IEEE Power Electronics Magazine - December 2015 - 44

Parameters considered
To decide which ferrite shield material will become superior, we have considered several parameters, which are
shown in Table 2. Some of the parameters listed require
testing to determine their performance, such as quality
factor ^Q h, permeability ^n h, emissions, and efficiency
^h h. For the testing, we considered two sets of transmitters and receivers, each with prefractured and solid ferrite materials. The testing was done on Linear Technology's demo board number DC1969A, which uses LTC4120
"wireless power receiver and buck battery charger."

Some of the important electrical parameters considered
are given next.

Quality Factor
The quality factor is the ratio of reactive impedance to the
resistive impedance, or, in other words, the ratio of apparent power to the resistive power losses:
~L
Q= R ,
where R = Rac + Rdc, and, at a higher frequency,
Rac 22 Rdc.
Since the resistance is inversely proportional to Q, in
a low-power wireless charging coil, achieving very low dc
resistance is difficult. This is due to the demand for a thin
profile and sometimes high inductance. The Q value of the
transmitter used here for testing is 100 and 80 for the solidmaterial-based coil WE-WPCC 760308101302 and competitor's prefractured material based coil, respectively. The Q
value is 16 and 25 for receiver coils with solid and prefractured materials, respectively.

Permeability and Resistivity
FIG 5 The field concentration at the center of WE-WPCC
760308101110.

The permeability of the coils has a very significant effect on
the coil's performance as it affects the inductance of the coil.
The prefractured material will have a lower permeability
compared with the solid ferrite material due to its properties
and method of construction. The emission performance on
one prefractured-material-based coil, WE-WPCC
760308103202, and one solid-material-based coil, WE-WPCC
760308201, has been verified, and the result is described in
the "Emissions" section. The permeabilities of prefractured
and solid ferrite materials are 2,000 ! 25% and 2,400 ! 25%,
respectively, and the resistivities are >6.5 and 4, respectively.

Emissions

FIG 6 The WE-WPCC 760308101216 is a flexible coil with a
6-mm diameter.

Table 2. Test conditions and material combinations
-
for receiver and transmitter coils.
conditions

combinations
receiver
transmitter

Receiver material
compared with solid
transmitter
Receiver material
compared with flex
transmitter
Transmitter compared
with solid receiver

Solid
prefractured

Solid

Solid
prefractured

Prefractured

Solid

Solid
prefractured

Transmitter compared
with flex receiver

Prefractured

Solid
prefractured

IEEE PowEr ElEctronIcs MagazInE

z December 2015

Ferrite material used in the coil shields the electromagnetic
radiation. Due to its high permeability and low reluctance,
the coil confines the magnetic flux within its volume. Also,
the use of a ferrite magnetic shield protects other nearby
components, such as the battery, from heating and prevents
circulating current in the metallic parts. When the coils
were tested for electromagnetic emissions, the result
showed a significantly better performance for the prefractured-material-based coil than for solid ferrite (Figure 7).
With limited experiments, the reason for this performance can be best explained as follows: the power loss
per cubic volume in the prefractured material is about
17% higher, and, hence, the permeability is lowered by
17% at 100 °C when compared with solid ferrite; due to
this, the harmonics of fundamental frequency do not
gain enough field strength to radiate compared with
the radiation using solid ferrite. Therefore, the level of
emissions is lower compared with solid-ferrite-based
coils. The other reason could be that the solid ferrite
has a homogenous field, whereas the prefractured ferrite cuts the field into smaller fields due to the small

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2015