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BQ51013B: Wireless charging RX side, clikcing noise. Please help

rongwei mao
Prodigy 40 points
Part Number: BQ51013B

Hello

I am using the BQ51013B to develop a wireless charging the battery. The output of the BQ51013B is 5v, so there is another chip for power management for battery.

Everything looks fine. But the noise really bothers me.

I tested a few coils. One coil is slightly smaller, 10uH. Another one is 17uH coil(much bigger in size).

Both of them works for receiving power. Apparently the larger coil can provide higher current and large tolerance in alignment.

But the board using large coil (17uH) has significant Clicking noise, about 1click/second. You can hear the noise very clearly even at 30cm away.

By limiting the current to a smaller value, it helps, but not much.

Try to hold the coil with tape or something else, it doesn't help. 

Not sure where exactly does the noise come from.  I am sure it is around the charging chip. 

Please help.

  • 0
    TI__Guru* 75656 points

    Hello

    The noise is probably coming from the ceramic capacitors on the RECT pin when communications packets are sent.

    Piezoelectric Noise from Multi Layer Ceramic Capacitors (MLCC) is common and a number of articles can be found on the web.  Voltage on the capacitor changes and capacitor vibrates, the PCB acts as a speaker diaphragm an audible noise. Also on the web a number of articles on how to reduce the noise.

    The voltage change is coming from communication packets sent every 250ms.  Changing the coil and Cs tuning will change the amount voltage change during communications.  Output load will change operating point and also have an impact on voltage.

  • 0 in reply to Bill Johns
    Prodigy 40 points

    by following your suggestion.

    I removed the capacitor after RECT. And use flying wire to connect the capacitor to the board, which means the capacitor is floating.

    The noise reduce a lot. There is stll some noise. But for sure, it is much much smaller.

    But in a practical product, what is the good option? Using special capacitor? 

    By the way, except the capacitor connected to RECT, what else I need to pay attention?

    Appreciate your help.

  • 0 in reply to rongwei mao
    TI__Guru* 75656 points

    Hello

    One solution is to cut slots under the capacitor the pcb can then flex and noise is reduced.  We tested this with good results.

    Other solutions are available.

  • 0 in reply to Bill Johns
    Prodigy 40 points

    Hello Bill

    I replace the capacitor using special capacitor such as GJ8 series from murutu company or tantalum capacitor. 

    It does reduce the noise a lot without cutting a slot. This may be a good solution for existing boards.

    Please mark as the solved in the title.

    Appreciate your assistance. Saved me lots of time.