Safety and Reliability Verification Process of Coil Module in Wireless Power Transfer System

using Circuit Level Sensitivity Simulation

Jonghoon Kim, Senior Member, IEEE, Jonghoon J. Kim, Hongseok Kim, Chiuk Song, Jinwook Song, Yeonje Cho, Sukjin Kim, Sunkyu Kong, Seungtaek Jeong, and Joungho Kim, Senior Member, IEEE Terahertz Interconnection and Package Laboratory, Dept. of Electrical Engineering, KAIST

291 Daehak-ro, Yuseong-gu, Daejeon, South Korea jonghoon@kaist.ac.kr

Abstract—High-power wireless power transfer system (WPT) with high voltage and/or high current can be very dangerous to both engineers and consumers because the probability of explosion is much higher than that of low-power WPT system. Therefore, safety and reliability issues of a coil module in WPT systems should be verified before the actual experimental test. In this paper, a coil module verification process using a circuit level sensitivity simulation in the design stage will be proposed. The proposed verification process can be useful to the development of WPT systems with more safe and reliable coil modules.

Keywords—wireless power trnasfer, coil module, reliability, safety, sensitivity

I. INTRODUCTION Wireless power transfer (WPT) technology has been

adopted in various applications, such as charging mobile products like smart phones, as shown in Fig. 1. The transferred power using WPT system varies from mobile devices to electric vehicle and high-speed train [1], [2]. As the number of products using WPT is increasing, consumer request for more convenient, reliable, and safe WPT system is also increasing.

Fig. 1. Examples of applications of wireless power transfer technology

II. SAFETY AND RELIABILITY ISSUES ON COIL MODULE WPT system can be divided into 3 modules, which are

power transmitting module, power receiving module, and coil module as shown in Fig. 2. Coil module includes transmitting (TX) and receiving (RX) coils, as well as tuning capacitors for resonances.

Transmitting module can be modeled using either equivalent constant current (C) source or constant voltage (V) source model. Receiving module can be modeled using resistive (R), capacitive (C), inductive (L), or their combination. CSSR topology means coil module with constant current (C) source, series (S) resonance scheme for transmitting coil, series (S) resonance scheme for receiving coil, and effective resistive (R) loading condition. [3], [4]

Fig. 2. Definition of coil module with seires (S) or parallel (P) resonance topology in WPT system.

High power WPT systems require high voltage and/or high current on coil module which, as previously mentioned, consists of a transmitting inductor (TX coil), a TX tuning capacitor, a receiving inductor (RX coil), and a RX tuning capacitor. Because of the high voltage, TX or RX tuning capacitors have the risk of exploding with not only a terrific

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0028680 and No. 2010-0029179).

explosion but also a bad odor. The excessive current on the TX or RX coil can also cause an ignition, which can be highly dangerous to both WPT engineers and consumers.

Moreover, WPT system should operate normally under the extreme environmental conditions such as the variation of equivalent load resistance by normal operation of load system, the variation of magnetic coupling coefficient by the change of relative position between the TX and RX coils, the variation of capacitance due to aging, and so forth.

Consequently, safety and reliability issues for a coil module in WPT systems should be evaluated in the design stage before the actual experimental test stage. In this paper, a coil module verification process using a circuit level sensitivity simulation in the design stage is proposed.

III. SAFETY AND RELIABILITY VERIFICATION PROCESS

A. Overall Process First of all, a coil module should be modeled using an

equivalent circuit model as shown in Fig. 4. After the sensitivity simulation of 13 variables listed in Table II, 17 parameters in Table I should carefully be considered. There are 221 (17 x 13) sensitivity graphs for the verification of the safety and reliability. If all parameters meet the design specification, the coil module can achieve ‘pass’ judgement. If not, the equivalent circuit model should be improved.

The 17 parameters can be used for the characterization of a coil module. Sensitivity simulation should also be conducted for 13 variables in Table II. If we know the relation between the parameters and variables, we can easily optimize the design of a coil module in a WPT system. Sensitivity simulation was conducted using CS4WPT_v104a.p which is a self-developed software as shown in Fig. 3.

TABLE I. PARAMETERS FOR VERIFICATION OF SAFETY AND RELIABILITY.

