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BQ51050B: BQ51050B: Charging not happening when the distance between receiver coil and wireless charger transmitter pad is zero (Receiver module placed directly above charger pad)

Hafiz Haja
Intellectual 490 points
Part Number: BQ51050B

Hi,

We are facing an issue with a module having wireless charger and receiver IC BQ51050B. The issue is, when we are placing the receiver module directly above the charge transmitter pad charging is not happening. But when we are slowly keeping the receiver module above the charge transmitter pad with an air gap of around 3mm, charging is initiating and we can put the receiver module freely above the charge transmitter pad, the Battery will charge (sometimes charging stops before full charge and then we have to take the receiver module from the pad and place it again like explained above). What may be the reason for this?.  What we have tried is we have placed a thermocol of thickness more than 3mm and placed the receiver module above the thermocol. Irrespective of how many times and different directions we tried, the Battery is charging suddenly; without any interruption Battery is fully charging. Receiver coil self inductance (Ls) is around 28uH and Ls' with Ferrite sheet, Battery and primary shielding is around 32uH.

Expecting support as soon as possible.

Regards

Hafiz

  • 0
    Intellectual 490 points

    I have read in datasheet that maximum received power allowed is 375mW and if received power is greater than 375mW, receiver will send error to transmitter so that transmitter won't stop transferring power. How to measure the received power and check whether this is causing the issue, since we are facing issue when receiver is closely placed to transmitter.

    Attached the RECT output waveforms during the non-charging condition.

  • 0 in reply to Hafiz Haja
    TI__Guru* 75786 points

    It looks like the voltage at RECT is getting high enough, should be greater than 2.7V to start.  But it also looks like it may be too high max is 15V, OVP point.

    What value of Cs tuning caps are you using?  Also Comm cap value?

  • 0 in reply to Bill Johns
    Intellectual 490 points

    Hi,

    Ls = 28uH (Approx.) & Ls' = 35uH (measured with Ferrite sheet stuck behind the coil). Cs --> C1  = 0.067uF (series first resonant cap) &  C2 = 890pF (parallel resonant cap). COMM 1 = 0.022uF (50V) & COMM 2 = 0.022uF (50V).

    Suppose if RECT voltage is too high our inductance value should be high. Any suggestion for reducing the magnetic coupling?.

    The behavior is depending on the distance b/w receiver coil and primary coil as if I am maintaining a gap of greater than 3mm charging will suddenly start.

    Regards

    Hafiz

  • 0 in reply to Hafiz Haja
    TI__Guru* 75786 points

    Hello

    For Ls' =35uH a Cs (C1) value of 67nF is about right.

    Try reducing the Comm1 and Comm1 capacitor to about 10nF.  The comm caps should be about 10% of Cs (C1) value, you are using a high L coil that will reduce Cs.

    Coil to coil distance should be 2.5mm to 5mm.

  • 0 in reply to Bill Johns
    Intellectual 490 points

    Hi,

    We have changed the COMM 2 & COMM2 capacitors to 0.01uF. The charger detection has been improved a lot when placing the charge receiver module above transmitter module. But still we are feeling a latency in the procedure. We have to place the module above the transmitter slowly. If we are casually placing the module above transmitter it is not charging. We think the distance issue has been reduced to less than 1mm. Do we still need to tune the COMM capacitors. Because 10% of 67nF (Cs) is 6700pF = 0.0067uF.  Shall we try this value for improving the result.

    Regards 

  • 0 in reply to Bill Johns
    Intellectual 490 points

    Hi,

    You said high L coil will reduce Cs. Can we measure the reduced Cs value? Is there any relation between coil inductance and effective Cs value?

    Regards

  • 0 in reply to Hafiz Haja
    TI__Guru* 75786 points

    Hi

    The Cs value is determined by the Ls' of the RX coil.  Ls' is the measured value of the coil when installed in device case and on the TX.  Coil L value in data sheet is free air with no influence from surrounding metal.

    Ls' and Cs are tuned to 100kHz. 

    As the Ls' value goes up the Cs value will go down to keep tuning at 100kHz.