department of electrical and electronic engineering · department of electrical and electronic...

Department of Electrical and Electronic Engineering

FYP Thesis

Title:

Wireless Charging Platform

Student: James Cuddy

08641277

Project Supervisor:

Dr Maeve Duffy

Co-Supervisor:

Dr Eddie Jones


ii

Abstract

This project investigates the use of wireless power to charge a Lithium ion battery. The aim

of this project is to build a wireless charging circuit to test the efficiency of wireless power

transfer. This circuit will also be used to investigate the effectiveness of using shielding in

the circuit. With the increase in use of smartphones and other portable devices, wireless

charging could solve the problem of short battery life. Long battery life wouldn’t be

essential if charging took place every time a device was left on a table (charging circuit built

into table). The system that was built takes approximately hours to charge a lithium ion

battery.


iii

Declaration of Originality

I declare that this thesis is my

original work except where stated.

Date:

Signature:

Acknowledgements

I would like to thank my supervisor Dr Maeve Duffy, and co-supervisor Dr Eddie Jones for

their help and guidance throughout the project. I would also like to thank the staff of the

electronic engineering department of NUIG for their help and knowledge.


Table of Contents

Abstract ................................................................................................................................................. ii

Declaration of Originality .................................................................................................................. iii

Acknowledgements............................................................................................................................. iii

Table of Contents ................................................................................................................................. 1

Table of Tables ..................................................................................................................................... 2

Table of Figures .................................................................................................................................... 3

Glossary ................................................................................................................................................. 3

1. Introduction .................................................................................................................................. 4

1.1. System overview .................................................................................................................. 5

1.2. Wireless power .................................................................................................................... 5

1.3. AC-DC conversion ................................................................................................................ 6

1.4. Battery Charging characteristics ....................................................................................... 6

1.5. Project development plan ................................................................................................... 8

2. Inductors ....................................................................................................................................... 9

2.1. Air PCB coil inductors ......................................................................................................... 9

3. DC-AC Conversion ...................................................................................................................... 12

3.1. Full Wave Rectifier ............................................................................................................ 12

3.2. Circuit testing and simulation.......................................................................................... 13

4. Optimal load resistance and power characteristics ............................................................. 15

4.1. Without Full Wave Rectifier ............................................................................................. 15

4.2. With Full Wave Rectifier ................................................................................................... 17

5. Battery charging integrated circuit ......................................................................................... 19

5.1. Bq24075EVM ...................................................................................................................... 19

5.2. Battery charging characteristics ..................................................................................... 20

6. Further plans .............................................................................................................................. 22

7. Conclusion ................................................................................................................................... 23


2

8. References ................................................................................................................................... 24

9. Appendices .................................................................................................................................. 25

Table of Tables

Table 1: Project development plan ......................................................................................................... 8

Table 2: Inductor characteristics ............................................................................................................. 9

Table 3: Resonant frequency and voltage output. ............................................................................... 10

Table 4: Full wave rectifier test results ................................................................................................. 13


3

Table of Figures

Figure 1: System Diagram ....................................................................................................................... 5

Figure 2: Li-ion Charging Characteristics ................................................................................................. 7

Figure 3: Resonance test circuit and image of inductors. ..................................................................... 10

Figure 4: Resonance Simulation Results ............................................................................................... 11

Figure 5: Full Wave Rectifier Circuit ...................................................................................................... 12

Figure 6: Full wave rectifier simulation results ..................................................................................... 13

Figure 7: Full wave rectifier test circuit................................................................................................. 14

Figure 8: Rl test circuit .......................................................................................................................... 15

Figure 9: Max power simulation results ............................................................................................... 16

Figure 10: Max power test results ........................................................................................................ 17

Figure 11: PSim simulation circuit ......................................................................................................... 18

Figure 12: Psim simulation results ........................................................................................................ 18

Figure 13: bq24075 application circuit. ................................................................................................ 19

Figure 14: detailed system diagram ...................................................................................................... 20

Figure 15: V1 and V2 at resonance ....................................................................................................... 20

Figure 16: dc output voltage ................................................................................................................. 21

Figure 17: battery charge time ............................................................................................................. 21

Glossary

PCB – printed circuit board

AC – alternating current

DC – direct current

USB – universal serial bus


4

1. Introduction

Recently, there have been significant developments in wireless charging systems for mobile

devices, with several charging platform products emerging on the market. The aim of this

project is to build a working prototype of a wireless charging platform and to get the best

efficiency from the inductors provided. This will be achieved by performing tests to

maximise the AC output. Testing will also be done using a full bridge rectifier to convert the

output to DC. The DC output will be maximised and a circuit implemented that will charge a

lithium ion battery. Further plans are to analyse this demonstrator circuit using different

types of shielding. After reading [1], it became clear that using ferrite plates coated with

copper is a very good shielding solution. A shielding effectiveness of 34dB was achieved

compared to 4dB when using ferrite plates with no copper. This finished system needs to be

reasonably efficient compared to a standard charger. The demonstrator circuit will consist

of a DC power supply, an inverter, inductors for wireless power transfer, a full wave

rectifier, battery charging circuitry and of course, a lithium ion battery.


