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Original question:

BQ51050B: charging doesn't stop, and it gets hot

BQ51050B: BQ51050B FOD resistors

Frank Buss
Prodigy 110 points
Part Number: BQ51050B

This is related to this question:

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/977143/bq51050b-charging-doesn-t-stop-and-it-gets-hot

I can charge it to 4.17 V, if the coil is outside the device in the air. If it is mounted under the accumulator and PCB, it charges only to about 4 V.

I guess this is related to FOD? I can't find any information about the Ros and Rfod resistors in the datasheet. In the devkit, they are 42k2 ohm and 200 ohm, which I've used currently in the circuit. How do the resistors change FOD, is there a formula for it? Can I just increase or decrease one of the resistors to increase the friendly metal offset, to avoid triggering FOD when the coil is mounted under the battery?

I also downloaded the Foreign Object Detection GUI and read the user manual. But unfortunately I don't have the $20k equipment listed in the manual to do all the required measurements. I have a regulated power supply,  a good Agilent oscilloscope, and various multimeters. Is there a cheaper way to do it? I don't need the full WPC conformance test, it should just work with most chargers.

  • 0
    TI__Guru* 75516 points

    Hello

    The thermal rise and FOD could be separate issues.

    FOD -- Can keep 42k2 ohm (R-OS). The 200 ohm (R-FOD) can be increased to 225 ohms to increase margin (friendly metal) for testing.  

    Thermal -- With the RX coil and PCB separated thermal is OK.  But assembled and on the charging pad thermal rise is a problem.  This points to the AC field entering the unit and heating PCB and possibly using magnetic shielding material will help.

  • 0
    Prodigy 110 points

    I can charge it to 4.19 V now with 470 ohm for R-FOD, and R-OS leaving open. I guess this effectively disables FOD? But at least it works with this setup. Will leave the footprints in, then later I can verify it again, if needed.