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BQ40Z60: Resistor Sizing for Radiated Emissions Issue

Zach
Prodigy 195 points
Part Number: BQ40Z60

We have been having radiated emissions problems using a BQ40Z60 in a new design, on further investigation it seems like many others have had similar issues with this tightly integrated package.


We were able to get a pass by (among other things) increasing the switching MOSFET drive resistors from 0 Ohms to 39 Ohms (R23 and R24 in the typical application circuit from the BQ40Z60 datasheet).


Everything looks to be working fine, however I wanted to see if TI could weigh in on any unintended repercussions of changing this drive resistor before we finalize the design. Is 39 Ohms an acceptable drive resistance for the BQ40Z60?

  • 0
    TI__Expert 6575 points
    Hi Zachary,
    Changing the R23, R24 values from 0 ohms to 39Ohms should not significantly impact the performance of your charger, but it will reduce the EMI and will slow down the turn on and turn off slew rate for the FETs.
    I think this should actually be recommended if you use low gate capacitance FETs, otherwise you will have ringing and EMI issues.
    The only downside I see is that you will have a slight efficiency hit, but it should be well below 0.5%.
    Regards,
    Swami
  • 0 in reply to Swaminathan Ramanathan75031
    Prodigy 195 points
    Swami, can TI comment on maximum allowable drive resistance for switching MOSFETs used in a BQ40Z60 design? We are considering raising it beyond 39 Ohms to 100 Ohms or higher and want to understand at what point we will have issues.