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BQ40Z60 + EMC

Robert Ploeg
Prodigy 240 points
Other Parts Discussed in Thread: BQ40Z60

We've designed a board with theBQ40Z60 for a 4S battery but cannot get the EMC levels at charging correct. Even after redesigning, adding extra filter components, shortening paths, following all the layout guidelines, it did not help at all. Charge current is set at max 3A. Everything works as designed excepet the EMC, the 1MHz switching frequency is visible over a measured spectrum up to 300MHz.

Does anyone have an idea or has an actual working charging design with the BQ40Z60?

Thanks,

Rob P.

  • 0
    TI__Guru 73830 points
    The charger has limited options to reduce emissions from switching. You can try adding some series resistance slow down the rise time, but too much will cause shoot through problems. Also, make sure that you use a shielded inductor. If all else fails, then you may have to place a can over the components. Maybe someone else on the forum can offer advice as to what they have done to reduce emissions.
  • 0 in reply to ThomasCosby
    Prodigy 240 points
    Thanks Thomas.
    We've tried the series resistor but no effect at al.
    The inductor is shielded.
    We are performing more measurements with following results:
    Measured on the gate of the hdrv fet: overshoot to 28V with overshoot to 35V, ringing at about 22.2Mhz. Same with undershoot. Same ringing at the PH input pin of the BQ.
    Changing the 100nF capacitor from PH to BTST into 100nF +10E resistor in series, solves the overshoot but does not affect the undershoot.
    Increasing the series resistor in the gate of the HDRV and LDRV does not affect the overshoot and ringing frequenty at all, only result is an increased temperature on the BQ.
    Strange is the ringing frequenty of about 22.2MHz. We have 2 designs with exactly the same result where we would expect differences in the ringing due to different lengths of tracks giving different parasitic induction. Indicating the ringing is not caused by the parasitic inductor and fet capacitor. This is also underlined by no effect of adding an extra snubber network over the fet. Only increased temperatue of the BQ.

    Any idea where this ringing is coming from?

    BR.
    Rob
  • 0 in reply to Robert Ploeg
    Prodigy 240 points

    Hi Thomas,

    in the pdf file slusaw3c are under 10.2.3 Application Curves, a measurement of the Vph, figure 11. Is this measured on the EVM board of the BQ40Z60 or coming from some other board? Our measurement of the Vph has the overshoot with 22.2MHz. If it is from another board, can we get the schematics of this?

    Thanks,

    Rob

  • 0 in reply to Robert Ploeg
    TI__Guru 73830 points

    Rob,

    The waveforms were collected on the bq40z60EVM. I do not recall seeing that overshoot in the signals on my EVM, but I can check when I get a chance. I assume that you have a good high frequency ground for the probe?

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 240 points
    Hi Thomas,
    fyi, we've completely re-designed the layout and got the EMC levels correct.

    Rob
  • 0 in reply to Robert Ploeg
    TI__Guru 73830 points
    Robert
    That is excellent news. Can you share the PCB and other improvements that allowed you to pass EMC?
    Tom
  • 0 in reply to ThomasCosby
    Prodigy 240 points
    Hi Tom,
    Cannot share it, but as a tip for others:
    we've physically split the 2 different grounds on the pcb, left - right, and coupled these grounds with several capacitors. Managed this on a 4-layer board. Also the distance between top-layer and 1st inner layer is set to 100um same for the bottom-layer and inner-layer 2.

    Rob