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THE 10
th
INTERNATIONAL SYMPOSIUM ON ADVANCED TOPICS IN ELECTRICAL ENGINEERING
March 23-25, 2017
Bucharest, Romania
Using WEBENCH Coil Designer for Wireless
Power Transfer coil generation
Catalin Bibirica 1, PHD student, Sandu Cristian2, PHD student, Lucian Ene3,PHD student,
prof. dr. ing. Mihai Iordache4, IEEE member
bibirica.teodor@yahoo.com
,
sanducristian06@gmail.com, ene.lucianupb@yahoo.com, mihai.iordache.44@gmail.com
Abstract- This paper describes the use of WEBENCH Coil
Designer for generating wireless power transfer PCB inductors
for low and medium power portable systems without the need of
complex calculations or complex layout design.
Keywords: WEBENCH, coil, designer, wireless, power, transfer,
inductor, generator
I. I
NTRODUCTION
In the past few years, there has been a particular emphasis
on wireless power charging, from charging mobile phones to
charging medical [1] and industrial equipment. While the
typical solution is widely accepted, using copper strands to
form coil inductors, the necessity for cheaper and easier to
implement inductors has increased the use of PCB (Printed
Circuit Board) coil inductors.
While these PCB inductors add little to no additional
production cost, they are sometimes hard to implement due to
the numerous difficulties engineers face while designing such
inductors, mainly PCB layout and coil parameters
calculations.
This paper addresses those issues by using WEBENCH
Coil Designer, a free tool from Texas Instruments (TI), [2], to
design, calculate and export the desired PCB inductor so it
can be directly used in the final production of PCB for WPT
(Wireless Power Transfer) applications. This procedure is
very useful for designing magnetic coupled coils that are used
for recharging medical implants batteries.
II. PCB
VS
.
C
OIL
I
NDUCTORS
Coil inductors have been used initially because they were
relatively easy to produce, have good power transfer
efficiency and the space they occupy was not an issue.
They are made from litz copper wire isolated with varnish,
paper or fibers with or without iron core which made them
resistant to heat and the environment but their properties, like
inductance, L, resistance, R, and quality factor, Q, were not
strictly defined.
In modern systems, this leads to variations in the oscillating
frequency of the LC main oscillator that, in turn, changes the
wireless power transfer frequency and may lead to an audible
buzzing of the WPT system or a decrease in the efficiency of
the WPT system. While manufacturing more strictly
controlled inductors is possible, the new inductors become
more expensive and occupy more space.Switching to PCB
inductors is more cost effective and creates more stable
inductors with precise electrical parameters and good
repeatability.
Those inductors are made by creating circular patterns on
the copper layer using a layout software (such as Altium
Designer). This pattern is electrically connected to the rest of
the circuit and when electrical current passes through it, it
acts as a traditional inductor. Because of the limited number
of available layers (usually two layers, but it can go up to 8 or
more), the actual inductance is less than in the case of a
traditional inductor. Modern systems compensate this by
increasing the transmission frequency.
These types of inductors have to be greatly simulated in a
specialized simulation program (such as ANSYS Maxwell,
[3]) with great results but with additional cost and time.
So, to minimize the cost and design time for new products,
the use of a specialized tool, such as WEBENCH Coil
Designer, is recommended for new and experienced
engineers.
III. WEBENCH
C
OIL
D
ESIGNER OVERVIEW AND USAGE
WEBENCH Coil Designer is a free online tool that can be
used by anyone designing PCB inductors for different
applications. This tool was designed to generate PCB coil
sensors for different TI sensor devices [3] [4] but this paper
will describe an alternative use: generating low and medium
power inductors with a maximum radius of 149mm.
This tool is composed of (fig. 1):
1. LDC part selection
2. Coil type selection
3. Output graph
4. Coil geometry and other parameters selection
5. Design export
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In order for any designer to use the WEBENCH Coil
Designer tool, the following steps should be followed.
A. Select LDC part
The first step in using WEBENCH Coil Designer is to
select "Custom" for the type of inductor needed. This is
always necessary because it will allow for free selection of all
possible parameters (Fig. 2).
The voltage range shown by the tool only applies to the
TI's LDC sensors and can be ignored. The operating
temperature range represents the minimum and maximum
temperature at which the inductor can operate safely without
decrease in reliability.
B. Select Coil Type
At this step, the coil type must be selected from 4 possible
types: circular, square, hexagonal and octagonal. Each
inductor type offers advantages and disadvantages like bigger
inductance value for square inductors and smaller occupied
space for the others.
For testing purposes, a round inductor was designed,
created on a PCB and the resulting inductance and resistance
was tested with an RLC meter.
