Key Factors for Wireless Power Charging Coils

In our previous post, we discussed the rise of wireless power and its various compliance standards. But what makes a wireless power solution truly effective?

In order to get the best possible solution for low loss energy transmission, a series of key factors are at play.

In this post, we’ll cover the six key factors for wireless power charging coils, including placement, coupling, skin effect, proximity effect, loss factor, and field pattern. We’ll wrap things up with a summary of the improvement and optimization of wireless charging coils.

1. Placement of the Coils

The transmitter and receiver coils need to be properly aligned to minimize losses. Lateral, canted, and vertical misalignments are particularly detrimental to energy transmission.

Good coupling and maximum energy transmission depends on the size of the effective area of the receiver coil in the magnetic field of the transmitter coil.

A coupling factor of 1 is ideal. This means the effective area of the transmitter and receiver coils is equal.

2. Coupling

To compensate for misalignment losses, it is important to design coils with high quality and coupling factors.

Wurth Electronics air coils with ferrite plates used as transmitters have a typical quality factor between 80 and 220. The resistance in the coils, as well as the ohmic resistance of the wire, are influenced by various factors.

3. Skin Effect

The displacement of electrical current from the center of a wire to its surface is called the skin effect. This phenomenon occurs in conductors which carry alternating current.

The skin effect depends on the frequency of the alternating current. When the frequency of the alternating current increases, the concentration of current on the surface of the conductor also increases. Therefore, high-frequency alternating currents displace, or “push,” the majority of the current to the surface. In such cases, the current density in the center of the conductor is lower than on its surface.

By using high-frequency wire strands in the transmitter and receiver coils, the energy losses from the skin effect can be significantly reduced.

High-frequency wires consist of thin wires braided in groups of two or more. Each of the wires in the group carries a portion of the total current. This helps minimize the skin effect, so more energy can be used effectively.

4. Proximity Effect

Another factor which influences the energy losses in the coils is the proximity effect. The proximity effect causes current constriction or current displacement in closely spaced conductors as a result of the magnetic flux leakage in each of the conductors.

Wire strand configuration, winding technology, and the structure of wire insulation can reduce unwanted eddy currents in the coils.

5. Loss Factor

Wireless energy transmission is limited by the loss factor in the system.

The main goal is to minimize the loss factor in the system as much as possible. If the transmitter and receiver coil configuration is optimized, a minimal loss factor can be achieved. However, the loss factor is still inherently affected by the quality factor and coupling factor of the system as a whole.

6. Field Pattern

Another important element which crucially influences the coils efficiency is the magnetic field pattern. The magnetic field pattern determines the unwanted radiation into the surroundings. Unwanted radiation into the surroundings has significant effect in system efficiency.

By using a suitable ferrite shield, the magnetic field in the inductive coupling is limited to the area between the coils; therefore, additional shielding measures are not necessary.

In addition, the Qi standard regulations prevent greater losses arising from misalignment of the coils. For example, if the energy transmission efficiency of the connection drops below 70%, then the power management system aborts transmission. Only when the alignment of the coils allows for energy transmission with an efficiency of >70% will energy transmission start.

Moreover, the geometries and materials of the transmitter coil are specified in detail in the Qi standard. Since the components (such as the transmitter coils, power management system, and chipset) need to comply with the standard, interoperability of the overall system can be guaranteed. Qi certification of devices involves independent test laboratories verifying interoperability of new products with previously certified devices.

There are currently many different designs stipulated in the Qi standard for the transmitter coils. These designs are divided into two classes:

  • One class consists of designs with ferrite plates and wound wire coils.
  • The other class consists of circuit boards with printed windings or hybrid solutions.

Within each class, the coils are classified by number of coils (single or array), size, shape, voltage, and the type of control mechanism (voltage, duty cycle, or frequency). There are also designs with a central permanent magnet in the coil. The permanent magnet is intended to automatically center very lightweight receivers on the transmitter coil.

A disadvantage of this alignment method is the permanent magnet in the magnetic field of the coil severely impacts the quality factor of the system, because eddy currents form within the magnet. These currents consume useful energy.

Improvement and Optimization of the Standard Coils

Those are the six main factors affecting the energy transmission of wireless power coils. To further improve and optimize the standard coils, a few measures can be taken.

The parasitic resistances in the coils can be minimized by using high-quality materials, designing efficient wire strand insulations, and optimizing winding technologies. Improvements in these areas ultimately increase the quality factor of the system.

Such efficient coils provide better performance in industrial or medical applications. Wurth Electronics offers a wide range of transmitter and receiver coils which meet the Qi requirements with a low RDC and a high quality factor. Check them out!

Summary of Wireless Power Transmission

Wireless energy transmission is most effective when it complies with a clearly defined standard.

In particular, in the Qi standard, there are various ways to positively influence the energy transmission of the coils. The selection of optimized coils, careful system design, and clear guidance for the user in placing the device on the transmission station can all constitute a competitive advantage for the device manufacturer.

From the standardization body perspective, better optimization definitions are a work in progress which should lead to an improvement in the performance of the next system generation. Nevertheless, the transmitter and receiver coils remain the key components in wireless energy transmission and are essential for the overall system efficiency.

More Wireless Power Charging Resources

Want to learn more about wireless power? You’re in the right place!

Check out our many other wireless charging resources, including: