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Frequency selection in wireless charging – what’s efficient and safe?

July 31, 2012 / 0 Comments / 1324 / Technical

When we talk about wireless power, one of the first design questions we consider is… “at what resonant frequency should the system transfer its power?”.  Selecting the system’s resonant frequency helps increase the distance for power transfer and improves the efficiency of the wireless power solution. But what are the health implications? and are wireless charging products being accurately measured?

Resonant frequency refers to the frequency at which an object naturally vibrates or oscillates.  It is at this frequency that objects are able to achieve their maximum amplitude – whereas it is difficult to get them to vibrate at other frequencies.  In the case of inductive power transfer (IPT), the principle of resonance is applied to induce an electrical current between coils of the same frequency over greater distances.

Naturally the resonant frequency will depend on the problem or application, however there are other factors which influence this decision.  As with all electronic products, the foremost issue is safety.

Usually safety is determined by how much radio frequency (RF) exposure can be applied without being harmful to human health.  Guidleines do exist which provide reference to what are acceptable and safe limits of exposure…or so we think.

Most would consider the 1998 and the 2009 ICNIRP (International Council on Non-Ionizing Radiation Protection) guidelines as a reference to those limits. Yet the guidelines lack any specific mention or reference to the behaviour of wireless power systems.  What this means is that the “limits” identfied may not accurately apply to RF radiation created by wireless charging.

That is not to say that wirelss power systems operating under these guidleines are unsafe –  electromagnetic fields created by this technology are considered to be at the lower end of the exposure spectrum. But surely more specific and conclusive measurements of IPT applications are required to understand the appropriate exposure limits?

Otherwise how do we know if limits are too harsh? (impacting the quality of solutions), or  conversely, not harsh enough?

Fady Mishriki is Co-Founder, EVP and Chief Tesla Officer at PowerbyProxi