1. What are wireless charging systems?
2. Where wireless charging systems can be used?
3. How wireless charging systems work?
   1. Inductive coupling
   2. Power classes for wireless charging systems
4. RF exposure testing of wireless charging systems
   1. Why is electromagnetic radiation measured?
   2. Standards for testing wireless charging systems in EU, USA and Canada
      1. Testing guidance for EU
      2. Testing standard for USA
      3. Testing standard for Canada
   3. EMF measurement procedure
5. Verkotan and wireless charging system testing

What are wireless charging systems?

Traditionally the charging of your phone, electric car or a laptop has been done with a cable plugged in to your device, connected to the mains power. However, you may have already used a wireless charging system, also known as wireless power transfer (WPT) system to charge your devices. Wireless charging systems transfer electrical energy without any physical wires between the charger and the device. This means you can, for example, charge your phone just by placing in top of the charging pad.

One of the advantages of wireless charging systems is that they can reduce the use of batteries and wires. This in turn will ease the mobility, safety and convenience for users, give future proof for products and more design options for product developers.  The market for wireless chargers for consumer devices is expected to grow in the future. Wireless charging systems require testing to ensure the safe use these devices.

Where wireless charging can be used?

There are many consumer devices that use wireless charging systems. Phones, Bluetooth speakers and electric toothbrushes to name a few. Also, for example, electric vehicles might have a wireless charging option. Wireless charging systems typically require quite a short distance to the device that is being charged. For example, a phone must be placed on top of the wireless charger for charging.

Wireless charging systems can be used in a variety of different applications:

• Portable and mobile devices: laptops, phones, headphones, TV’s
• Home appliances: electric toothbrushes, toasters, blenders, coffee makers
• Logistics application: industrial trucks
• Electric vehicles: cars, electric bikes
• Medical equipment: insulin pumps, defibrillators and hearing aids

Even though it is called wireless charging, the transmitter device (charging pad) needs to be plugged in to the mains power. However, it is called wireless charging because the receiver device (for example a phone) does not need to be plugged in anywhere.

How do wireless charging systems work?

But how do these systems work? In wireless power transfer systems, there is a transmitter device and a receiver device. When electric current passes the primary coil in the transmitter device, it produces a time-varying electromagnetic field. The time-varying electromagnetic field creates electric current in the secondary coil of the receiver device that is usually located close to the transmitter. The receiver device then delivers the electricity to the electrical load, for example to the phone or the car. For the wireless charging of a smartphone, the primary coil is built into the charger and secondary coil is built into the phone.

Wireless power transfer technologies can be divided into two categories: far field and near field.  However, the wireless charging systems that require testing are operating in the near field due to a small separation distance between the charger and the device in charging.

Inductive coupling

The most common technology used for power transfer in WPT systems is inductive coupling. Inductive coupling is usually used in WPT systems due to its high efficiency at short separation distances. Its operational frequency is between 85-205kHz.

In inductive coupling a transmitter and a receiver coil are used to transfer energy using magnetic fields (Etemadrezaei, 2018). Electromagnetic induction is based on Faraday’s Law on induction according to which electromotive force is created with changing magnetic field when magnetic field and electric conductor, such as a wire, move relative to one other. There are also capacitive coupling and non-capacitive coupling techniques, but they are not used as often as inductive coupling.

Qi wireless power transfer standard is the most common application for inductive charging and it is maintained by Wireless Power Consortium (WPC). Most of the wireless chargers we test at Verkotan comply with the Qi wireless power transfer standard. The Qi standard defines charging distances up to 4 cm and provides wireless charging of 5-15 watts of power to small electronic devices. WPC members have work in progress to increase the maximum charging power of Qi wireless power transfer standard up to 60 watts in the future, mainly for laptop and tablet charging. Products that have the Qi logo are Qi-Certified and they have been tested to show compatibility and safety.

Power classes for wireless charging systems

According to ETSI TR 103 493 V1.1.1 (2019-02), WPT systems are categorized in several power classes that define the maximum allowed transmission power for a wireless charger, as described in the table below. Power classes may also have an influence on the transmission frequency range.

Wireless charging of consumer devices usually belong to a power class P < 5 W or 5 W < P ≤ 31.5 W. For these power classes there are currently several different frequency bands where WPT systems are available according to ETSI TR 103 493 V1.1.1 (2019-02) as shown in the table below.

RF exposure testing of wireless charging systems

For RF exposure evaluations, EMF measurements of WPT systems are typically performed on low frequency devices that operate below 13.567 MHz and are intended to be used closer than 20 cm to the end user. In EMF testing, E- and H-field values are measured from all directions of the device with the required separation distance to make sure it is compliant.

EMF measurements can be done in three main zones: reactive near field, radiating near field and far field. Most WPT devices are designed to be operated at reactive near field, as it has a high transmitting efficiency at small separation distance.

The strength of the electric fields is possible to be measured with probes or sensors. These probes are basically antennas with different kinds of characteristics. Measurements are possible to do, for example, with a calibrated antenna that is exclusively sensitive to electric fields and are connected to measurement equipment. A 3-axis isotropic probe is the preferred option for EMF measurements, however a single axis probe can also be used if the measurements are repeated and the results are summed over three axis.

Measurement equipment for electric fields can generally be divided into two classes: broadband and frequency selective. A broadband measurement probe detects all signals within the frequency range supported by the measurement probe. Frequency selective probe makes it possible to measure values at the frequency range of interest instead of the whole frequency range supported.

Why is electromagnetic radiation measured?