Fig. 3 shows the basic geometrical parameters of the
selected inductor: the width of the trace (W), the spacing
between traces (S), the inner diameter (Din) and the outer
diameter (Dout) of the coil. Those parameters will be manually
introduced in the next step
C. Select coil geometry and other parameters
Carefully selecting the write parameters for the designed
coil is an important factor for a successful WPT design. For
example, if the WPT is done with strongly coupled resonant
inductors, it is important that both the transmitter and receiver
inductors are identical. This will allow for the best match of
magnetic fields when the coils are close together and will
increase efficiency.
Both Metric and Imperial measuring standards are
supported.
The selectable parameters include the LC sensor
capacitance (C) that has a maximum value of 10nF, too little
to achieve lower resonant frequencies (like 100kHz for the QI
specifications [5]) and small form factor inductors. So, for
WPT inductors, the RL capacitor value can be ignored.
Unfortunately, this will cause some resulting parameters, like
resonant frequency and quality factor, to not be accurate.
However, some of those parameters can be calculated using
simple formulas.
Depending on the size of the final product, the maximum
possible size of inductor should be used. The tool will only
allow for a maximum size of 149.86mm [2], a big enough
diameter for medium power portable devices (like tables).
Fig. 2. Step 1: Always select "Custom" for the any type of wireless powe
r
transfer inductor needed
Fig. 3. Step 2: Selecting coil type from the drop down top menu. Roun
d
inductors look similar to the traditional inductors but square inductors offe
r
the biggest inductors in the same square space.
Fig. 1. WEBENCH Coil Designer overview showing the main components
(and steps) of this tool: 1. Select LDC Part, 2. Select Coil Type, 3. Outpu
t
Graph, 4. Select Coil Geometry and Other Parameters, 5. Export Design.
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With the outer diameter selected, the number of layers must
be selected. This will generally be limited by the number of
layers the rest of the circuit requires (if the inductor is on the
same PCB) or limited from 1 to 8 (if any number of layers is
acceptable). The most common number of layers in 2 (it's the
best value for money).
The number of turns per layer and trace width will
determine the power capability of the inductor. The biggest
selectable trace width is 1.016mm, [2], and the number of
turns per layer will determine the DC resistance.
The spacing between traces is determined, in general, by
the minimum distance the manufacturer can produce, but a
0.3mm spacing can usually be manufactured by most PCB
producers.
Copper thickness is usually limited by the rest of the PCB
or by the manufacturer, but the thickest copper possible
should be selected.
The final selectable parameter is the operating temperature.
While selecting this, keep in mind the ambient temperature
where the inductor will be kept and the self-heating generated
by losses in the coil.
Fig. 4 shows all the parameters that can be selected.
D. Output parameters
After all desired geometrical parameters have been
inserted, the output parameter box can be studied. Here, the
default results are: Total Inductance, AC resistance, Coil fill
ratio and Coil inner diameter (Fig. 5). There are two other
parameters (Sensor frequency and Q factor) that are shown
based on the selected capacitor, but for low frequency WPT
circuits, those values have to be calculated.
Some parameters are only shown by clicking the
"View more" button under the main parameters. The
hidden parameters are not crucial for the functionality
of the WPT system, but may be of interest to some
engineers.
The most important parameters that cannot be
determined by the WEBENCH Coil Designer for lower
frequency applications are the LC oscillator capacitor
and quality factor, Q, based on a fixed frequency.
Assuming the frequency, f, is fixed (to align to a
predefined standard, like the QI standard [5]), the
capacitance needed can be calculated using (1).
(1)
The quality factor, Q, can also be calculated using a simple
formula (2):
(2)
1
∙4∙
∙
2
Fig. 4. Selectable coil geometry and other parameters
Fig. 6. All available output parameters returned by the WEBENCH Coil
Design tool. Some parameters, like Current and Power dissipation are baste
d
completely on the basis that the inductor is destined for a sensor coil an
d
should be ignored.
Fig. 5. Basic output parameters returned by the WEBENCH Coil Designe
r
tool. Sensor (oscillator) frequency and quality factor (Q) cannot be used due
to the limitation of the maximum oscillator size and must be manuall
y
calculated.
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Equations (1) and (2) are based on the coil inductance, L,
and coil resistance, R.
This quality factor, Q, can be used to compare different
types and inductor sizes. The bigger the quality factor is, the
more efficient the inductor will be and the power loss will
decrease [6].
Another way to analyze the newly created inductor is using
the provided graphs. WEBENCH Coil Designer allows for a
multitude of parameters that can be used on the Y-axis and Y-
axis.
Fig. 7 shows an example on how to chose the inductor with
the desired capacitance by consulting the Total inductance
graph by the number of layers and inductor type. The
designer can chose, for example, a square inductor of 100µH
using 4 layers or any other type of inductor by increasing the
number of layers or by increasing the diameter.
E. Export design
The final step in using WEBENCH Coil Designer is
exporting to the preferred design program. The options are:
Altium Designer, Cadence Allegro, CadSoft EAGLE PCB,
DesignSpark PCB and Mento Graphics PADS PCB.
