7V and a maximum current of 1A.

A power amplifier circuit (part C) was built by a

wireless power transmitter chip (T5336) and a

capacitor of 47µF/16V. This circuit generates

controllable high‒frequency power signal with

voltage gains of four times. Its output voltage is

around 32 Volts.

The operating frequency/oscillator (part D) was

controlled by the voltage difference between the DC

voltage and the T5336 output. The oscillator circuit

was built of LC tank, TX coil (13.9µH), and

capacitors bank of 39nF. The capacitor bank defines

the operating frequency of proposed TX of around

110kHz. This frequency complies with WPC standard

operation frequency between 100 – 205 kHz.

The operating frequency was obtained by

Equation (10):

2π

√

(10)

where f is the oscillation frequency (Hertz (Hz)), L is

the coil inductance (Henry’s (H)), and C is the

capacitance (Farad (F)).

The circuitry and Tx coil built on PCB (80 × 20 mm).

Figure 3: Wireless power transmitter apparatus prototype.

3.3 Wireless Power Receiver Apparatus

The Rx coil (13.9µH) on wireless Rx apparatus will

absorb a high‒frequency oscillation wave from

wireless Tx apparatus. A voltage rectifier and input

filter circuit (part A) converts into a DC voltage. A

voltage rectifier was built by a capacitor of 27nF and

a diode of SS34. While input filter built by a capacitor

of 10µF/25V, and resistors of 10kΩ and 6.2kΩ.

The core of Rx apparatus (part B) is a wireless

power receiver chip (T3168) and voltage adjustment

(a voltage divider circuit composes of 6.2 kΩ and 2kΩ

resistors).

The output filter and protection (part C) was built

using a diode of SS34 and an inductor of 22µH

(feedback protection), and a capacitor of 10µF/16V

as an output filter.

The wireless Rx apparatus generates output

voltage at 5V and a maximum current up to 1.4A.

Figure 4 shows the integrated Rx coil and wireless

power receiver PCB (80 × 17 mm).

Figure 4: Wireless power receiver apparatus prototype.

4 EXPERIMENTAL AND

DISCUSSION

4.1 Experimental Method

The experiment was conducted to examine the

developed wireless Tx apparatus performance. The

experiment configuration is composed of a 2.5 ‒ 3.0 V,

2.5A DC power supply, wireless power Tx apparatus,

wireless power Rx apparatus, two multimeters, and an

Oscilloscope as shown in Figure 5.

Figure 5: Experimental setup. A is wireless power Tx

apparatus, B is wireless power Rx apparatus, C is DC power

source, D and E are multimeters.

Wireless Power Transmitter Apparatus for Low-Voltage DC Power Transmission Applications

The wireless power Tx and wireless power Rx

coils are configured at vertical alignment. The

wireless power Tx apparatus, which powered by DC

power supply, emitted power wirelessly to the

wireless power Rx apparatus coupled with either a

resistor or smartphone as an electric load.

Multimeters measure input power of wireless power

Tx apparatus and output power of the wireless power

Rx apparatus output. An Oscilloscope capture and

record the electromagnetic wave generated by

wireless power Tx apparatus.

The experiment will perform with configuration

of open load circuit, short circuit, and closed circuit

crossed the wireless power Rx apparatus output

terminal. A 4Ω resistor is used as an electric load. In

between range from 0 to 2 cm, the gap between Tx

and Rx coils is changed by the 0.5cm step. The

current, voltage, and power crossed the wireless

power Tx apparatus input terminal and the wireless

power Rx apparatus output terminal were measured

respectively. Moreover, the power delivery efficiency

also obtained using input/output power measurement

results.

The experimental also conducted to evaluate the

developed wireless power transmitter apparatus

compliance with the standard. A standard wireless

receiver charger module which couples with a

smartphone and a smartphone with standard wireless

receiver charger module built-in are used to perform

this experiment

4.2 The Characteristic Performance

The generated waveform by the radio frequency

power supply chip (XKT-412) and the wireless power

transmitter chip (T5336) conjunction with an

Oscillator part are shown in Figure 6. The recorded

operating frequency is around 110kHz.

(

)

(

)

Figure 6: Generated waveforms; (a) the radio frequency

power supply chip waveform; (b) operating frequency

waveform.

