Feature: Wireless power
Since there is no known standard procedure to measure the AC rated current of a WPT coil, this measurement procedure is the fi rst of its kind
Designing a wireless power transfer system with AC rated current
By Deepthi Poonacha, Design Engineer in the Custom Magnetics department at Würth Elektronik, and Christian Merz, Design Engineer and Technical Lead, Wireless Power Transfer at Würth Elektronik
W
e are increasingly seeing wireless power coils used to charge not only consumer electronics, like smartphones and watches, but also electric cars, medical devices such as implants and wearables, and industrial systems such as sensors
and mobile robots. Here, a transmitter and receiver coils are used for the transfer of energy via a magnetic fi eld, a process known as electromagnetic induction. One of the most important parameters for the electrical characterisation of these wireless power coils is the rated current,
22 November 2025
www.electronicsworld.co.uk
which indicates the maximum at which the coil can be operated. Other important characteristics of a coil are its inductance, quality factor and ohmic resistance. T e rated current of WPT coils has always been specifi ed as a
direct current (DC) value in datasheets. However, here we will present a new measurement method that accurately determines the rated current of wireless coils.
Rated current T e rated current is defi ned as the current at which the temperature of a WPT coil has increased by 40K in comparison to the ambient temperature. However, since the operational frequency range for wireless power applications is usually in the 100-200kHz range, the rated current values in terms of alternating currents (AC) are more realistic for users. T e principle of wireless power transfer technology is that an
AC current fl ows through the transmitter coil, which generates an alternating electromagnetic fi eld, inducing a current in the receiver and enabling power transfer over a short distance using the principle of inductive coupling. T us, it is important to determine the impact of AC current in a wireless coil. T e new measurement method can be used to fi nd AC rated
currents at 125kHz for wireless power coils. Since there is no known standard procedure to measure the AC rated current of a WPT coil, the measurement procedure is the fi rst of its kind.
Standardised DC measurement T e datasheets of all Würth Elektronik (WE) wireless power charging coil (WPCC) products defi ne an IR(DC)
value, which is
the DC rated current. T is value is found by the measurement procedure described here, and similar to the one outlined in the IEC 62024-2 standard for inductors. 1. T e coil under test is measured inside a cubic enclosure that is about 20cm long on each side, to prevent any temperature fl uctuations linked to air fl ow. T e enclosure may contain top openings to avoid heat build-up. 2. T e thermocouple must be positioned correctly for temperature measurement. It should be placed at the point where the maximum
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