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1.1. System overview

The system built in this project is broken into 3 main sections.

DC-AC conversion

Wireless transfer(inductor coils)

AC-DC conversion

Battery charging circuitry

Figure 1: System Diagram

A DC power supply is used. This DC input is then converted to AC (an AC signal is required

for the wireless power transfer). The AC signal is then converted back to DC on the load side

of the circuit. Batteries operate on a DC voltage and hence, require a DC input for charging.

The battery charging circuit regulates the current and voltage to the battery during charging.

1.2. Wireless power

Wireless power works by use of inductors. Electric current flowing through a primary

coil creates a magnetic field that acts on a secondary coil producing a current within it. As

the distance between the two coils increases, the power transfer is less efficient. Current

applications for this type of power transfer are kettles and electric toothbrushes. It is

advantageous for these products to transfer power wirelessly due to the presence of water.

This technology would be useful for smartphones and other portable devices. Due to their

small size, battery size is restricted and thus, battery life is an issue. If wireless charging was

ubiquitous (chargers built into tables, desks etc), users wouldn’t need to consciously charge

their phones and they would have more opportunities to charge their phones.


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1.3. AC-DC conversion

AC-DC conversion is achieved using a full wave rectifier (see below). This configuration of

diodes inverts the negative parts of the signal, creating a DC signal. This signal is not

sufficient for battery charging and needs to be smoothed using a capacitor in parallel to the

output of the bridge.

1.4. Battery Charging characteristics

Lithium ion batteries charge under very specific conditions. There are 4 stages involved in

the charging process. [2]

Constant current

- The voltage rises at constant current until the battery reaches the

recommended voltage (4.2V).

Constant voltage

- Once the max voltage has been reached, the charging switches to constant

voltage. At this stage, the current is reduced until the battery is fully charged.

At this point there is no current flow.

Charge complete

- No current is flowing in the circuit. When there is no current, the voltage of

the battery will start to drop. A full charge should take about 3 hours.

Topping

- Once the battery voltage has dropped below a certain voltage, current will

flow again and a topping charge will be applied to the battery.


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Figure 2: Li-ion Charging Characteristics


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1.5. Project development plan

This is a rough outline of how the project will progress during the year. The college year is

very busy and some changes may occur to the plan. This plan is subject to revision as new

challenges are encountered.

Table 1: Project development plan

September Completed: - Summarised several papers in the fields of wireless power transfer -build basic circuit to test and demonstrate the concept of wireless power. - Start Project Website: Layout, Contents, etc.

October Investigate properties of inductors. Understand charging characteristics of lithium ion batteries. Design a battery charging circuit configuration.

November Identification of the maximum AC power transfer possible for a given pair of PCB coils.

- Determine details of optimum frequency, voltage and current levels. - Demonstrate maximum power possible with a function generator

power source. Prepare progress report.

December Specification of a demonstrator battery charging system for a lithium ion battery Design, circuit modelling, implementation and test of a suitable rectifier stage on the load side of the inductive coils. Design, circuit modelling, implementation and test of a DC/AC inverter stage on the source side of the inductive coils. Finish progress report. Prepare oral presentation.

January Prepare final project thesis. Demonstration of a complete wireless charging system, including protection circuitry and interconnects required to enable charging of a standard lithium ion battery.

February Comprehensive system testing and analysis of battery charging efficiency

March Prepare for bench demonstration and open day. Complete final project thesis.


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2. Inductors

In this chapter, we will look at the characteristics of the inductors used in this project and

how the wireless power transfer can be optimised by getting the resonant frequency of the

circuit.

2.1. Air PCB coil inductors

An inductor is a wire wrapped into a coil. When a wire is made into a coil and a voltage is

applied, its magnetic properties can induce a current in nearby inductors. An AC signal is

required since it is the change in voltage that induces a current on the load side. Air coil

inductors were used in this project. The characteristics of the coils are as follows:

Table 2: Inductor characteristics

Width Thickness Resistance Turns Radius Depth of coils

0.4mm 0.0175mm 0.8 Ohms 10 1.15cm .85cm

These coils were tested at a resonant frequency for different capacitor values and the

voltage out was measured. Initially, both capacitors were in series in the circuit. After

testing it was found that the output improved when the capacitor was in parallel to the

inductor on the load side (fig 3).


