Feature: Wireless power


Once the measurement setup is completed, and aſter choosing


the coil for measurement, the same coil is connected on the Tx and Rx


side.


We chose coil 760308101. Te Tx


using Equation 3: (3)


where, RL


ω0


= load resistance of the WPT system, k = measured coupling factor between Tx


= 2πf,


f = operation frequency of the WPT system, L0


= inductance of the receiver coil. Te operating frequency is set to 125kHz. Tis frequency


is used as the switching frequency for all our measurements, because it is an ISM frequency that is suitable for use in industrial, scientific and medical applications. Te Tx


and Rx side is turned on and the electronic load is calculated


and Rx


coil,


coils are aligned until a sine waveform appears


on the oscilloscope. Te RMS current for the coil observed on the oscilloscope is as shown in Figure 3. Te distance between the coils can be optimised until the desired operation point is reached. Te heat generated by the coil is measured using the infrared


thermal camera. Te temperature of the hottest point on the ferrite surface is measured by the camera and displayed on the FLIR camera program on the PC; see Figure 4. At the same time, the ambient temperature is measured using a handheld temperature measuring device. Te temperature steadily increases from the beginning of


the measurement until aſter a certain time period at which the temperature starts rising very slowly. At this stage, when the temperature rises 40K higher than the ambient temperature, the current value displayed on the oscilloscope is recorded as the AC rated current for that specific coil. In the case that the temperature rise from the start of the


measurement continues to rapidly increase without stabilising, then the RMS current on the oscilloscope can be adjusted by altering one of the mentioned parameters. Once the temperature rise stabilises, the AC rated current can be measured and recorded.


Simulation and results To validate some of the AC rated current measurement results, the Ansys Maxwell simulation soſtware was used with the ICEPAK module. Ansys Maxwell is a high-performance soſtware package that uses finite element analysis to solve electromagnetic problems. It calculates the electromagnetic fields by solving Maxwell’s equations in a finite region of space. ICEPAK is an Ansys module, which is a solver for the thermal management of electronic components using computational fluid dynamics. It is used to simulate the heating of a WPT coil at the surface of the ferrite at a certain AC current flowing through the coil.


Table 1: Simulated and measured values of rated AC current For 60 WE WPCC coils, the values for the AC rated current


were determined by measurement using the procedure described earlier.


Te measured values were verified with Equation 2. For nine


WPCC Qi coils, the measured and calculated value was also verified with the Ansys ICEPAK simulation program. In addition, the deration (the ratio of the AC to the DC rated current values) for each of the measured coils was found, and the deviation between the calculated and measured value of each coil was determined. Te overall deviation between the measured and calculated of the 60 WPT coils is about 6% and the overall


values IR(AC)


derating value is 0.8. Table 1 shows nine different coils with their measured (m.) or values and their verification by simulation.


calculated (c.) IR(AC) the measured or calculated values of IR(AC)


Te deviation between the simulated and the measured (m.) or calculated (c.) value is also shown in the table. Te table shows that the deviation of the simulated values of of the nine verified coils is on average only 4% compared to .


IR(AC) Te measured value of AC rated current here is slightly lower


than the DC rated current because of the proximity and skin effects.


AC rated current improves WPT design Finding the AC rated current at a certain frequency is very useful for engineers because they will know during the design stage the maximum AC RMS current the coil can handle. With these measurement results, we can now roughly approximate the AC rated current for any coil from the DC rated current value. Tis saves time and costs during the design stage.


www.electronicsworld.co.uk November 2025 25


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