The past year has seen an infiltration of wireless charging technologies in the consumer electronics space. A number of mobile phone operators are rushing to build wireless capabilities for their products as the promise of true, flexible wireless charging becomes a reality. As is often the case with new technology, one of the first questions raised is how safe is it for use by humans – in this situation: how safe is wireless charging? The post that follows presents our case as to the safety of our technology and specifically our own loosely coupled charging pad (Proxi Smartphone Solution) by independently measuring the level of emissions in relation to international safety guidelines for consumer electronic devices.
In order to measure the emissions level and respective safety of the product, we have looked to verify emissions in terms of the electric, magnetic and electromagnetic emissions compliance with the major American and European safety guidelines. In the United States of America, the Federal Communications Commission (FCC) mark is used to show safety compliance. In the European Union, the European Commission CE mark is used.
- The FCC and CE marking schemes each have their own standards for emissions safety. For a wireless power product to be legally sold in the United States of America, it must be compliant with the FCC regulations. For European Union countries and internationally, compliance with International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines is generally required.
PowerbyProxi’s inductive power transmitter (IPT) systems are designed to operate far into the near field, using magnetic loops to generate the magnetic flux field. Therefore, only magnetic field emissions need to be considered when determining emissions safety. Electric field emissions are very low due to the near field operation of these magnetic loops. Further, because PowerbyProxi’s wireless power systems are designed to operate so far into the near field, far field emissions – electromagnetic waves – are not relevant from a safety perspective, as this far field will only occur at distances of around 100m away from the transmitter, by which point the emissions will be very weak.
It is also important to note that testing of the Proxi Smartphone Solution was conducted by an independent tester – EMC Technologies Melbourne (an accredited FCC testing and CE testing laboratory). The maximum magnetic field strength observed was 0.28 A/m, or equivalently 0.352µT.
USA Emissions Safety Compliance
In the United States, electric, magnetic, electric and electromagnetic emission safety is demonstrated through compliance with FCC standards. The relevant FCC parts are covered in detail in this section.
Radiofrequency (RF) radiation exposure limits (FCC Part 1.1310)
The FCC Par 1.1310 RF exposure limits are given in Figure 1 below. The operating frequency of the Proxi Smartphone Solution is 285kHz – at the nearby frequency of 300kHz, the magnetic field exposure limit is 1.63 A/m.
Independent tests conducted on the Proxi Smartphone Solution showed that the system does not emit more than 0.28A/m field strength, which is approximately 83% under the FCC limits. Likewise the electric field strength for the Proxi Smartphone Solution is very low and is estimated to be around 0.066 V/m at 10cm distance, which is 99.99% under the FCC MPE limit.
Radiofrequency radiation exposure evaluation: portable devices (FCC Part 2.1093)
This measures the specific absorption rate (SAR) of radiation allowed into human tissue. In practice, measuring SAR at frequencies as low as 285kHz (the operating frequency of the Proxi Smartphone Solution), is challenging. This is because, for a given field strength, the SAR falls as frequency decreases and so the SAR is very low at 285kHz. The most stringent SAR limits on both the FCC and ICNIRP standards are the same, at 0.08W/kg for general public exposure. The Proxi Smartphone Solution is already compliant with ICNIRP reference levels by a wide margin, so it can be inferred that it will comply with the ICNIRP SAR limit of 0.08W/kg also.
The FCC regulation states:
European Union emissions safety compliance
In the European Union, magnetic, electric and electromagnetic emission safety is demonstrated through compliance with ICNIRP standards. The relevant ICNIRP documents are covered in detail in this section.
ICNIRP 1998 reference levels and basic restrictions
Most European Union members still use the ICNIRP 1998 standard for emissions safety, even though ICNIRP 2010 is the more recent standard.
Within ICNIRP 1998, there are two ways to check emissions safety: the reference levels and the basic restrictions. The reference levels, shown in Figure 3 below, are by far the more stringent of the two, and serve as a “quick check” that a system will be compliant. The reference levels are chosen by ICNIRP such that if a system passes the reference levels, it will definitely pass the basic restrictions. Compliance with the reference levels is relatively straightforward to measure as the reference levels are given in terms of E-field strength, H-field strength and B-field strength, at the frequencies relevant for wireless power systems. Measuring these quantities is relatively straightforward using standard test equipment.
As mentioned previously the electric field strength for the Proxi Smartphone system is very low and is estimated to be around 0.066 V/m at 10cm distance, which is 99.92% under the ICNIRP reference level limit. At 285kHz (the operating frequency of the Proxi Smartphone system), the maximum magnetic field strength allowed under the ICNIRP 1998 reference levels is 2.56A/m, and the maximum allowed flux density is 3.22µT. From the testing data alluded to earlier, the Proxi Smartphone system does not emit more than 0.28A/m field strength and just 0.35µT flux density, both of which are around 89% under the reference level limit.
If a wireless power system fails to pass the reference levels, the basic restrictions, shown in Figure 4 below, can be used to assess compliance. The quantities given in the basic restrictions cannot be easily measured and must be assessed using sophisticated numerical modelling techniques, taking into account a range of possible consumer use cases and human body models. While some of PowerbyProxi’s competitors in the consumer space are forced to fall back on these basic restrictions in order to prove safety compliance, PowerbyProxi’s system is designed with consumer safety in mind and as such passes the more stringent reference levels test by a wide margin.
ICNIRP 2010 reference levels and basic restrictions
At 285kHz, the reference levels for magnetic field strength and magnetic flux density of ICNIRP 2010 are significantly less stringent than those in ICNIRP 1998. In ICNIRP 2010 and tested at 285kHz, the maximum magnetic field strength is 21 A/m, and the maximum magnetic flux density is 27µT. Since the Proxi Smartphone system is compliant with ICNIRP 1998 reference levels, it will also be compliant with ICNIRP 2010 reference levels.
The ICNIRP 2010 reference levels and basic restrictions are given in Figures 5 & 6 below.
Figure 6: Basic restrictions for human exposure to time-varying electric and magnetic fiel
Tests have shown that the Proxi Smartphone Solution system is compliant with electric field, magnetic field and electromagnetic field emissions safety regulations, in both the US and Europe. Given that the FCC and ICNIRP regulations represent the benchmark for emissions safety testing, the Proxi Smartphone Solution is considered safe for use by the general public.