Although the technology for wireless charging has been around since Nikola Tesla demonstrated it in the late 19th century, its use has only become popular fairly recently. Due to this popularity, as of April 2021, the FCC announced that for near field charging devices operating at 13.56 MHz, RF testing services are now the preferred method for showing compliance. These devices were previously excluded from physical testing for SAR compliance. 

In our effort to keep up with the latest updates to SAR requirements and regulations, RF Exposure Lab has now received equipment that allows us to test near field charging devices at 13.56 MHz. 

What are Wireless Charging Systems and Wireless Power Transfer

Wireless charging systems, also known as wireless power transfer (WPT), are becoming an increasingly popular tool for charging the wireless devices we rely on. WPT is a method used to move electrical energy between a charger and the device being charged. This is most commonly applied to phone charging, but WPT has a variety of applications, like electric vehicles.

One of the main advantages of wireless charging is that they significantly reduce the number of batteries and wires that we need in our daily lives. This improves convenience and mobility for users, and as more applications for this technology are developed, WPT will provide even greater convenience for users. However, to prove that these devices are safe for consumers to use, RF testing services are required to prove compliance.

FCC Requirements for Near Field Charging

The FCC had previously stated that wireless battery chargers and wireless power pads that operate at frequencies above 9 kHz qualified as intentional radiators. This made them subject to either Part 15 or Part 18 of the FCC’s rules for RF exposure.

Whether the device is subject to Part 15 or Part 18 depends on how the device is operated as well as whether there is communication between the charger and the device being charged. For example, devices that broadcast information are required to be certified under Part 15. In contrast, a device that operates in a charging mode, as well as a communications mode, may be approved under both Part 18 and Part 15.

However, regardless of what rules the device is subject to, the FCC requires an appropriate RF exposure evaluation.

 

What is Near Field Charging?

The most common technology for wireless charging uses an electromagnetic field between two copper coils, but this requires the device to be close to the charging pad. 

There are three types of wireless charging devices:

• Charging pads, which use tightly coupled electromagnetic inductive or non-radiative charging.
• Charging bowls or through-surface type chargers, which loosely couple or use radiative electromagnetic resonant charging that is capable of transmitting a charge within a few centimeters.
• Uncoupled radio frequency (RF) wireless charging, which is able to transmit a trickle charge at distances of many feet.

Tightly coupled and loosely coupled devices operate on the principle that a time-varying magnetic field induces a current in a closed loop of wire.

Here’s how it works: a copper coil, or magnetic loop antenna, is used to generate an oscillating magnetic field, which then generates a current in receiver antennas. If the proper capacitance is added to make the loops resonate at the same frequency, the amount of current in the receivers will increase. This is known as inductive charging or magnetic resonance, and it enables power to be transmitted at large distances between the transmitter and receiver.

The size of the coil also influences how far this power can be transmitted. The larger the coil or the more coils are contained in the device, the greater the distance at which power can be transmitted.

Uncoupled radio frequency wireless charging devices allow a mobile device to be charged without it needing to be put down in a specific spot. They work similarly to wireless routers, sending out RF signals that are received by enabled mobile devices. These devices are made up of a small RF antenna, an application-specific integrated circuit, and software. 

Testing for Wireless Charging Devices

RF testing is usually performed on wireless power transfer systems that operate at or below 13.567 MHz and are used less than 20 cm away from the body. During measurement, E- and H-field values are measured from all directions of the device in the reactive near field, radiating near field, and far field. Because WPT devices are typically designed for operation at the reactive near field, they have a high transmitting efficiency at small separation distances. 

RF Testing Services for Wireless Charging Devices at RF Exposure Lab

At RF Exposure Lab, we are always working to stay up to date on the latest developments in SAR regulations so that you don’t have to. We also go above and beyond to make sure our clients understand what we do and assist them in understanding how FCC regulations affect the testing required for your device. We strive to be as communicative and transparent as possible throughout the RF testing process so you always know what’s going on with your product and are never left in the dark.

We offer RF testing services for a variety of wireless devices including cell phones, laptops and tablets, medical products, survey equipment, wireless modems, millimeter wave devices, and near field charging devices. If you’re looking for SAR testing help or have any questions about SAR testing for near field charging devices, please contact us.