Wireless power transfer systems, more commonly known as wireless chargers, emit electromagnetic radiation when they are being used, as all electrical devices do. WPT systems transmit electrical energy by electromagnetic fields. Electromagnetic fields consist of magnetic fields (typically called H-field) and electric fields (typically called E-field). The unit of the E-field is volts per meter (V/m) and the unit of the H-field is amperes per meter (A/m). WPT systems are tested to ensure the safe use of these devices and the devices on the market must comply with the requirements.

Public concern about the possibility of health risks resulting from exposure to electromagnetic fields (EMFs) emitted from various wireless devices and their infrastructure continues to be an issue all over the world. It is essential that the public is not exposed to too high levels of electromagnetic radiation. Thus, EMF measurements are performed to ensure a device’s compliance to international guidance.

Standards for testing WPT systems in EU, USA and Canada

There are international and regional RF exposure requirements that are scientifically defined to ensure the safe use of the devices. For the determination of product compliance, the measurement results are compared against reference levels. The reference levels are defined by different region’s authorities. The council of European Union determines the limits for EU, Innovation, Federal Communications Commission (FCC) for USA and Science and Economic Development Canada (ISED) for Canada. If a product is going to be sold in these areas, the product must comply with the limits determined by the region’s authorities.

Testing guidance for EU: New IEC Publicly Available Specification for testing of WPT systems

International Electrotechnical Commission (IEC) published a publicly available specification (PAS) considering the testing of wireless charging systems in May 2021. The new IEC PAS 63184:2021 specifies the assessment methods for evaluating the RF exposure compliance for dynamic and stationary wireless power transfer systems in the EU. The IEC PAS 63184:2021 covers frequencies from 1kHz to 30MHz. EN 62311:2008 that includes E and H field measurement assessment methods does not provide any specific guidance considering wireless power transfer systems.

The new IEC PAS 63184:2021 defines

• Measurement methods
• Assessment procedures
• Numerical assessment methods
• Assessment combining measurement and numerical methods

for the testing of wireless power transfer systems.

Before the publication of the new IEC PAS, there were some guidelines determined in an ITU-R report that described the non-definitive methodologies to assess the RF exposure of WPT systems. IEC has described an exposure assessment method of WPT’s for electric vehicles. However, there is no other product standard for other WPT systems.

Limits and measuring standards

WPT systems must comply with certain limits defined by the European Union for CE marking. The council of the European Union has adopted the ICNIRP limits for occupational/controlled exposure and General Population/uncontrolled exposure maximum limits to electromagnetic radiation. The reference limits vary depending on the frequency of the device, as can be seen in the table below.

By occupational (also known as a controlled) environment is meant environments where the person is aware of the possible RF exposure or situations, where a person is transmitted through a location in which the occupational RF exposure limits are used and the person is warned about the possible exposure. Controlled environment can be for example an area fenced around an antenna. Uncontrolled environment/general population exposure means situations where the people are not aware of the exposure or do not have a control over it. An example of an uncontrolled exposure could be a workplace where uncontrolled RF exposure limits are not expected to exceed. (28 FCC Rcd, p. 3558, 2013.)

The testing standard for USA

For USA, the reference levels for testing wireless charging systems are specified in 47 CFR § 1.1310 by FCC. The limits vary depending on the frequency of the device. There are also different limits for EMF exposure for occupational/controlled exposure and General Population/uncontrolled exposure. WPT devices however are evaluated against general public limits.

There are certain requirements for how the testing should be done. FCC KDB 680106 D01 RF Exposure Wireless Charging App has requirements for how the RF exposure evaluation should be conducted for WPT devices.

• FCC KDB 680106 D01 RF Exposure Wireless Charging App defines:
• PART 15 AND PART 18 Requirements
• RF Exposure Requirements
• Considerations for Inductive Transfer Techniques
• Equipment Approval Considerations

Testing standard for Canada

For Canada, the nerve stimulation (NS) reference levels are specified by RSS-102. The NS reference levels for electromagnetic radiation depend on the frequency and there are different reference levels for controlled and uncontrolled environment. WPT devices are evaluated against general public limits.

SPR-002 Supplementary Procedure for Assessing Compliance with RSS-102 Nerve Stimulation Exposure Limits has general test methods applicable to performing assessments on the nerve stimulation exposure limits for devices operating in the frequency range of 3 kHz to 10 MHz.

SPR-002 defines:

• Test environment and equipment
• Measurement methods
• Measurements for sources with multiple frequencies
• Spatial averaging
• Considerations for floor-standing, floor-mounted, hand-held, wall-mounted and tabletop devices

EMF testing procedure for wireless charging systems

The EMF measurement procedure is similar between the CE, FCC and ISED. There are few differences when evaluating against general public reference levels.

The Device Under Test (DUT) is placed on a table free from metal objects. The measurement probe is placed on the table with the required separation distance(s) to the DUT. There are differences in how the separation distance is defined between CE, FCC and ISED.

For CE and ISED, the separation distance is defined from the edge of the measurement probe to the edge of the DUT. For FCC the measurement distance is defined from the center of the measurement probe to the edge of the DUT.

The E- and H-field values are measured from all sides of the DUT. In the CE and FCC testing, the 6-minute average of the E- & H-fields are measured. In ISED testing, the instantaneous (peak) of the E- & H-fields are measured with increased monitoring period. If the value obtained from the measurement is below the reference level limit, the device is compliant.

Verkotan and wireless charging system testing

Verkotan provides accredited EMF testing for wireless charging systems to be tested according to IEC 62311:2008, KDB 680106 D01 and SPR-002 at our laboratories. These WPT systems require Electromagnetic field (EMF) testing from which Verkotan professionals have an extensive experience. We have tested various wireless power transfer systems according to CE, FCC and ISED requirements. Read more about our EMF services.

References

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