How Würth Elektronik Constructs Wireless Power Charging Coils

Now that we’ve provided a thorough overview of wireless power transfer and the compliance standards set by the Wireless Power Consortium (Qi) and the AirFuel Alliance, let’s discuss how our wireless power charging coils at Würth Elektronik are constructed -- either for Qi or any other power transfer application.

Construction of Thin Receiver Coils

Since the transmission coils are very basic, we won’t dwell on those. However, the thin receiver coils did take more engineering to find the best mechanical and electrical solutions, so let’s go over that construction process.

To construct the thin receiver coils, we used a pre-fractured, slightly flexible ferrite sheet, which we placed between two adhesive layers. We then added a layer of PET for increased shock and vibration performance and placed a copper coil on top. This construction ensures that if the receiver coil is dropped or chipped, the ferrite sheet will maintain its magnetic properties and still have a high rate of efficiency.

All Würth Elektronik wireless charging coils use Litz wire, regardless of whether they are transmitters or receivers. We’ll touch on that more later in this post.

Use of Shielding and Copper Wire

To determine the permeability levels we need for a Qi-compliant application, we simply go by the defined frequency set by Qi (100-205kHz); we typically use the 354 xx series. For other applications, we might use ferrite materials with higher permeability or ones that perform better at higher frequencies.

Outside of shielding, the other critical component for wireless charging coils is the copper wire. There are two main hurdles that need to be overcome in copper wires: the skin effect and the proximity effect. To combat the current density caused by the skin and proximity effects and achieve a high Q factor for wireless coils, Würth Elektronik uses Litz wire.

Litz wire is basically many thin wire strands, individually insulated and twisted together. This both shields each strand against each other and shields the entire cable unit together. For our transmit coils, we use 108 strands of Litz wire per cable; you can’t even define the individual strands within it. This gives our coils excellent performance and efficiency.

Qi only requires the use of Litz wire in general; it doesn’t specify which kind of Litz wire must be used. Würth Elektronik uses a higher-performance Litz wire to ensure the best efficiency possible. That gives us a very robust solution.

Coupling of Wireless Coils

The next most important aspect of wireless power transfer is coupling. We can’t overstate how critical coupling is to your design. Misalignment can quickly and completely undo a perfect circuit design for wireless coils.

The ideal coupling factor is 1, but that isn’t very achievable. We consider a ~0.9 factor to be very good, or an “optimized alignment.”

There are three main types of misalignment: lateral, angular, and vertical. Angular is possibly the most detrimental sort. We have seen a 40-degree angular misalignment reduce the coupling factor from 0.9 to 0.4 -- that’s a drop in efficiency of over 50% just from that one misalignment. To help take the guesswork out of the equation and pull coils into optimized alignment, Würth Elektronik adds permeate magnets in some of our wireless power coils.

Coupling efficiency is based on three factors:

  • Diameter ratio between transmitter and receiver coils (1:1 is ideal, with a maximum usable ratio of 1:6)
  • Displacement of receiver from transmitter
  • Coupling coefficient

Summary of Wireless Power Transfer Compliance

While compliance standards like Qi, Rezence, and PMA help if you want universal interoperability, you don’t have to be compliant with a standard to transmit power wirelessly. If you want to be able to control both your transmitting and your receiving sides, you have a lot of freedom in your wireless power design.

If you’d like specific help in creating that design, we have excellent FAE (field applications engineer) and product management teams here at Würth Elektronik . For more information, please reach out to us at wirelesspower@we-online.com.

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