[FAQ] Coil frequently asked questions for wireless power RX

Which RX Coil should I use?

We recommend to start with the EVM coil or similar. This is because debugging both the coil and the rest of the circuit at the same time can be difficult. When everything is working correctly replace with new coil and adjust the series (C1), parallel (C2), and COMM1/2 capacitors.

Receiver coil design can be difficult and you should expect to modify your coil design to meet performance requirements. Test the RX with a known good coil and test the coil with a known good RX. 

What size coil should I use?

RX coil should be close in size to the TX coil for best performance. This is typically 35X40mm coil with L of about 12uH (Qi certified TX). As the RX coil is reduced in diameter, the efficiency and max coil-to-coil distance will be lower. When a small coil is needed higher coil L is necessary, for 20mm round coil go for L in the range of 25uH.

What is the smallest coil size that I can use?

The smallest size coil possible is around 10mm. At this size, the RX will be difficult to get working with all Qi transmitters. It is not big enough to capture the magnetic field being transferred from the TX. If your system does not need to be Qi-compliant, then a smaller TX coil would help (closer to the RX coil size).

What is the max coil-to-coil distance?

All TI RX (and Qi systems) have a max recommended coil to coil distance of 5mm. This is in line with the Qi standard. Beyond this is possible but higher coupling and fine tuning is needed. Expect lower performance at higher coil-to-coil distances.