Part 17 Parameters Unit Description

Power Source

part

Z11ALL Ω Input impedance at source to TX part

VSRC V Output voltage of source

ISRC A Output current of source

PSRC W Output power of source

PF % Power factor at source to TX part

TX Coil part

VLTX V Voltage of TX coil (inductor)

ILTX A Current through TX coil (inductor)

VCTX V Voltage of TX tuning capacitor

ICTX A Current through TX tuning capacitor

RX Coil part

VLRX V Voltage of RX coil (inductor)

ILRX A Current through RX coil (inductor)

VCRX V Voltage of RX tuning capacitor

ICRX A Current through RX tuning capacitor

Load part

VLD V Transferred voltage to load

ILD A Transferred current to load

PLD W Transferred power to load

EFF % Efficiency of real-power transfer

TABLE II. VARIABLES FOR SENSITIVITY SIMULATION

Part 13 Variables Unit Description

Power Source

part

FREQ Hz Operation frequency

VSRC V Output voltage of source, only V*** typology

ISRC A Output current of source, only C*** topology

RSRC Ω Internal resistance of source

TX Coilpart

LTX H TX coil inductance

CTX F TX tuning capacitance

RLTX Ω Internal resistance of TX coil (inductor)

RCTX Ω Internal resistance of TX tuning capacitor

TX~RX K Magnetic field coupling coefficient

RX Coilpart

LRX H RX coil inductance

CRX F RX tuning capacitance

RLRX Ω Internal resistance of RX coil (inductor)

RCRX Ω Internal resistance of RX tuning capacitor

Load part RLD Ω Load impedance (resistance)

Fig. 3. Practical sensitivity simulation procedure using MATLAB program

B. Equivalent Circuit Modeling for Coil Module Fig. 4 is an example of an equivalent circuit model of a coil

module with CSSR topology. The targeted power to be transferred to the load is 300 W. The circuit elements are described in Table II.

Fig. 4. Equivalent circuit model of a coil module (example circuit diagram)

C. Examples of Sensitivity Simulation Results After sensitivity simulation, we can obtain sensitivity

graphs such as the ones shown in Fig. 5 to Fig. 12.

Sensitivity graphs with frequency variation are depicted in Fig. 5 to Fig. 8. As can be observed from Fig. 5, for power source part, the PSRC is 310 W, the ISRC is 23 A, the VSRC is 14 V, and the power factor is 97% at the target frequency of 60 kHz. If the frequency increases, the control voltage VSRC should be increased for constant current source. Accordingly, the PSRC increases and the power factor decreases rapidly. In Fig. 6, for the TX coil part, the ICTX and the ILTX are found to be 23 A, whereas the VCTX and the VLTX are approximately 850 V at 60 kHz, which can be very dangerous and needs further verification. In Fig. 7 for the RX coil part, the ILRX and the ICRX are 12 A, the VLRX is 50 V, and the VCRX is 40 V at 60 kHz. In Fig. 8 for load part, the PLD is 300 W, the VLD is 28 V, the ILD is 11 A, and the efficiency is 97% at 60 kHz. If the frequency is decreased, PLD and VLD also decrease. However, if the frequency is increased PLD converges to 140 W.

Fig. 5. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : FREQ] and [parameters : Power Source part]

Fig. 6. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : FREQ] and [parameters : TX Coil part]

Fig. 7. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : FREQ] and [parameters : RX Coil part]

Fig. 8. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : FREQ] and [parameters : Load part]

Sensitivity graphs for load part with selected ISRC, k, CRX, and RLD variation are described in Fig. 9 to Fig. 12. As shown in Fig. 9, if the ISRC is increased, the VLD will be increased with 20 dB/decade scales and the PLD will be increased with 40 dB/decade scales; however, the efficiency will not be changed. It means that the PLD can be controlled by controlling the ISRC. Furthermore, in Fig. 10 with magnetic coupling coefficient k variation, the VLD and the PLD trends are found to be the same as they were with the ISRC variation, but the efficiency is very different. If k is decreased below 0.03, the efficiency will be decreased with 20 dB/decade scales. In Fig. 11 with the CRX variation, the trends of the graphs are similar to the ones with FREQ variation. If the CRX is decreased due to such reasons as aging, the PLD will be decreased with 40 dB/decade scales. Lastly, in Fig. 12 with the RLD variation, if the RLD is increased, the VLD has almost constant voltage, the PLD decreases with -20 dB/decade scales, and the efficiency also decreases. The RLD variation is normal operating condition because the power consumption of the load can be varied in actual operation.