Figure 6(a) shows 7V square waves with frequency

of about 68kHz generated by radio frequency power

supply chip (XKT‒412). XKT‒412 chip input

voltage is a step-up output power of the DC‒DC and

boost regulator converter (MT3608) part. This power

is a suitable input power driving for XKT‒412 chip

to generate a square wave signal for the wireless

power transmitter chip (T5336). Figure 6(b) shows

T5336 chip and oscillator wave output. The

waveform is a sinusoidal wave with an amplitude of

32 Volt and the frequency (operating frequency) of

about 110kHz. This operating frequency compliance

with the WPC standard.

Figure 7 shows the developed wireless power

apparatus performance characteristic.

(a) (b)

Figure 7: The developed wireless power characteristics; (a)

the output voltage characteristic; (b) the output current

characteristic; (c) the output power characteristic; (d) the

efficiency characteristic.

The experiment result revealed that maximum output

voltage, current and power on the wireless power

receiver output are obtained at the smallest distance

(tightly coupled) between Tx and Rx coils whatever

the load configuration (open circuit, short circuit,

4Ω). Those will be decreased gradually with the

increase in the distance between the Tx and Rx coils

because of the high transmission coupling loss as

shown in Figures 7(a), 7(b) and 7(c).

The output power was measured at the wireless

power receiver output terminal crossed a resistor of

4Ω as shown in Figure 7(c). The power chart shows

the peak efficiency is around 70% which is obtained

at the maximum power delivery of 5.2W as shown in

Figures 7(d) and 7(c).

4.3 Battery Charging Testing

This experiment goal is to evaluate the developed

WPT apparatus compliance with the WPC standard.

Figure 8 (A is device under charge, B is wireless

power Tx apparatus, C is wireless power Rx

iCAST-ES 2021 - International Conference on Applied Science and Technology on Engineering Science

apparatus) shows the developed WPT apparatus

charged a smartphone battery (3.8V Li‒ion battery

5.7Wh). A 2.5‒3.0 V, 2.5A DC power supply was

connected to the developed wireless power Tx

apparatus, and the developed wireless power Rx

apparatus was coupled with the device under charge

(smartphone). This experiment result shows the

device under charge can be charged with suitable

power.

Figure 8: The battery charging experiment setup first

configuration.

Figure 9 shows other battery charging experiments.

The developed wireless power Tx apparatus (Figure

9(a)) was connected with a 2.5‒3.0 V, 2.5A DC

power supply. A smartphone couples with WPC

standard wireless Rx charger module (Figure 9(b))

was charged by the developed wireless power Tx

apparatus as shown in Figure 9(c).

(a) (b)

Figure 9: The battery charging experiment setup second

configuration: (a) developed Tx module; (b) cell phone

couple with standard wireless Rx charger; (c) charging

process with standard wireless Rx charger; (d) charging

process with built-in standard wireless Rx module.

While Figure 9(d) shows the developed wireless

power Tx apparatus charged a smartphone with built-

in wireless power Rx module. The experiment results

show the developed wireless power Tx apparatus

works properly and it complies with WPC standard.

The charging performance test of the developed

apparatus was conducted using a 3.8V, 5.7Wh Li-ion

rechargeable battery. The charging voltage of the

battery under test is shown in Figure 10. In 15

minutes, the battery under test voltage reached a

maximum and the charging test was stopped due to

the battery voltage remained constant. Actually, the

charging process can be remained to achieve

maximum battery power capacity.

Figure 10: The Typical Charging Curve using the

Developed Apparatus.

5 CONCLUSIONS

The WPT design provides high design freedom in

achieving interoperability. In this research, a wireless

power transmitter apparatus prototype was developed

and implemented based on the magnetic inductive

coupling and complies with WPC standard. There are

various electrical dependent devices on the market

and specific applications have the potential to use this

apparatus.

According to the experimental results, the

developed wireless power transmitter apparatus can

work to meet the interoperability of various

applications that comply with international standard

either commercially or functionally. Its operating

frequency is 110kHz with a maximum transmission

distance of about 10mm. It achieves maximum power

transfer at 5W with voltage to be transmitted coming

from 2.5 ‒ 3.5 V at a maximum current of 2.0A DC

power source. From an efficiency point of view, it has

a peak power transfer efficiency of around 70%.

The coil design needs more attention in the design

process to improve the power transfer efficiency.

ACKNOWLEDGEMENTS

The authors thank the Ministry of Science and

Technology of Taiwan for the funding support with

grant number of MOST 110-2222-E-167-003-MY3.

Wireless Power Transmitter Apparatus for Low-Voltage DC Power Transmission Applications

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