IEEE Circuits and Systems Magazine - Q3 2019 - 25

The concept of the simultaneous power and data transmission is shown as a schematic overview in Fig. 1. The
primary as well as the secondary side each has a block for
the power electronic and the communication electronic.
They are connected to the associated power coil (L 1 or
L 2) and described in section III and IV, respectively. The
communication concept is outlined in section V.
For the compensation of the IPT system a double
sided serial compensation topology is chosen, because
it is easy to control and the output voltage is equal to
the input voltage. This means on each side a compensation capacitor (C 1 and C 2) is connected in series to the
power coil, which can be seen in Fig. 3. This ensures a
defined minimum load impedance for the communication electronics.
As transmission coils two 10 nH double-layer charging coils from Wurth Elektronik are chosen. With an air
gap of 2.5 mm, which relates to a standard IPT use case.
The primary as well as secondary inductor can be measured to L 1 = L 2 =14.0 nH and the leakage inductances
to L 1v = L 2v = 3.2 nH. This results in a mutual inductivity
M of 10.8 nH and a coupling factor k of 0.77.
With first harmonic approximation, which depends
on k, the normalized output voltage M O and the phase
between the normalized input voltage M i and current
J i can be calculated. The results are shown in Fig. 2 for
different normalized loads Q L [47].

f0
1
=
2r L 1/2 C 1/2 (1 + k)
(1 + k)

(2)

To provide a similar communication channel in both
cases, this frequency has to be outside of the communication band. Furthermore, for the communication
system, the switching frequency of the IPT should be
as high as possible to allow a better suppression and
a higher and wider frequency band for the data link.
THIRD QUARTER 2019

(3)

This leads to a interference resonance frequency of
202 kHz. In order to reach a similar channel attenuation
with IPT on and off it is necessary to keep a reasonable

Power
Electronic

−

Power
Electronic

(1)

The horizontal axes show the normalized frequency,
which is the ratio of the inverter switching frequency fs
to the characteristic frequency f0 at which the system
is fully compensated. For a ratio of about 2, the system
reaches a point of constant output voltage independent
of the load. An inductive behavior (phase greater than
zero) is achieved as well, which ensures Zero-VoltageSwitching (ZVS) [47].
The communication channel is changing depending
on whether or not the IPT is active. This is because of
the switched inverter and rectifier. In order to calculate
the interference resonance frequency fz for symmetric
coils, equation (2) was derived.
fz =

1
2r L 1/2 C 1/2

Comm.
Electronic

Figure 1. Schematic overview of the system.

100
50
0
-50
-100

0 1 2 3 4 5
Normalized Frequency

Normalized Output Voltage

C2
L2

f0 =

Phase (°)

Q L = 82 R L

For the IPT system, a higher frequency has multiple impacts. A advantage is, that the system becomes smaller
which saves assembly space. Also the input and output
current has less ripple and the dynamic range rises so
the system can be regulated faster. Disadvantage is, that
the losses in the core and in the MOSFETs rise with the
frequency. Furthermore, the litz wire must be finer, the
voltage rating of the compensation capacitors drops
and suitable drivers for the frequency are needed.
Consequently, the switching frequency is chosen to
500 kHz and the characteristic frequency to 269 kHz.
This offers a good trade-off between a wide frequency
band for the data communication and a reasonable frequency for the power transmission. To archieve this frequency, the capacity compensation capacitors have to
be choosen by equation (3) to 25 nF [47].

3
2
1
0

0 1 2 3 4 5
Normalized Frequency

(a)
QL = 0.5

QL = 1

(b)
QL = 1.5

QL = 2

QL = 3

Figure 2. The (a) phase and (b) output voltage of the first
harmonic approximation.

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IEEE Circuits and Systems Magazine - Q3 2019

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