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AC

Cs

Ls LlCl

Vout

Figure 3: Resonance test circuit and image of inductors.

A circuit (fig 3) was built to test the characteristics of the inductors and to get the resonant

frequency for various capacitance values. The results were as follows:

Table 3: Resonant frequency and voltage output.

Capacitance Frequency Inductance Vin Vout

270pF 8.5MHz 1.298uH 10Vpp 12.6Vpp

560pF 6.3MHz 1.139uH 10Vpp 17.5Vpp

680pF 5.3MHz 1.326uH 10Vpp 25Vpp

820pF 4.8MHz 1.286uH 10Vpp 15Vpp


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The calculation below was used to calculate the inductance:

This test was also performed using a PSim simulation. For the optimal capacitance (680pF),

the simulation showed a Vout of 250Vpp (fig 4). This is very different to the test circuit

which yielded an output voltage of 40Vpp. This could be due to the distance between the

inductors in the test. Also a 1 phase transformer was used in the simulation.

Figure 4: Resonance Simulation Results

In the next chapter we will look at DC-AC conversion. The battery requires a DC voltage to

charge, but the wireless part of the circuit needs an AC signal. A full bridge rectifier will be

used for this conversion.


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3. DC-AC Conversion

In this project, DC-AC conversion was achieved using a full wave rectifier. Lithium ion

batteries operate on a DC voltage and hence, require a DC voltage for charging.

3.1. Full Wave Rectifier

A full wave rectifier consists of 4 diodes configured as shown in figure 5. Shottky diodes

were used because of their low threshold voltage of 0.3V. Normal diodes have a voltage

drop of 0.7V. A lower threshold voltage yields a higher voltage at the output.

Full Bridge+

-

~

Figure 5: Full Wave Rectifier Circuit

This circuit alone will not produce a smooth DC voltage. To smooth the signal a capacitor is

added in parallel to the output of the bridge.


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3.2. Circuit testing and simulation

The full bridge was tested for various frequencies and the results were as follows:

Table 4: Full wave rectifier test results

capacitance frequency Vpp Vdc power Rload

820 4.6 10.7 12.5 62.5mW 2.5K

680 5.3 9.8 11 48.4mW 2.5K

560 5.9 9 11 48.4mW 2.5K

Again the best results were achieved using 680pF capacitors. This circuit was also tested in

PSim and the results can be seen in figure 6.

Figure 6: Full wave rectifier simulation results

Looking at the simulation results, we see the red AC input signal is converted to the blue DC

voltage of approximately 8 Volts. The battery charging circuitry requires an input voltage

range between -0.3 and 28 volts, so this is well within the range.

The purpose of the full bridge rectifier is to make the output voltage DC. A capacitor (Cpar)

can be put in parallel after the bridge to smooth the signal (fig 7). Without the capacitor

there is some noise in the signal.


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AC

Cs

Ls Ll Cl Full

Bridge

Cpar Vout

Figure 7: Full wave rectifier test circuit

In the next chapter we will look at finding the optimal load resistance in order to maximise

the output power. This will be done using Psim and Matlab simulations and confirmed by

testing.


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4. Optimal load resistance and power characteristics

This Chapter investigate how varying the load resistance can improve the output power.

Simulation and testing were used to find the best resistance in order to get max power.

4.1. Without Full Wave Rectifier

Matlab was used in this project for solving various equations. It is very good at complex

computation. This project used Matlab to find the optimum values for specific variables.

The following circuit (fig 8) was used to calculate the best value for Rl so as to optimise the

power out.

LlCl

Rl

Figure 8: Rl test circuit


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The following equations were used:

This was simplified to:

√

Where Ri = resistance of Ll(inductor)

Different values of Rl, ranging from 0Kohm to 5Kohm were tested and yielded this graph (fig

9) of Power out verses Rl for 3 frequencies. The 680pF and the 560pf seem to peak for a

resistance of 2.5KOhm, whereas the higher frequency (820pF) peaks at about 2Kohm.

Figure 9: Max power simulation results


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4.2. With Full Wave Rectifier

Tests were performed including the full wave rectifier using 680pF capacitors (fig 10). Max

power occurred at 1KOhm providing 0.12W of power. This is a lower power and resistance

than the matlab simulation, but the voltage drop across the bridge would explain that. It is

reasonably consistent with the simulated data. A Psim simulation was also performed

including the full wave rectifier. The simulation circuit and results can be seen in figures 11

and 12 respectively. The AC signal was 10Vpp and 5.3Mhz. The load resistance was

2.5KOhm. From the results (fig 12) we see that Vout is just under 10V, but the power is

quite low (.03W). This is significantly lower than what was achieved through testing.

