Instrumentation & Measurement Magazine 23-4 - 72

Fig. 8. Transmission efficiency with different offset angle.
Fig. 6. Resonant wireless charging prototype.

respectively. Node F is the input square voltage signal from
signal generator. As the system operates at a high frequency of
13.56 MHz, all nodes adopt the Sub Miniature version A type
(SMA) pins.

Power Transmission Efficiency Test
The two coils are placed in parallel (Fig. 6). When the signal
source inputs a sinusoidal signal with a value of 15 Vpp, the
transmission efficiency is measured with each coil followed by
the same load resistor of 50 Ω. The measured transmission efficiency with different coils distance is shown in Fig. 7.
In Fig. 7, the distance between the coils is changed from 10
mm to 70 mm with the inner radius of 20 mm, 40 mm and 50
mm, respectively. The coil with the larger radius has higher
transmission efficiency at the same distance. But, the relationship between transmission efficiency and distance is not
monotonous, and transmission efficiency will approximate
the peak with the distance of 20 mm.When the distance is 20
mm and the coil's inner radius is 50 mm the transmission efficiency achieves the maximum value about 79.3% which is
lower to theoretical calculations of 82%, due to the parasitic capacitors and resistors on the mounted board. Once the distance
surpasses 20 mm, the efficiency starts to decrease, eventually
to half of the initial value.

two coils is set to 25 mm. The transmission efficiency is obtained by changing the offset angle of the two coils, as shown
in Fig. 8.
When the offset angle changes from 0° to 90° (Fig. 8), the
larger the coil offset angle, the lower the transmission efficiency of the system. When the coil offset angle is 90°, the
transmission efficiency is 0. At this time, the direction of the
magnetic field is parallel to the receiving coil, and no magnetic
flux passes. Fig. 8 verifies electromagnetic theory: in order to
maximize the transmission efficiency of the system, the angle
is set to 0°.

Receiver Node Voltage
When the input voltage of rectifier (also the voltage on receiving coil) is gradually increased, so do the corresponding
output voltage of rectifier and LDO (Fig. 9). When the input
voltage is changed from 0 to 20 V, the output voltage of rectifier is increased linearly with input voltage, due to the 2*VON
voltage drop of the rectifier. The LDO did not work until the
output voltage of rectifier is 3.74 V, which is the turn-on voltage of LDO. The output voltage of LDO can be stabilized at
3.34 V when the input voltage varies from 7 V to 20 V and a distance of 70 mm.

Dynamic Load Measurement
Offset Angle Analysis
The coils with inner diameter of 20 mm, 40 mm and 50 mm
send a sinusoidal signal with a value of 20 Vpp and a frequency of 13.56 MHz. The horizontal distance between the

A signal generator is used to generate the voltage ramp. A 5 V
square wave is injected into OPA, and a 5 mA current flows to
ground through a given 1 KΩ resistor. The transient waveform
of the LDO is shown in Fig. 10.

Fig. 7. Transmission efficiency with different coils distance.

Fig. 9. Output voltage of LDO and Rectifier.

IEEE Instrumentation & Measurement Magazine

June 2020

Instrumentation & Measurement Magazine 23-4

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 23-4