Wireless Power Transfer and Wireless Charging
What is Wireless Power Transfer (WPT)?
Wireless Power Transfer, the process behind wireless charging bases and wireless charging devices, has forever changed how we think about ‘smart’ products. Today’s product designers are busy exploring exciting new applications for the technology, from cell phones (wireless Qi) to Electric Vehicles (EV). In the U.S. alone, the WPT market is expected to reach $12 billion by the end of 2022, with much of the demand coming from industrial and healthcare-related products. Due to a multitude of end-use possibilities and benefits for end users, such as convenience, reduced energy use, and prolonged battery, the consumer market will continue to drive demand for wireless charging.
Wireless Power Transfer takes several different forms, but the most common today is the use of magnetic resonance to provide a safe, flexible, and convenient option to electrically charge products. While some manufacturers are beginning to look at using higher frequencies for WPT, this article focuses on the devices that operate less than 1 MHz.
The radio frequency function of these wireless charging devices is the wireless transmission of energy between a wireless power source and a receiver. Because these devices are used in close contact with humans, product safety considerations are imperative. These devices must adhere to FCC guidelines for RF exposure. EMC compliance for wireless charging devices is also required due to the transmission of energy during use.
FCC guidelines for Radio Frequency (RF) exposure
The KDB 680106 of the FCC Guidelines states that devices specifically intended for use for wireless power transfer, or inductive charging, require FCC guidance for frequency exposure review. This includes Part 18 devices. It may be necessary for the responsible party (manufacturer) to seek support from the FCC on specific WPT devices by submitting a KDB inquiry. More information on this here.
How to proceed with FCC approvals
In the U.S, product manufacturers must seek guidance from the FCC by submitting a wireless charging application inquiry, unless the device meets the following requirements per KDB 680160 section 5 (outlined below). Medical devices are not covered under KDB 680160, so inquiries for these devices would be needed on a case-by-case basis.
(1) Power transfer frequency is less than 1 MHz.
(2) Output power from each primary coil is less than or equal to 15 watts.
(3) The transfer system includes only single primary and secondary coils. This includes charging systems that may have multiple primary coils and clients that are able to detect and allow coupling only between individual pairs of coils.
(4) Client device is placed directly in contact with the transmitter.
(5) Mobile exposure conditions only (portable exposure conditions are not covered by this exclusion).
(6) The aggregate H-field strengths at 15 cm surrounding the device and 20 cm above the top surface from all simultaneous transmitting coils are demonstrated to be less than 50% of the MPE limit.
The inquiry to the FCC should include the following:
- Complete product description, including coil diameters, number of turns and current
- FCC Rule Part(s) the device will operate under and the basis for selecting the Rule Part(s)
- Planned equipment authorization procedure (i.e., SDoC or certification)
- Drawings, illustrations
- Frequency of operation
- Radiated power
- Operating configurations
- Conditions for human exposure
RF exposure guidelines in Canada
Canada takes a different approach to RF exposure for wireless inductive charging devices, separating products and sub-components into three type categories per RSS-216:
- Type 1: Does not transmit any intelligence and does not require certification
- Type 2: Category II Radio - includes modulation on WPT frequency but is certification exempt
- Type 3: Category I Radio - requires certification
WPT devices must also comply with the Health Canada’s Safety Code 6 for RF exposure. When assessing compliance, all transmitters must be at maximum power. This includes those not used for power transfer.
EMC compliance for wireless charging devices
For the FCC, a radiator would need to be tested and certified under FCC part 15C (FCC 15.209, FCC 15.225, etc.) if it is intended for telecommunications applications involving the transmission of data. FCC Part 18 is allowed where no intelligence is transmitted, and RF is used to perform work heating, welding, treatment of materials, and charging. Only in specific cases, and by very limited means, is charge status (but not temperature) allowed to be transmitted via Part 18.
Requirements cited per KDB 680160 section 2a:
Systems that use load impedance changes, also called load modulation, on the client device at the fundamental transfer frequency for the sole purpose of load management may be authorized under Part 18. The load modulation must be integral to transfer system power management and control, and must be used only to the extent necessary to enable safe and efficient operation—such as rapid shut-down in response to over-voltage conditions, reporting of charging status, and identification of invalid devices.
For devices authorized under Part 18, such load modulation may not be used to communicate any other information, such as prioritization of devices for charging and the transfer of any other data, for example: extended system data, images or music. For such designs, both Part 15 and Part 18 requirements must be satisfied for equipment approval. Similarly, devices that use a 680106 D01 RF Exposure Wireless Charging App v03 Page 2 secondary frequency for load management, control and data functions must be authorized according to both Part 15 and Part 18 requirements, as appropriate.
For Canada, per RSS-216, devices without communication would be tested to ICES-001, which references CISPR 11 group 2. Devices with communication would be tested to the relevant product standard (RSS-210, RSS-310, etc.).
Similar to the US and Canada, a pure wireless power transfer system would not fall under the RED in the EU. See the definition in the RED guide. However, ANY form of communication, such as load modulation, would fall under the RED.
Products and applications that use electromagnetic waves exclusively for purposes other than radio communication and/or radiodetermination (products that propagate electromagnetic waves in space, but are not intended for the purpose of radio communication or radiodetermination) are not covered by the RED, for example:
- Inductive warming or heating appliances (including cookware)
- Pure wireless power transfer (excluding any communication or radiodetermination)
- High frequency surgical equipment
- Test equipment if intended to use radio waves exclusively for testing other devices
Generally, a device with communication would be tested to EN 303 417 or EN 300 330 along with the applicable EN 301 489 product standard. The intentional RF portion of the device without communication would fall under EN 55011 (CISPR 11 Group 2).
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