October 19, 2020
Guidance for RF Exposure Assessments of Portable Devices with Wireless Power Transfer (WPT) Capabilities
This notice provides guidance to ensure that RF exposure from WPT-enabled portable devices is properly assessed according to ISED’s rules.
Wireless charging is increasing in popularity among portable devices, such as tablets and smartphones. While early portable devices were only capable of receiving power, new devices are being released with the capability of charging accessories or other devices.
Most WPT implementations for portable devices operate between 100 kHz and 10 MHz. As a result, they must comply with the limits preventing both thermal and nerve stimulation (NS) effects.
2. Assessment Methods
WPT-enabled portable devices shall comply with the basic restrictions for internal electric field and specific absorption rate (SAR) to prevent NS and thermal effects, respectively. While the NS basic restriction is defined from 3 kHz to 10 MHz, SAR is defined from 100 kHz to 10 MHz. However, there are no commercially available systems to measure these quantities in the range of 100 kHz to 10 MHz. As a result, there are two options for assessing compliance of WPT-enabled portable devices:
- Computational assessment against the basic restrictions. The applicant shall consult with ISED for guidance in this regard.
- Both electric (E) and magnetic (H) field measurements for comparison with the SAR and NS-based reference levels summarized in Table 4 of RSS-102. If any of the reference levels are exceeded, a full computational assessment is required in accordance with (a).
It is important to note that WPT-enabled portable devices do not meet the condition for exemption from routine evaluation outlined in section 6.4.4 of RSS-216. In addition, they should not be treated as table-top devices, since charging may occur when the devices are in a body-worn configuration.
3. Separation Distances
For SAR compliance, WPT-enabled portable devices should be evaluated under body-worn conditions. In accordance with section 3.1.1 of RSS-102, when assessing SAR compliance, the maximum allowed separation distance is 15 mm.
Section 6.4 of SPR-002 states that NS compliance should be assessed at a separation distance which is logical based on normal usage conditions, and declared by the manufacturer in the user manual. In the case of WPT-enabled portable devices, the separation distance for NS compliance testing should also be 15 mm or less.
For WPT-enabled portable devices being considered for certification, the user manuals shall specify the separation distance when the WPT function is enabled, which shall not be larger than 15 mm.
4. Measurement probe
All field measurements should be performed using a probe which meets the requirements outlined in Section 5.2 of SPR-002 (additional clarifications can be found in Notice 2020 – DRS0012).
5. Test conditions
Applicants shall consult with ISED for guidance regarding computational assessments.
When performing field measurements for comparison with reference levels, the following guidance provided in section 6.4.4 of RSS-216 is relevant:
“RF exposure shall be evaluated with the client devices charged/powered by the source device at maximum output power. Additionally, all transmitters, including those not used for wireless power transfer, must be active simultaneously and at maximum power.”
In the context of NS, the relevant procedures outlined in SPR-002 should be followed.
The measurements should capture the highest fields to which a user may be exposed. This may require several measurements, taken from each side of the equipment under test.
6. Future considerations
ISED will be modernizing its guidance with respect to WPT devices. Computational assessments will be preferred in the near term; ISED's internal guidance will be further developed and generalized for this purpose. As internationally accepted assessment methods and measurement systems become available, they will be evaluated and incorporated into ISED's guidance accordingly.