IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

International Journal of Research in Information Technology (IJRIT) www.ijrit.com

ISSN 2001-5569

A Review on Wireless Inductive Power for Mobile Devices Chetan Kawley, Prashant Chatur Student, Electronics and Telecommunication Department, Govt. College of Engineering Amravati, Maharashtra, India [email protected] Head of Department, Electronics and Telecommunication Department, Govt. College of Engineering Amravati, Maharashtra, India [email protected]

Abstract Wireless power provides convenience of charging mobile phones and devices. Just place the device on the pad and that's it, no cable and plugs. In this work wireless inductive power transmission is explained. Also limitations with respect to efficiency of whole magnetic system is discussed. Inductive charging allows free positioning of the device on the pad. Inductive power transmission at a surface can be efficient as the conventional power supplies.

Keywords: Inductive Wireless Power Transmission, Efficiency, Mobile devices, SAR.

1. Introduction Wireless power transmission based on inductive power got into the focus of attention in the recent past. Since data communication has become wireless, users expect similar use comfort also for powering of their mobile devices. In this report, an inductive power transmission pad is presented, which is intended to charge devices like mobile phones. Wireless power transmission suggest the freedom of placement for power transmission. However, efficiency and emitted magnetic fields limit the inductive power transfer to close to a surface. This paper gives a review on consideration about feasibility and limitations of such systems. A major part of this section refers to a previous detailed publication [1] on efficiency limits and cites from a further one [2], but new aspects about resonance operation and magnetic emissions are also added. In a further part of this work, an inductive power transmission pad is presented, which is intended to charge devices like mobile phones.

2. Resonant Operation To discuss the wireless inductive power system, a closer look at the system is necessary. Fig 1 shows the typical arrangement of the transmitter coil and receiver coil. An AC current supply to the transmitter coil is induced to the secondary coil and creates the power for the load. However, to increase efficiency of power transfer the resonant operation is used, a series resonant capacitor is connected to the load.

Chetan Kawley,IJRIT


IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

Previous schemes for wireless power transmission included attempts by the late scientist Nikola Tesla and the Microwave power transmission. Both Tesla's design and the later microwave power were forms of radiative power transfer. Radiative transfer, used in wireless communication, is not particularly suitable for power transmission due to its low efficiency and radiative loss due to its omnidirectional nature. An alternative approach is near field interaction between the source and device so that efficient power transfer is possible. The approach is resonant inductive coupling.

Fig. 1 Typical arrangement of wireless inductive power system

3. Limitations Due to Efficiency Efficiency can be evaluated by calculating the losses in the magnetic system. Losses in the source and in the rectifier are comparable to any other switch mode power converter. Also due to low frequency radiation losses can be neglected. Hence, losses in the magnetic systems only appear as the ohmic losses in the winding. These ohmic losses can be determined by coil's magnetic coupling factor k and the quality factor Q: (1)


The quality factor Q depends on the applied frequency 2πf = ω, inductance value L and resistance of coil R. This quality factor Q can be influenced by coil technology, their sizes, shape and amount of conducting material used. Higher the Q, better the coil. Technically, it is difficult to obtain the value of Q greater than 1000. Values less than 10 are not very useful. For mass production values around 100 can be expected. The coupling factor k gives the amount of magnetic flux penetrating into the receiver compared to the total generated flux. k varies between 0 and 1. For k = 0, the two coils are completely decoupled, while k→ 1 means very well coupled coils. The arrangement of two coils determines the coupling factor. The ohmic losses in the coils depends on matching between load resistance RL in the receiver and coil impedance ZC. If load resistance is too low then high output current is needed to provide the output power, which causes losses in the receiver. If the load resistance is too high, a high output voltage is needed, which requires a high magnetizing current which induces losses in the transmitter. The optimal loss factor

λopt defined as the ratio of losses to transferred power: (2) λopt =

Chetan Kawley,IJRIT

2 . 2

[1+ 1 

.  2 ]


IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

In wireless power transfer, efficiency is the function of distance. Over larger distance efficiency is less. Only low power levels can be wirelessly transmitted without significant losses. Possible applications can be industrial sensors, mobile phones.

