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BQ2057W: Battery IC geeting Heated.

Gajendiran Arumugam
Prodigy 30 points
Part Number: BQ2057W
Other Parts Discussed in Thread: BQ2057, CSD25404Q3

We are Using BQ2057 battery charger IC in our product to charge the 8.4V battery through 9V-1A adapter. As per the datasheet to enable/disable the charging process, we used AO4409 FET in between adapter and battery.

Whenever the beginning of battery charging (Charging Current 0.5A) this FET and BQ2057 getting very hot, because of across FET dropout voltage is more.

For this issues we are facing more failure case of broads.

  • 0
    TI__Guru* 75656 points
    The BQ2057 will use and external pass transistor to regulate the current to the battery, in data sheet this is Q1. The power dissipation on this device is high and it will need good PCB layout to manage the thermal rise. See data sheet page 15 Application Information. Also see EVM layout and heat sink for Q1.

    The A04409 FET is a small package with no heat sink features, it will have thermal problems in this application. Power dissipation will be over 1W.

    You could experiment with reducing the charge current, this would reduce power dissipation on Q1 but best solution would be to looks at a larger device.
  • 0 in reply to Bill Johns
    Prodigy 30 points

    Thank you Bill Johns, But this FET (AO4409) will support 2W @ 70°C and 3.2W @ 25°C.

  • 0 in reply to Bill Johns
    Prodigy 30 points

    Hi Bill Johns,Would you recommended sutiable FET for this application.

  • 0 in reply to Gajendiran Arumugam
    TI__Guru* 75656 points

    Hello

    I estimate the power dissipation on the MOSFET (Q1) to be 1.5W at beginning of charge when the battery voltage is low (6V).  Charging current of 0.5A.

    The PCB would need to be large enough to dissipate or spread this heat and keep the temp rise at an acceptable level.  

    On the EVM copper area available for Q1 is over 20mm X 20mm. 

    A couple of options:

    1.) If more copper area is available for heat sinking Q1 this may help the current device.  Use solid copper areas at the drain leads, not traditional pads.

    2.) Switching to a MOSFET with lower Junction to Lead or Junction to Pad C/W will help reduce the device temp.  But only if copper area is available to take advantage of the improved thermal impedance.

    3.) Reduce charge current, 0.5A will result in 1.5W dissipation.  A charge current of 0.25A will cut that in half.  But the charge time will increase also.

     A MOSFET with lower Junction to Pad impedance is the CSD25404Q3, this device has a metal pad for improved device to PCB contact.  Other similar devices are also available from other manufactures.  A drain to source voltage of -20V is probably enough if the external adapter is well regulated.