Design Considerations in Modern Wireless Power Transfer Systems: Frequency of Operation

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
Table 1

The relationship between frequency, energy and distance.

The relationship between frequency, energy and distance.

Table 2

ICNIRP Summary of SAR limits for general population 2010

ICNIRP Summary of SAR limits for general population 2010

19 comments on “Design Considerations in Modern Wireless Power Transfer Systems: Frequency of Operation

  1. vasanjk
    October 31, 2014

    Hi Fady,


    Nice post as your earlier one. I would like to know if there are any specific standards in force for using wireless power charging in medical devices. One of my clients is pushing me to implement wireless charging in a medical device.

  2. samicksha
    October 31, 2014

    I am sure but i guess wireless power charging is not recommendable for Medical devices, infact even in mobiles phones this method of charging is not very efficient.

  3. vasanjk
    October 31, 2014

    Samicksha May be it is possible that the docking stations could use wireless charging to facilitate charging when the device is not on the patient.

  4. fasmicro
    November 1, 2014

    >> I would like to know if there are any specific standards in force for using wireless power charging in medical devices. 

    I do not think this has been worked out and finalized with any standard. Wireless charging is not mainstream yet. It has to have decent maturity before it gets into life and death applications like biomed.

  5. fasmicro
    November 1, 2014

    It would be a very dangerous proposition to use a highly immature technology to power systems in very fragile patients. Wireless charging is not advanced. When I see that in iPhone first, then I will expect to see that in clinics.

  6. goafrit2
    November 1, 2014

    Your big problem with biomedical is not just the technology. It is the FDA. You can make it work in your phones but that does not mean FDA will buy into it.

  7. Davidled
    November 2, 2014

    Wireless charging has not been fully validated in Auto industry, even though a few OEM has released charging pad. So far, it was unpopular in the residential area. Engineer might look carefully at medical device which is very potential sensitive electronic component to human body.  

  8. vasanjk
    November 2, 2014

    Fasmicro This is what everyone said when defibrillators were invented. Today it is an indispensable device. Offline charging with wireless may not affect the patient any way.

  9. vasanjk
    November 2, 2014

    Though medical device design is a sensitive issue, the fear of unknown takes prevalence more than rational design thinking, sometimes. This could cause roadblocks in technology evolution. Medical device = Other similar device+patient protection+regulatory compliance+….

  10. uchiha
    November 3, 2014

    @vasanjk: Yes with time only things will improve. Anything new that comes to the market does have its negative feedbacks. That is how any product improves. So the same theory applies for this as well.

  11. vasanjk
    November 3, 2014

    uchiha, Wireless charging technique has grown above the level of being called 'unproven' or 'primitive'. Standards are available from almost day one. Good agencies are monitoring this technology. I guess it is here to stay for long long time.

  12. uchiha
    November 4, 2014

    @vasanjk: Yes indeed its improving day by day. Things are looking good but its important to maintain the stability to retain in the market. Also I think we need to look into the security aspects too. What do you think mate ? 

  13. samicksha
    November 9, 2014

    When device is not on patient, it can easily be charged with help of wirres why add cost with wireless charging pads or dockets stations.

  14. vasanjk
    November 9, 2014

    Samicksha Wireles charging can be a boon where you don't want any openings on the equipment and thus void dirt,corrosion, etc. Coat is not a factor in such situations but life of the equipment.

  15. ue2014
    November 10, 2014

    >>>>>>>>>>>>> vasanjk – Wireles charging can be a boon where you don't want any openings on the equipment and thus void dirt,corrosion, etc.

    Very True… Wireless Charging could provide many benefits to industries specially in health sector if it would be used effectively. 

  16. uchiha
    November 10, 2014

    @UE2014: Yes it can be used for many sectors but if you are using it for sensitive sectors you should consider closely on the levels of security involved in it. 

  17. ue2014
    November 10, 2014

    @uchiha – Yes indeed. Security comes first, specially when it comes to sensitive and high important sectors such as Health. No one would take a risk with highy sensitive and important areas such as Human Life. 

    That is why it is very much important to have strickt testings on these technologies to identify any negative impact on Human Beings or on its usage. 

  18. uchiha
    November 13, 2014

    @ue2014: Very true mate but on the other hand these things will have to be tested only after its being put into to practical scenarios. So the risk is there at any point. I guess there is no other option too.

  19. ue2014
    November 13, 2014

    @uchiha – >>>>>>>>>>>>> So the risk is there at any point. I guess there is no other option too.

    I do agree. It is not something that could be avoided 100%. But could be minimized to minimum levels through extensive Testings. 

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