4. Power Restrictions Due to EMF Limits A. Power extraction from magnetic field Wireless inductive power has some limitations due to effect of alternating magnetic field on human being (EMF) and on technical devices (EMI). Hence, it is necessary to know how much power can be derived from limited magnetic field. Consider a loop inductor in homogeneous magnetic field generated by transmitter. Let φTx be the magnetic flux linking with the loop inductor. Let N be the number of turns of the coil and magnetic flux density BTx is homogenous over the area of loop A then output voltage Uout is equal to induced voltage: (3) Uind = N . A . jω . BTx where ω = 2πf is circular frequency of magnetic field. If loop is shorted, the short circuit current will flow which generates flux φRx in opposite direction to the transmitter flux φTx.. Ideally, it just cancels out transmitted flux inside the loop.

Fig. 2 Magnetic flux in loop a) Open loop b) Shorted loop c) Resonant loaded loop

Fig. 3 Equivalent circuit of a loaded resonant inductor coil

If loop is the part of loaded resonance circuit, the magnetic flux of the receiver may even exceed the transmitter flux (fig 2c). Fig. 3 shows the related equivalent circuit. The voltage source represents the induced voltage Uind according to equation (4). The voltage drop at the receiver inductor LRx corresponds to magnetic flux of receiver φRx. In case of resonance the inductor and capacitor voltage cancels each other. The current is only limited by series resistance of resonant circuit Rs and the load resistance RL. The inductor voltage UL may exceed the induced voltage by a factor equal to loaded quality factor of resonance circuit. Hence, receiver flux φRx is much higher than transmitter flux φTx. In case of resonance maximum power can be taken when RL is matched to RS i.e. RL = RS . Then the maximum output power is :


Chetan Kawley,IJRIT

| |




IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

Now AL can be defined as LRx = N2 . AL , the maximum output power can be directly calculated from magnetic flux density BTx by combining equation (3) and (4) :

 .| | 

Pmax=Q . . 


Inductance AL value can be calculated similar as in previous chapter according to [1]. Depending on the winding width w the inductance for a ring coil inductor may vary slightly. So for inductance value, an approximation is calculated according to equation given in [7] : (6) L= N2 . 0.35


. dout . 10$%&

,valid for din/dout ≤ 0.9

where dout is outer diameter and din is the inner diameter . The loop is approximated with diameter of ratio din/ dout = 0.9. B. Limitations Due to EMF Standards ICNIRP (International Committee Non Ionizing Radiation Protection) derived guidelines for human exposure to electromagnetic radiation. Considering magnetic field as a limiting value, the maximum possible power reception can be calculated according to equation (5) and equation (6), taking into account resonant operation. Calculations are based on the homogenous magnetic field of the transmitter and the position of receiver. However, in reality magnetic field decays with the distance. In order to maintain safety limit for the user in the area closer to the transmitter, the power level has to be decreased. Also, due to the resonant operation field level increase in the vicinity of the receiver. If user has access to this area, the power level has to be decreased further. Applications like wireless power space requires a magnetic field at an arbitrary position in the space. But then whole body of user is exposed to magnetic field without restriction. Hence, strict ICNIRP guidelines for public exposure have to be applied. But in case of wireless power surface, magnetic field is concentrated in the area between the transmitter and receiver. The user is only exposed to strongly decaying stray magnetic field. Also, only extremities of human body are exposed to magnetic field (e.g. the hand ) and exposure time is limited for short period (e.g. while placing a device in the power area). This allows higher magnetic flux levels for the power transfer. Analysing the exposure is tedious in this case. ICNIRP defines reference levels for fields only for homogenous fields and exposure of the whole body. Localised inhomogeneous fields, which affect only extremities requires application of the basic restrictions defining limits for induced current dissipated heat (SAR) in the body. These values cannot be measured but must be simulated with an appropriate method like finite element method for which simplified models example of hand are defined as shown in figure.

Chetan Kawley,IJRIT


IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

Fig. 4 Illustration of FEM calculation of induced currents in user hand

Concluding from EMF point of view, power transfer in open space is not recommended but at surface reasonable wireless power transfer is possible.

5. Application According to these findings, inductive wireless power transfer is promising along a surface. A possible application can be the charging of mobile devices. Here, wireless communication already has become standard, and the consumer expects that charging would also be possible without the hassle of cables and plugs. For such an application, pads that charge wirelessly are proposed. One example is the Powerpad (see Figure 5), which was presented by Philips Research [1] [2]. It provides very simple functionality to the user. Just place the mobile somewhere on the pad and it will charge.

Fig. 5 Inductive Powerpad for charging of mobile phones

C. Power Transmission System For charging the mobile phones, normally power transmission of 5 W is required. Nominal operating frequency is 110-210 kHz. Resonant operation is used to optimize power transfer. Typical configuration of inductive power transfer is shown in Fig. 6. The main components are high frequency inverter and resonant network.

