BQ51013B: Design of Rx coil

Thanks for posting to the E2E forum and your interest in wireless power.

Special authorization is required for TI components in life-critical medical equipment such as pacemakers. Use of the bq51013B in this application would need to be addressed separately from coil design.

Regarding the technical question of coil design it would depend on several factors.
1.) Coil to coil distance and alignment—lower coupling factor (k) between the coils will make it harder to start power transfer (start-up) also reduce system gain resulting in lower efficiency. Coupling factor decreases with coil to coil (Z) distance and X-Y alignment. For Qi system with larger (EVM) coils 5mm is max Z distance and X –Y is about 10mm Radius
2.) Size of coil (typically RX)—As coil size is decreased the coupling factor will decrease due to ratio between RX and TX coil. Also the performance with increased Z distance, X-Y distance will decrease.
3.) Current in the coil—For 5W applications coil current will be 1A this requires a coil with DCR similar to the EVM coil. But as output power is reduced a lower DCR can be tolerated but TDK coil has 1.21 ohms this looks high, current would need to be low.
4.) To offset the increase coupling factor a higher RX coil inductance can be used, the TDK coil has a 27.9uH this is going in the right direction.