The export functionality has a few limitations. The first and
major limitation is the number of layers that can be exported:
2 and 4. This can easily be bypassed from the PCB design
program, where more layers can be copied or deleted.
Another limitation is that, when exporting, a free account is
required.
IV. E
XPERIMENTAL RESULTS
In order to test and validate the WEBENCH Coil Designer
tool, some inductors were manufactured with the manual
toner transfer method.
The first inductor was designed for low power applications
at high frequency (Fig. 8). In this case, the online tool was
able to calculate the resulting inductance as well as the
quality factor, AC resistance and oscillator frequency (the
chosen parameters were sufficiently high so that no manual
parameter calculations were needed).
The inductor was tested using a push-pull transistor circuit
controlled by a square ware signal generator. The frequency
of the signal was varied from 500kHz to 1MHz and the
oscillator node voltage was observed and measured with an
oscilloscope. The maximum voltage amplitude oscillation
was observed for 760kHz, a value close to the predicted
740kHz, thus validating the initial prediction.
Another reason why this inductor was made was for testing
the accuracy and shape of the automatically generated
inductor. In this case, a round inductor was chosen because of
the usual difficulty in designing those types of inductors and
the trace width and spacing was as small as most PCB
manufacturers will design at a reasonable cost.
The resulting inductor, although round in the overall shape,
is made out of straight lines 0.305mm thick with the
0.305mm spacing kept constant.
This coil design was also tested under load, and a
maximum of ~0.5W was transferred, but with low efficiency
caused by the inductor's high resistance (AC and DC).
Another inductor was designed for bigger wireless power
transfer (Fig. 9). Because it was designed to work at a
frequency of 100KHz (QI frequency standard, [5]) with the
same outer diameter as the previous one, the WEBENCH
Coil Designer was only used for determining the inductance
and resistance.
This inductor had the traces as thick as possible (1mm)
with a small spacing between them of 0.3mm.
The inductance and resistance of this coil were measured
using a RLC meter and the results were again similar with the
predicted values.
The inductance measured value of 5.53µH was close to the
theoretical value of 5.525µH (with an error of 0.09%). The
measured value for the resistance was 1.02 Ω (including DC
and AC resistance measured at 200kHz) was also close to the
predicted value (considering the measured frequency).
Fig. 7. Example of possible graph generated by the WEBENCH Coil
Designer tool. Using this visual representation, design engineers can
optimize the WPT inductor according to their needs and constraints.
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To test the power transfer capabilities of this inductor, the
same push-pull transistor circuit was used at a frequency of
100kHz. A maximum output power of 2.5W was transferred
successfully without any major heating problems on the
receiver side and minor heating on the transmitter side (2
identical inductors were made, one for the receiver and one
for the transmitter system).
The final inductor was designed so that the online tool
would be able to display the results at 200kHz. This meant
that the inductor had to be big (100mm x 100mm) and had the
maximum inductance possible (so a square type was chosen)
(fig. 10).
With the inductance of 62.681µH and 10nF capacitor, the
resonant frequency was calculated to be 200.692kHz.
However, the toner transfer method did not yield good
results. The small spacing between turns of 0.2mm was too
small and short-circuits between turns was inevitable.
However, this exercise has proven that an industrial
manufactured PCB inductor should be used for future coil
production.
V. C
ONCLUSION
This paper described the use of WEBENCH Coil Designer
for creating PCB coil for wireless power transfer applications
without the need of complicated simulation programs.
Besides giving the most used parameters of the designed
inductor, this tool also exports the new inductor to a number
of PCB layout programs thus eliminating the humar error part
of creating PCB inductors.
The WEBENCH Coil Designer tool is not perfect but,
because of it's simplicity in operation, it can be used along
with other, more precise programs to offer a complete design
and layout solution for new wireless power charged devices.
In the future, the problem with low inductance values will
be adressed by increasing the number of layers and adding
ferite sheets on the PCB coil.
R
EFERENCES
[1] Uei-Ming Jow, Maysam Ghovanlo, “Design and Optimization of
Printed Spiral Coils for Efficient Transcutaneous Inductive Power
Transmission“, September 2007.
[2] https://webench.ti.com/wb5/LDC/#/spirals
[3] Ansys Maxwell Application Brief, “Wireless Power Transfer“, 2012.
[4] Texas Instruments Application Report, “LDC Sensor Design“, March
2015.
[5] Wireless Power Consortium, “The QI Wireless Power Transfer System
Power Class 0 Specification, Part 4: Reference Designs“, version 1.2.2,
April 2016.
[6] https://www.wirelesspowerconsortium.com/technology/quality-
factor.html
Fig. 9. Wireless power transfer inductor made with the WEBENCH
Coil Designer online tool using the manual toner transfer method fo
r
low power wireless transfer systems.
Fig. 10. Wireless power transfer inductor made with the WEBENCH
Coil Designer online tool using the manual toner transfer method fo
r
medium power wireless transfer systems.
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