Chetan Kawley,IJRIT


IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

Fig. 6 Minimum configuration of inductive power transfer system

1) High Frequency Inverter Fig. 7 shows full-bridge inverter topology, which consists of two bridge legs; the two power switches on each bridge leg operate in an alternating fashion. There are two opposite polar power pulses transferred during each PWM cycle, enabling flux hysteresis loop of output transformer to operate in quadrants 1 and 3; so duty cycles from zero to one are possible.

Fig. 7 Full-bridge inverter topology

Full-bridge inverter is widely used in DC-to-AC conversion applications, due to the following features: • High DC voltage utilization to support wide input voltage range • More control variations for different application conditions • Unipolar fixed-frequency PWM control to reduce EMI • Phase-shifted control strategy for possible soft switching operation to improve system efficiency • Small power component stresses for medium/high power applications • Simple inverter topology with output transformer 2) Resonant Network Resonant network is used to convert AC square wave to sinusoidal wave, which is targeted to assist the converters to operate in soft switching mode, and reduce EMI from power switches in converter. The output transformer fed with sinusoidal wave will also improve power transfer efficiency and reduce harmonic radiation from the transformer side. So, the single-stage full-bridge inverter with parallel LCL resonant network (shown in Fig. 8) is used as power stage of transmitter to meet the specifications.

Fig. 8 LCL resonant network

Chetan Kawley,IJRIT


IJRIT International Journal Of Research In Information Technology, Volume 2, Issue 5, May 2014, Pg: 559-565

6. Conclusion From an efficiency point of view, wireless inductive power transfer is feasible for general power applications only if transmitter and receiver coils are in close proximity to each other. Inductive power transfer in a larger space is not feasible due to the very low efficiency. Resonant operation and limitations of power transfer due to efficiency and EMF limits are discussed. An inductive power pad to charge mobile devices is presented and main components for power transmission are described.

References [1]

D. Kurshner Christian Rathge, and Ulrich Jumar, Design Methodology for High Efficient Inductive Power Transfer Systems With High Coil Positioning Flexibility , IEEE Trans. on Industrial Electronics, Vol. 60, Jan 2013.


Eberhard Waffenschmidt, Toine Staring, Limitation of inductive power transfer for consumer applications, 13th European Conference on Power Electronics and Applications ,paper no. 0607. Eberhard Waffenschmidt, Wireless power for mobile devices, VDE Kongress 2010, Leipzig, Germany.



International Commission on Non-Ionizing Radiation Protection (ICNIRP), Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields, Health Physics April 1998, Volume 74, No 4.

[5] R. Hui, W. Ho, A new generation of universal contactless battery charging platform for portable consumer electronic equipment, 35th IEEE power electronics specialists conference. [6] Wireless Power Consortium website. http://www.wirelesspowerconsortium.com/index.html.



[7] X. Liu, W. M. Ng, C. K. Lee and S. Y. R. Hui, Optimal operation of contactless transformers with resonance at secondary circuit, conference proceedings APEC'08, USA, pp. 645 to 650. [8] Eberhard Waffenschmidt, Shielding properties of soft-magnetic layers for planar inductors, 14th International Power Electronics and Motion Control Conference - EPE-PEMC 2010, Ohrid, Republic of Macedonia. [9] Xun Liu and S.Y.(Ron) Hui (2006), Optimal design of a hybrid winding structure for planar contactless battery charging platform, IAS. [10] Joaquin J. Casanova, Zhen Ning Low, Jenshan Lin and Ryan Tseng, Transmitting coil achieving uniform magnetic field distribution for planar wireless power transfer system, Proceedings of IEEE Radio and Wireless Symposium 2009, p.530, paper no. TU4B-5. [11] S. C. Tang, S. Y. Hui, and H. S.-H. Chung (1999), Coreless printed circuit board (PCB) transformers with multiple secondary windings for complementary gate drive circuits, IEEE Trans. Power Electron, vol. 14, no. 3. [12] S. Y. Hui, S. C. Tang, and H. S.-H. Chung, Optimal operation of coreless PCB transformer-isolated gate drive circuits with wide switching frequency range, IEEE Trans. Power Electron., vol. 14, no. 3, pp.506 to 514.

Chetan Kawley,IJRIT


A Review on Wireless Inductive Power for A Review on ... - IJRIT

To discuss the wireless inductive power system, a closer look at the system is .... winding width w the inductance for a ring coil inductor may vary slightly.

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