Freq [Hz]102 103 104 105 106

Impe

danc

e [

], Vo

ltage

[V],

Cur

rent

[A],

Pow

er [W

], PF

[%]

10-3

10-2

10-1

100

101

102

103

104Z11ALL(green), Vsrc(cyan), Isrc(blue), Psrc(red), Power Factor(magenta)

Freq [Hz]102 103 104 105 106

Volta

ge [V

], C

urre

nt [A

]

10-3

10-2

10-1

100

101

102

103

104VLTX(magenta), ILTX(red), VCTX(cyan), ICTX(blue)

Freq [Hz]102 103 104 105 106

Volta

ge [V

], C

urre

nt [A

]

10-3

10-2

10-1

100

101

102

103

104VLRX(magenta), ILRX(red), VCRX(cyan), ICRX(blue)

Freq [Hz]102 103 104 105 106

Volta

ge [V

], C

urre

nt [A

], Po

wer

[W],

Effic

ienc

y [%

]

10-3

10-2

10-1

100

101

102

103

104VLD(cyan), ILD(blue), PLD(red), Efficiency(magenta)

850 V It can be dangerous.

Fig. 9. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : ISRC] and [parameters : Load part]

Fig. 10. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : k ] and [parameters : Load part]

Fig. 11. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : CRX ] and [parameters : Load part]

Fig. 12. Sensitivity graphs for the coil module with the circuit model in Fig. 4. [variable : RLD ] and [parameters : Load part]

IV. CONCLUSION In this paper, a verification process of the safety and

reliability for a coil module in a WPT system was proposed. The verification can be easily done using a circuit level sensitivity simulation. The 17 parameters for consideration and 13 variables for variation used in the process of verification are defined. Firstly, the equivalent circuit model of a coil module should be prepared. Secondly, the sensitivity simulation for all 13 variables can be easily done by using the automated simulation software. Finally, the 17 parameters should be carefully considered to check whether the parameter values meet the specifications or not.

The authors hope that the proposed verification process of the safety and reliability can be helpful to the design of coil modules in WPT systems.

ACKNOWLEDGMENT The authors thank Prof. Seungyoung Ahn who gave lots of

valuable discussion for development of sensitivity simulation program CS4WPT_v104a.

REFERENCES [1] Chwei-Sen Wang, Oskar H. Stielau, and Grant A. Covic, “Design

Considerations for a Contactless Electric Vehicle Battery Charger,” IEEE Trans. on Industrial Electronics, vol. 52, issue 5, pp. 1308-1314, Oct. 2005.

[2] Seonghwan Kim, Hyun-Ho Park, Jonghoon Kim, Jingook Kim, Seungyoung Ahn, “Design and Analysis of a Resonant Reactive Shield for a Wireless Power Electric Vehicle,” IEEE Trans. on Microwave Theory and Techniques, vol. 62, no. 4, pp. 1057-1066, 2014

[3] Jonghoon Kim, Hongseok Kim, In-Myoung Kim, Young-Il Kim, Seungyoung Ahn, Jiseong Kim, and Joungho Kim, “Reduction of Electromagnetic Field from Wireless Power Transfer Using a Series-Parallel Resonance Circuit Topology,” Journal of the Korean Institute of Electromagnetic Engineering and Science, vol. 11, no. 3, pp. 166-173, Sep. 2011.

[4] Jonghoon Kim, Hongseok Kim, Mijoo Kim, Seungyoung Ahn, Jiseong Kim, and Joungho Kim, "Analysis of EMF Noise from the Receiving Coil Topologies for Wireless Power Transfer," Asia-Pacific International Symposium on Electromagnetic Compatibility (APEMC), pp. 645-648, TH-AM-PE2-2, May 2012.

Isrc [A]10-1 100 101 102 103

Volta

ge [V

], C

urre

nt [A

], Po

wer

[W],

Effic

ienc

y [%

]

10-3

10-2

10-1

100

101

102

103

104VLD(cyan), ILD(blue), PLD(red), Efficiency(magenta)

k [ ]10-4 10-3 10-2 10-1 100

Volta

ge [V

], C

urre

nt [A

], Po

wer

[W],

Effic

ienc

y [%

]

10-3

10-2

10-1

100

101

102

103

104VLD(cyan), ILD(blue), PLD(red), Efficiency(magenta)

CRX [F]10-9 10-8 10-7 10-6 10-5

Volta

ge [V

], C

urre

nt [A

], Po

wer

[W],

Effic

ienc

y [%

]

10-3

10-2

10-1

100

101

102

103

104VLD(cyan), ILD(blue), PLD(red), Efficiency(magenta)

RLD [ ]10-1 100 101 102 103

Volta

ge [V

], C

urre

nt [A

], Po

wer

[W],

Effic

ienc

y [%

]

10-3

10-2

10-1

100

101

102

103

104VLD(cyan), ILD(blue), PLD(red), Efficiency(magenta)

Decreasing of power consumption ≡ increasing of load resistance RLD