The Qi wireless power standard has become ubiquitous for inductively charging smartphones and other portable electronics. Many smartphones offer reverse wireless charging (Battery Share) so users have constant access to a Qi compliant charger in their pocket. This opens up opportunities for powering implantable devices. Compared to NFC energy harvesting which can source 30-500 milliWatts of power, Qi can source 15-25 Watts of power which enables higher power applications like the Bodybytes. Standard Qi receiver coils that you can buy off-the-shelf are too large for an implantable device, so lets review some of the details of how you would design a low-profile Qi receiver coil into a custom PCB.
These commercial receiver coils have thick wires to carry large amounts of current and to lower the overall resistance of the coil. They are flat (only one winding thick) to maximize the inductance. They have a ferrite sheet backing with high magnetic permeability and low coercivity that maximizes the inductance while shielding anything behind the coil from the high energy field. They are also similar sizes and shapes as the transmitter coils to achieve a good coupling factor.
Here are some examples of PCB trace coils that attempt to find a sweet spot balancing all of these performance factors:
For an example of the math required to calculate the Q-factor of the coil and it’s resonant capacitor values we can review page 30 of the Texas Instruments BQ51013C Qi receiver IC.
Here is the dual resonant circuit schematic
and here is the calculations found in the datasheet
The inductance and resistance of the coil should be calculated during the design phase, but this will never be accurate even with advanced simulation software. You’ll need to have the design fabricated and test it. You can use an LCR meter to measure the inductance directly, but this doesn’t account for the interwinding capacitance of the coil itself. This can be measured with a vector network analyzer (coil inductance calculator and an interwinding capacitance calculator). In the equations above, Ls is the free-space inductance of the coil that you measure, and Ls’ is the inductance measured with any ferrite sheet or “friendly metal” that will be present in the final configuration.
The detection pulses for power contract negotiation and foreign object detection operate closer to 1MHz and more prominently involve the resonance with C2. Ls is used for calculations during this detection phase because the ferrite is not strongly involved due to the higher frequency and lack of power transfer.
The primary power transfer of Qi charging occurs between 100-200kHz. The power transfer phase more prominently involves the resonance with C1, and the Ls’ is used because the ferrite becomes very relevant.
Keep in mind that wireless charging is inherently inefficient due to losses to heat in the system. The Q-factor of a wireless charging coil will usually be between 10-100, with higher numbers indicating better power transfer efficiency.