WIRELESS POWER - Minimizes Interconnection Problems

8/12/2019 WIRELESS POWER - Minimizes Interconnection Problems

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DESIGNfeature

10 Power Electronics Technology| July2011 www.powerelectronics.com

U

se of wireless power involves themagnetic coupling of conductorsconfigured so that the change in

current flow through one conduc-tor transfers a voltage in anotherconductor through electromagnet-

ic induction. The amount of inductive couplingbetween two conductors is determined by theirmutual inductance.

You can enhance wireless power transmis-sion with the application of resonance effects.This technique employs transmitter and receiverinductors tuned to a mutual frequency. This

enables power to be transmitted over a distanceof up 1/4 or 1/3 times the size of the primarycoil. Transmitting and receiving coils are usuallysingle layer solenoids or flat spirals with seriescapacitors, which, in combination, allow thereceiving element to be tuned to the transmitterfrequency.

Electromagnetic induction is a function of thefrequency and the intensity of the conductorscurrent and voltage that produces the fields. The

higher the frequency, the greater the inductioneffect. Energy transfers from the conductor

WIRELESS POWER ALLOWS THE

TRANSMISSION OF ELECTRICAL

ENERGY FROM A POWER SOURCETO AN ELECTRICAL LOAD WITH-

OUT INTERCONNECTING WIRES.

THIS PROCESS IS USEFUL WHERE

INTERCONNECTING WIRES ARE

INCONVENIENT, HAZARDOUS, OR

IMPOSSIBLE.

SAM DAVIS,Editor-In-Chief, PET

WIRELESSPOWERMinimizes

InterconnectionProblems
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Fig. 1. Wireless transmitter uses a single layer winding array structure.

that produces the fields (the primary) to any conductor

on which the fields impinge (the secondary). You canincrease the coupling between two conductors by wind-

ing them into coils and placing them close together on

a common axis, so the magnetic field of one coil passes

through the other coil. Coupling must be tight to achieve

high efficiency. If the distance from the primary to the

secondary increases some of the magnetic field will miss

the secondary, lowering the coupling.

Common uses of resonance-enhanced electrodynamic

induction are charging the batteries of laptop computers

and cell phones, medical implants and electric vehicles.

A localized charging technique selects the appropriate

transmitting coil in a multilayer winding array structure.Fig. 1shows a typical wireless transmitter. Resonance is

used in both the wireless charging pad (the transmitter

circuit) and the receiver module (embedded in the load)

to maximize energy transfer efficiency. This approach hasbeen adopted as part of the Qi (pronounced chee) wire-

less charging standard developed by the Wireless Power

Consortium.

LINK EFFICIENCY

The effectiveness of the electrodynamic induction is called

link efficiency, dlink

. It is a measure of the primary and

secondary coils ability, to transfer energy from one coil to

the other:

(1)

Where:

k = Magnetic coupling factor

(2)

= Unloaded quality factor of the primary circuit

(3)

= Unloaded quality factor of the secondary circuit

RE= REopt

opt

kQ

1 10 100 1 x 103

1.2

1.1

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Fig. 3 Magnetic flux lines from primary and secondary coils where z = 6 mm

(vertical distance between coils) and k = magnetic coupling factor.

Without ferromagnetic shielding

r= 1 (relative permeability)

k = 0.45

With ferromagnetic shielding

r= 3000 (relative permeability)

k = 0.68

Fig. 2. Optimum link efficiency as a function of the figure of merit for the coils.
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(4)

= QE= Effective Q

Where:

RE= Effective load resistance which models the rectifier

including the output filter capacitor and the actual load

resistance RL. The relationship between REand RLis

(5)

If series resonance is used on the secondary side

t= Primary angular velocity (2f)

t2 =Secondary angular velocity (2f2)

L1= Primary inductance in Henriesr1= Primary dc resistance in Ohms

L2= Secondary inductance in Henries

r2=Secondary dc resistance in Ohms

The loaded quality factor is:

(6)

For maximum efficiency:

Tune the link to the secondary resonant frequency

Set the effective load resistance to:

(7)

To maximize kQ (figure of merit):

Q is called the system quality factor. It should be noted

that a low magnetic coupling can be compensated by an

increased system quality factor.

WIRELESS POWERtransmission

Quality factor (D47x1)

Q

350

300

250

200

150

100

50

0

Frequency (kHz)0 100 200 300 400

Q

350

300

250

200

150

100

50

0

Frequency (kHz)0 100 200 300 400

Q

350

300

250

200

150

100

50

0

Frequency (kHz)0 100 200 300 400

D47 x 1-24

Quality factor (P40x5) Quality factor (P52x6.75)

P40 x 5-65P40 x 5-10P40 x 5-15

P52 x 6.75-10P52 x 6.75-20P52 x 6.75-28


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WIRELESS POWERtransmission

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Fig. 2 plots the optimum link

efficiency as a function of the figure

of merit for the coils, which is doptvs. kQ.

One way to manage a high k value

is to use a ferromagnetic shield (core)

enhances the magnetic coupling fac-

tor and reduces the stray magnetic

field. Fig. 3 shows the magnetic

field resulting from a ferromagnetic

shield with and without ferromag-netic shielding.

Another way to manage a high

efficiency inductive solution is maxi-

mize the Q or Quality factor for the

inductive coils. Table 1 lists typical

wireless power core from RRC and

Fig. 4 are plots of Q vs. Frequency

for these cores.

Generally, for any type of coil

type the magnetic coupling factor, k,

can be improved by reducing the ver-

tical distance between the coils andaligning the coils vertically as shown

in Fig. 5a, lateral misalignment (Fig.5b) and angular misalignment (Fig.

5c) degrade efficiency.

EVALUATION BOARD

To help designers evaluate the feasi-

bility of developing a wireless power

transmission system, RRC Power

Solutions Inc. has introduced a wire-

less power evaluation kit to allow

OEMs to become familiar with the

inductive coupling technology. The

kit (Fig. 6) consists of a 5W (5V,1A)

Qi compatible wireless power trans-

mitter and receiver in a self-contained

testing module. Users can remove

the board and coil from the module

housing and embed them into a

design for hands-on device evalua-

tion. RRC will provide detailed

instructions on embedding the kit

into a device at www.rrc-ps.com.

Therefore, optimum efficiency =

(8)

Fig. 6. Evaluation kit

consists of a 5W (5V,1A)

Qi-compatible wireless

power transmitter and

receiver in a self-con-

tained testing module.
http://cui.com/powerhttp://www.powerelectronics.com/http://www.rrc-ps.com/

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WIRELESS POWER - Minimizes Interconnection Problems