For anyone contemplating a wireless solution to their powering needs, the question is no longer “When will the technology be sufficiently robust?” Recent innovations have led to a variety of wireless power systems that can be tailored to many industrial and consumer applications. The key question nowadays is “which type of wireless power setup will best meet my needs?” There are two major design considerations to keep in mind when planning a wireless power system. One is coil geometry, which I explored in Design Considerations in Modern Wireless Power Transfer Systems: Coil Geometry. The other is frequency of operation.
The consideration with frequency of operation is whether to use a low or high operating frequency to achieve the required power transfer. (The two main recognized frequency ranges are around 110kHz kHz and 6.78MHz MHz.) The frequency range affects a number of performance criteria. But the main considerations from a design perspective are system power efficiency and safety; particularly human exposure to Radio Frequency (RF). Other considerations when deciding upon the best operating frequency include interference with other devices and compliance with national and international standards for electromagnetic compatibility (EMC).
Human RF exposure (safety): RF exposure is measured by the specific absorption rate (SAR) and the induced current density, or J field. A system designer must decide on a frequency that allows the best power transfer level within the SAR and J field limits.
SAR is the rate at which energy is absorbed into the body, and its value is expressed in terms of watts per kilogram (W/kg) or milliwatts per gram (mW/g). The J field's value is expressed in milliamps per meter (mA/m).
SAR for electromagnetic energy can be calculated from the electric field within the tissue as:
σ is the sample electrical conductivity
Ε is the RMS electric field
ρ is the sample density
Other things being equal, the power that can be transferred for a given magnitude of magnetic field follows this relationship:
What we find is that when exposed to a magnetic field of a given intensity, both SAR and J field increase with the frequency (also see table 1). The J field is the most stringent limit on emissions. It has some headroom in the lower kHz frequencies, but it steadily becomes a higher percentage of the limit as frequency climbs into the MHz.
A kHz level operating frequency will allow more useful power transfer levels within SAR and J field limits compared to MHz level operating frequency.
EMC compliance: EMC radiation standards are used to qualify electronic devices against interference with other electronic devices. A piece of wire that has a current running through it will generate electric (E) and magnetic (H) fields. EMC radiation standards ensure that these fields do not interact with other electronic devices to impair their operation. CISPR11, sets limits for an array of industrial, scientific, and medical equipment. At certain frequencies, such as 6.78Mhz, wireless power faces an extremely tight “chimney” due to power limits drafted to protect other classes of devices.
Electromagnetic force (EMF) interference: Think of EMF as a fancy term for voltage. Electronics are sensitive to over-voltage. Some are more sensitive than others, but suffice to say that over-voltage can cause permanent damage to devices. The operating frequency of some electronic devices can also make them prone to interference from wireless power systems. Time-varying magnetic fields will also induce strong voltages in nearby metallic objects such as PCBs (printed circuit boards).
You should plan on testing or simulating how your planned wireless power device may induce EMF in devices with “non-intentional” receiver coils. (Keep in mind that applying more than -0.7V to the pins of most microcontrollers will cause permanent damage to the device.) By first principles, the amount of EMF interference will be less at lower frequencies. This is based on Faraday's law, which states induced voltage is proportional to frequency.
System power efficiency: Stay tuned. We will address this design consideration in an upcoming post.
Useful formulas :
- Table 1 shows the relationship between frequency, energy and distance.
- Table 2 shows the International Commission for Non-Ionizing Radiation Protection (ICNIRP) max SAR as a function of frequency. CNIRFP Fact Sheet