BQ51050B: Choosing Rx coil

Hello Nick

The coil has several tradeoffs, some of them below:

1.) Best to start with evaluation module to reduce number of problems.  Difficult to debug both the coil and circuit at same time.

2.) Large coil has better coupling to the TX coil, similar to the evaluation board.  This would be 35 X40mm coil with L of about 12uH.

3.) As coil is reduced in diameter the coupling is lower -- efficiency is lower, power out is lower and coil to coil distance is lower.

4.) Increasing coil L will help -- 20mm round coil and L in the range of 25uH.

5.) Qi system (BQ51050) will have a max recommended coil to coil distance of 5mm. 

What charging current will you use in your system?

Dear Johns,

Thank you for your prompt response. The max charging current would be 250mA.

Nick

But 5mm is a quite small gap and the Rx coil will be mounted inside the insole. Do you have any suggestions? Should I use a different IC module instead of BQ51050?

Nick

Hi Nick

Sorry, the BQ51050B and our other devices will be limited to 5mm.

Greater distances may be possible with devices from other companies but I do not have any recommendations.

Hi Bill,

Thank you for your response,

I was thinking that the coil to coil distance is related to the coils' physical dimension rather than the ic (the ic is just receives and manipulates the Rx power isn't it?). For example, if I use larger coils I couldn't increase the distance between them?

Nick

Hi Nick

The device has been evaluated with standard Qi TX coil which is about 50mm and standard EVM RX coil of 35mm X 45mm.  This will give good performance at 5mm distance. 

Larger coils and greater distance has not be evaluated. 

How large a distance do you need?

Dear Bill,

Thank you for your prompt response, I would be more comfortable with 7-8mm

Thanks in advance

Nick

HI Nick

The thing that would reduce the risk and make 7 to 8mm possible:

1.) good alignment between the TX and RX coils

2.) no drop in input voltage to TX, 5V or a little higher.

The most challenging part is start up, this is a low power ping to power housekeeping circuits and get power transfer started. 

One more inquiry Bill regarding the specification versioning. What I can see, there are plenty of WPC versions... Should I proceed with WPC 1.1 or 1.2? BQ51050 that I have chosen to proceed is according to WPC 1.1 and I am thinking whether or not it's a good idea to proceed with WPC 1.2 instead.

Nick

Hi Bill,

I found the TIDA-00669 evaluation board that incorporates the bq51050 ic from your website but I can not find where to purchase? Is it still available?

Do you have any recommendations for evaluation board?

Nick