IEEE Power Electronics Magazine - March 2018 - 32

Modified
Performance Factor (mT . MHz0.75)

Performance Factor (mT . MHz)

NiZn ferrites) and Ferroxcube (3F46
today, losses in the magnetic compoMnZn ferrite). Although these data
nents dominate over capacitor losses,
Losses in the magnetic
indicate the potential for substantial
motivating further consideration of
components dominate
improvements moving from 1 to 10
the losses in magnetic materials and
or 20 MHz, increased winding losses
inductive windings.
over capacitor losses,
at higher frequencies cancel some of
The contribution of a soft magmotivating further
the benefits. The performance facnetic material to the power-processtor can be modified to account for
ing capability of a transformer or
consideration of the
this. Although the appropriate modiinductor can be estimated using the
losses in magnetic
fication depends on the details of
product of peak flux density and frethe assumptions about the winding
quency at a specified loss density.
materials and inducdesign constraints, a broadly useThis performance factor has been
tive windings.
ful modified performance factor
measured for a wide range of materiis B·f 0.75 [8], as plotted in Figure 5(b).
als in [8]. Figure 5(a) shows the envelope of the performance-factor data
From this perspective, recent MnZn
[8] along with the envelope for commercial materials in a
ferrites operating at 1-2 MHz offer attractive performance
2013 data book [9] and estimates for recently introduced
that is almost as good as NiZn ferrites at 10-20 MHz. Howor improved materials from Fair-Rite (80 and 67 MnZn and
ever, this analysis also highlights the importance of highfrequency winding loss effects: if they could be effectively
mitigated in the 10-20 MHz range, the dramatic improvement in power density at higher frequencies predicted
by the data in Figure 5(a) would become practical.
200
Up to hundreds of kilohertz, the lowest winding loss is
usually achieved by using litz wire with strand diameters
150
much smaller than the electromagnetic skin depth d. With
this approach, litz wire can reduce ac winding losses by
100
more than 90% at 100 kHz, but, by 10 MHz, the finest commercially available litz strands (30-40 μm) have a much
smaller benefit [10]. Changes in the strand configuration
50
can help provide some benefits in this range [11], but more
substantial improvements will require a different approach.
0
One alternative is to use thin foil for conductors rather than
0.01
0.1
1
10
thin strands: economical foil thicknesses are much smaller
f (MHz)
than economical wire strand diameters. But making effec(a)
tive use of thin foil entails many challenges, including the
100
technical challenges of achieving equal current distribution
90
among many layers and avoiding lateral current crowding
80
as well as practical fabrication and termination challenges.
70
Some options for addressing these challenges are provided
in [12], but many questions remain regarding practical imple60
mentation and application.
50
One approach is to balance the currents between layers
40
using capacitance between layers as a ballast impedance,
while also using a magnetic core to shape the field lines
30
parallel to the foil layers to address lateral current distribu20
1
10
0.01
0.1
tion. This approach is particularly attractive where capacif (MHz)
tive impedance is needed for other purposes, such as in a
(b)
resonant circuit. It has been applied, e.g., to produce high-Q
resonators for wireless power transfer, resulting in a Q of
Commercial Materials 2013 [9]
1,180 at 7 MHz and an FoM six times higher than that of
Data from Hanson 2016 [8]
conventional wireless power transfer coils [13].
New Materials 2017
Similar resonant structures also show promise as passive components for resonant power-converter applicaFIG 5 (a) The performance factor B·f for available soft magtions. A prototype resonator constructed with commercial
netic materials based on a 500-mW/cm3 loss. (b) The modified
0.75
off-the-shelf ceramic capacitors was tested in [7]. At a size
performance B·f
for the same materials, modified to conof just 1.14 cm3, such a resonator would experience only
sider high-frequency winding loss effects.

IEEE PowEr ElEctronIcs MagazInE

z March 2018

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