BQ51003: How to increase the distance Qi

The problem is starting the power transfer. Most of the time you can start the unit at 5mm and then move it to 10mm position. This demonstrates the start up problem. System will start at 175kHz in accordance with the WPC standard. At larger distances this freq will not generate enough gain. During power transfer the system can change the operating freq to increase gain. Take a look at the RX RECT pin (V-Rect) this voltage need to be greater than 2.7V (UVLO) for the system to start. What you will see at 10mm is a voltage generated by the TX that is less than UVLO. Best TX to test with is the bq500212A.
Try changing the RX coil inductance to increase the gain and increase the RX coil size.
Do you have a target RX coil size?

Hello

i use the wurth coil 760308102207 and I have no place to increase the size of the coil.

I have monitored the Vrect (attached my screenshot).

Is there any other point to check or improve to increase this distance

thanks

Best regards

Thanks for the scope capture, this does show that the RX V-RECT voltage is not getting high enough to start operation.  Very close to exceeding UVLO but not quite.

Wurth coil 760308102207 is 40mm X 40mm this is a good size and does not need to increase.

This inductance value of 8uH will be a problem, it is low for a 5mm distance.  This needs to double (16uH) or possibly triple (24uH) for a 10mm coil to coil distance.

See "Designing a Qi compliant receiver coil"  it is still on the Mouser web site.

http://www.mouser.com/pdfDocs/TI-Designing-a-Qi-compliant-receiver-coil.pdf

Gain of the system is set by ration of TX coil L and RX coil L and the coupling factor between the two coils.  Coupling factor will go down with distance.  This can be offset by increasing the L of the RX coil.

Also coupling factor will go down with the physical ratio of the two coils, 1:1 is good that is about where you are at.  A bad situation is a small RX coil (20mm) and standard TX coil (50mm).