BQ51013B: Coil A works, coil B not

Hi Mathieu
Thanks for providing the sch and scope capture of RECT pin. The problem appears to be communication issue between RX and TX. We see that the RX is sending a communication packet and TX decodes the first packet but something goes wrong on the 2ed packet and TX stops power transfer. Odd that it is doing it that consistently.

Take a look at the TX COMM+ signal it should not go below ground, needs to be between 0V and 2.5V---around 1.5V is best.

If that is OK then try increasing the RX Comm capacitors, 22nF is standard but they can be increased to 68nF.

During decoding of the first three packets is any error is detected in the packet the TX will restart. After this the TX must receive one good packet every 1.5sec.

Hi Bill,

Thank you for accepting my post. In meanwhile I took a few more scope captures. This time I used the correct function to export, so X and Y scale are present.

I measured the COM+ at TX side. This does not go bellow 0V. DC offset is 1,5V and a package is modulated on it.

At receiving side I stacked 33nF on top of the 22nF capacitor. Apparently I'm out of 47 and 68nF capacitors SMD0603. So I'm not really 68nF, but getting close. However, this does no seem help very much.

RECT at RX with 22+33nF

COM+ at TX side

I think the problem may be with H-Bridge drive signal to DPWM-1A / -1B. The coil side of the H-Bridge must be driven with DPWM-1A and capacitor side with DPWM-1B. The communication sample point is synchronized to DPWM-1A and if the two signals are reversed it will cause communications problems.

Hi Bill,

So long story short. There were actually two issue's. Both were not related to each other but by coincidence they seemed not to work anymore.

The Tx side I was using suffered from a few reworks in the past and a wire connecting the DPWM-1A signal to the H-brige was broken loose, and probably it worked unstable. Repaired the board and energy transfer was a success. Unfortunately there was nog significant increase in the distance between the to coils for initiating energy transfer.

So I took another Rx board and attached yet another Qi Rx coil (WE 760308101303). On the WE website there is a note in the compliancy column, 'works with Qi Rx IC’s'. So it's not really Qi compliant but it should work. I have no ambition to certify my unit, so Qi or not. Don't care. But I want to increase the distance between my two modules so I'm trying to improve the EM coupling between the two, and this one might do the trick.
Unfortunately I doesn't, there is no energy transfer at al. While other two coils work perfectly. So I assume there is an issue with the capacitance (to low or to high), according to my calculations with some tolerance included it should work. What is best practice to counter this issue? Where should I watch the signal to see what is going on. How can I judge to increase or decrease, or is it really try and error?

Another question I'd like to ask is. What is the best way to increase the distance between the two? On one Rx board I changed C109 and C110 (the COMM1/2 capacitors) to 68nF. In my opinion that has a positive effect on the distance between the two. Can you confirm that?