BQ51013B: Coil A works, coil B not

Part Number: BQ51013B

Hi all,

Recently I developed a system that uses wireless power transfer to charge a battery. Therefor I'm using the BQ500212A on TX side and the BQ51013B at RX side. Both been proven good.

I originally designed the RX side with a ABRACON AWCCA38R32H09C01B coil. Which works perfectly with the TX reference design kit with the BQ500212A and my own design TX station. Once implemented in our application the distance between TX and RX is a bit critical, lets say charging starts from 4 mm and both enclosures have a wall thickness of 2mm. The is some self adhesive tape between the enclosure and the coils. And in some cases no energy is transferred.
To resolve this problem I though to used another coil wich fits better with the TX coil (WE 760308101103). The one I selected is WE 760308103205, I threw these value's in my calculation sheet to calculate new values for C1 and C2 (in ref with datasheet of the BQ51013B). I replaced those capacitors on my board, did a quick test, and to my surprise the system is not working at all. I looked to RECT with my scope and the induced voltage way above 2,7V. So I was expecting it would start transferring energy.

My TX side schematic

My RX side schematic

RECT on RX side:

RECT_1.tifRECT_2.tif

  • 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.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

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  • In reply to Bill Johns:

    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

  • In reply to Mathieu Hebbrecht96:

    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.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

    Note:  If this answer solves your question please click the "Verify Answer"  button.

  • In reply to Bill Johns:

    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?

  • In reply to Mathieu Hebbrecht96:

    Coil to coil distance will be driven by coils and the big challenge is getting enough coupling to start. TX will send a digital ping at 175kHz and this needs to generate 2.7V on RX RECT pin to exceed UVLO. Once started unit can move the operating point to compensate for coupling and other factors.
    You system will not be Qi certified?
    If that is the case you can change tuning and TX coil to give you more gain at start up.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

    Note:  If this answer solves your question please click the "Verify Answer"  button.