Figure 10: Max power test results

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

500 1000 1500 2000 2500 3000 4000 5000 6000

Po

we

r(W

)

Rl(ohms)


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Figure 11: PSim simulation circuit

Figure 12: Psim simulation results


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5. Battery charging integrated circuit

This chapter looks at the operation of the battery charging circuit and the battery charging

performance for the overall system.

5.1. Bq24075EVM

The bq24075 is made by Texas Instruments. It can be powered by USB or an AC adapter and

can support a maximum charge current of 1.5A. The max current the coils can drive is 1.2A

and the current required for charging is about 1 Amp. A typical application circuit is shown

below (fig 13)[3].

Figure 13: bq24075 application circuit.

In this project, there is no system output. A 4.7uF capacitor is connected between OUT and

ground. TS is not used (External NTC Thermistor Input) and a 10kΩ fixed resistor from TS to

VSS to maintain a valid voltage level on TS.


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5.2. Battery charging characteristics

The overall system (fig-14) is capable of charging a lithium ion battery in approximately 6

hours. The voltages (V1-V4) were measured during the system operation. When charging is

not taking place V3 is 25Vdc. Once the battery starts charging, the drops to 4.2Vdc. V1 and

V2 show the circuit resonance. As the battery charges, V4 increases at a rate of 0.004V per

minute. When the battery voltage is lower, the charge time is faster. When the battery

voltage is nearly charged, the current starts to drop.

AC

Cs

Ls LlCl

Full Bridge Cpar

LI ion battery

Battery Charging Circuit

(bq24075EVM)

IN

VSS

BAT

VSS

V1 V2 V3 V4

Figure 14: detailed system diagram

The results below show the circuit at resonance. The output (green) is almost 50Vpp.

Figure 15: V1 and V2 at resonance


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The next waveform shows the DC voltage after the full bridge rectifier. At this stage of charging it is

applying 3.7V to the battery charging circuitry. The voltage increases during charging as the battery

voltage increases.

Figure 16: dc output voltage

The graph below represents the voltage of the battery during charging. The rate of increase appears

to slow down as the battery approaches a full charge.

Figure 17: battery charge time

2.9

3

3.1

3.2

3.3

3.4

3.5

3.6

0 5 14 21 26 44 95

Vo

lts(

V)

Time(mins)

Battery Charging


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The next chapter will investigate further plans to improve the project by testing the effects

of shielding on the circuit and by adding an inverter to the source side of the circuit.

6. Further plans

Along with investigating the effects of using shielding in the system, this project will also

look at replacing the battery with a mobile phone or other portable device. This could be

done by having the load side of the circuit built into a protective case and connect the

charging unit to the phone using micro USB. Micro USB is the main connection in all

Smartphones as of 2011[4]. Testing the shielding could be done by testing different types

and measuring the output power and charge time of the battery. Also a DC-AC conversion

circuitry will be added to the source side of the project. This will enable a DC power supply

to power the circuit which can drive more current and should improve charging time.


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7. Conclusion

A wireless battery charger was successfully built as part of this project. This demonstrator circuit can

be used to test the effects of shielding on wireless power transfer. Testing with a full bridge gave an

output of 25V DC output. The battery will charge at a rate of .004V per minute. More research needs

to go into the source side of the circuit. Up to now, a signal generator has been used. This needs to

be replaced with a DC power supply and a DC-AC converter in the future. Some power conversion

will be required for this aspect of the project. Further goals of the project are to use what I have

learned to build a battery charging circuit for a mobile phone. This could be done by integrating the

load circuit into a Smartphone protective case.


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8. References

[1] S. C. Tang, S. Y. (Ron) Hui, Henry Shu-hung Chung, Evaluation of the Shielding Effects on Printed-Circuit-Board Transformers Using Ferrite Plates and Copper Sheets, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 17, NO. 6, NOVEMBER 2002

[2] http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries March 2012

[3] 1.5A USB-FRIENDLY Li-Ion BATTERY CHARGER AND POWER-PATH MANAGEMENT IC

http://www.ti.com/lit/ds/slus810g/slus810g.pdf March 2012

[4] http://techcrunch.com/2009/06/29/micro-usb-to-be-the-standard-phone-charging-port-of-

europe/ March 2012

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries%20March%202012

http://www.ti.com/lit/ds/slus810g/slus810g.pdf

http://techcrunch.com/2009/06/29/micro-usb-to-be-the-standard-phone-charging-port-of-europe/

http://techcrunch.com/2009/06/29/micro-usb-to-be-the-standard-phone-charging-port-of-europe/


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9. Appendices

Included on the submitted CD is the entirety of the FYP folder used in the development of this

project. Folders divide up different aspects of the project, from graphs to test to the matlab and psim

circuits used in the project.

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