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BQ24295: Radiated emission noise issue - FCC testing

Brian Krieger
Prodigy 130 points
Part Number: BQ24295

Hi,

I have a design undergoing FCC testing that uses the bq24295 for battery charging / power management. Functionality is fine and we based our circuit off of the reference / application circuits provided by TI. We are using a 2900 mAh li-po battery but using the default charge settings (1A bulk charge current). The charge source is a switching wall-adapter power supply rated for 5V / 3A output with USB-C plug type.

We are failing the radiated emissions test due to noise that is being emitted only when the charge source is connected. The frequency of the noise is about 1.4 MHz. Based on this, it seems very likely that the noise is connected to the charge control circuit. 

Has anyone seen this type of problem before? I'd be grateful for any suggestions regarding how to zero in more specifically on the the noise pathway and/or directions to explore for potential solutions. Thanks in advance for any advice,

Brian

  • 0
    TI__Guru**** 166626 points
    Brian,

    The buck converter switching frequency is 1.5MhZ typical so noise at around this frequency is expected. FCC are usually concerned with much higher frequencies than 1.5MHz. Which FCC standard are you measuring against?

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Prodigy 130 points

    Hi Jeff,

    Thanks for following up. I checked with our certification lab and they told me that the device failed 15.109 class B for non-intentional portion and 15.207 for the intentional radiator portion in restricted band (broad band nature).

    I'm wondering if our PCB design (or part of it) is acting as an unintentional antenna for the normal switching noise associated with the functioning of the charge controller - have you seen something like that before? Or please let me know if you have other ideas or thoughts. Thanks for your help,

    Brian

  • 0 in reply to Brian Krieger
    TI__Guru**** 166626 points

    Brian,

    I fully expect the charger to emit at its switching frequency, probably from the SW node that connects to one side of the inductor, even with a schielded inductor.  I do not think you will be able to eliminate switching noise around 1.5MHz unless you add metal sheilding.  You might be able to reduce the emission by adding 0.1uF decoupling capacitors at PMID and SYS, close to the IC, shrinking the size of the SW node trace/pour and not having ground directly under the SW trace/pour.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 130 points

    Hi Jeff,

    Thanks for following up and my apologies for the delayed reply from my end - we've been digging into this problem more. In addition to the suggestions you made, we are considering a common mode choke between the charge supply rail and the connector (to reduce emissions from the cable) and/or between the BAT pins on the bq24295 and the Vbat rail on the board. Do you think this approach might be helpful? Do you think that the use of a common mode choke at either location would undermine the functioning of the bq24295 circuit? Thanks,

    Brian

  • 0 in reply to Brian Krieger
    TI__Guru**** 166626 points
    Hi Brian,

    You will likely only need the choke on the input line as it is the noisy side of a buck converter. If you do add to the output, I would add only to the SYS rail.

    Regards,
    Jeff