Feature: Wireless power


Figure 1: (a) Cross-section of the physical model of a coil used, showing the temperature distribution for the ferrite rings; (b) Thermocouple configuration for measurement


temperature of the coil is expected; see Figure 1. 3. Measure the temperature of the coil (t1


temperature (ta1 ) and the ambient ) before applying direct current to the coil.


4. Te coil to be tested is supplied with direct current from a DC power supply. Aſter the temperature of the coil has become constant and has reached a plateau, the temperature of the coil under test (t2


) and the ambient temperature (ta2 again.


5. Te IEC 62024-2 standard states that the rate of change of temperature should be lower than 1°C/min for the coil to be considered in steady state. Tus, the temperature stability criterion is given as ∆T < 1°C/min. 6. Te value of the direct current applied must be increased step by step and the value of the temperature rise is then calculated using Equation 1:


(1)


where, t is the value of the temperature rise (°C), t1 t2


current is applied (°C), ta1


ta2 ) are measured


200W reference design from Würth Elektronik was used. Te measurement procedure is shown in Figure 2 as a block diagram. Te 200W design reference serves as transmitter and receiver ) board is used to convert the regulated


board. Te transmitter (Tx


DC current from the power supply to AC current through an inverter circuit, to conduct the AC current through the coil. Te Tx


board Mosfets are used in a full bridge inverter topology


and have a variable switching frequency range of 100-205kHz. Tis switching frequency is used to tune the circuit and find an optimum operating frequency at which the coils display maximum efficiency. Te receiver board (Rx


) has a rectifier circuit in a similar


topology as the transmitter, to convert back the AC to DC current. An electrical load is used on the receiver board. A current


AC current flowing through the coil, which is displayed on the oscilloscope. Te Tx


probe connected to the supply leads of the Tx and Rx


spacers. Te whole setup of the Tx Rx


is the initial value of the specimen temperature (°C), is the temperature of the specimen when a certain direct


is the initial ambient temperature (°C), is the ambient temperature when a certain direct current is


applied (°C). 7. Te DC current at which the temperature rise has reached the value of 40°C specified in the standard is found. Two consecutive temperature measurements are taken one minute apart, and verifying that they do not deviate more than 1°C. Based on this standardised measurement procedure, the


following novel measurement procedure has been developed to find the AC rated current at frequencies of 125kHz.


AC rated current measurement To verify the new measurement method, it must be designed as a standardised procedure, to maintain consistency during measurements for different coils. For this measurement, the


coils are separated using Plexiglas and Rx


boards with the Tx coils and current probe are placed in the glass enclosure as


mentioned earlier. Tis type enclosure is used for safety reasons and to reduce air flow. A cut out on top of the enclosure allows an infra-red thermal camera to be inserted, to measure the surface temperature of the ferrite of the Tx


coil. Te operation of the


infrared camera is controlled by a pre-installed camera soſtware on the PC. A handheld temperature measurement device is used to measure the ambient room temperature. Tere are different parameters that can be optimised to find coils and thereby


the required operation point for the Tx establishes the coupling coefficient;


• Te value of the load resistance; • Te switching frequency; • Te input voltage on the Tx


side;


• Te resonant circuit frequency. Te theoretical calculation of the AC rated current is given in Equation 2:


www.electronicsworld.co.uk November 2025 23 and Rx


influencing the variation in the AC current. • Te distance of separation between Tx


and Rx coils which and coil measures the


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44