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BQ24650: BQ24650 MPPT Charger Standalone board

Rajesh Prasad
Prodigy 80 points
Part Number: BQ24650

Hello,

We have designed a standalone 2 layer charger board based on the BQ24650 EVM layout. All the layout is as per the recommendations in the datasheet / EVM.

Panel Input: 19-21V

Battery: Li-Ion 14.8V / 12AH

All the Test Point / voltages are as per the test procedures. The battery charging current is set a 1A for now. The board is working fine and the charging is operational for the set values. The Charging Mosfets are working fine and there is no heating observed. However, the Charger IC BQ24650 is getting heated up to 60'C. The thermal pad is very well connected to ground plane and there is sufficient ground plane on the PCB.

If the Charging current is increased to 2A, the device is getting quite hot, but the MOSFET temperature is quite normal. Please confirm if this behavior is normal or there is some issue that we need to look into.

Thanks,

Rajesh

  • 0
    TI__Mastermind 24075 points

    Hi Rajesh,

      BQ24650 is a charge controller, meaning that there is internal gate drive to control external MOSFETs. As MOSFETs are external, all the current passes through them, and not through BQ24650. BQ24650 itself should not consume any current, so for just BQ24650 to heat up is not normal behavior. Do you have a thermal image of the board, when operating at 1A and 2A?

        Note that BQ24650EVM is a 4 layer board.

  • 0 in reply to Kedar Manishankar
    Prodigy 80 points

    Dear Kedar,

    Thanks for the clarifications. I agree with you and we were also thinking on the same lines. We have assembled 3 boards and incidentally all are behaving the same way. Except for the Controller device heating up issue, the board is working as expected. I understand the EVM being a 4 layer board has better ground planes and routing compared to 2 layer board.

    Unfortunately, we do not have the Thermal Imaging facility. We could only measure the temperature externally. Is there any other possibility of the device heating up? Attached schematic of the board for reference. Do let us know if there is anything that we could have missed.

    Thanks,

    Rajesh2626.SCHEMATIC1 _ PAGE1.pdf

  • 0 in reply to Rajesh Prasad
    Prodigy 80 points

    Dear Kedar,

    Also to add another point, the Device heating up starts once the Battery starts charging only.

    Thanks,

    Rajesh

  • 0 in reply to Kedar Manishankar
    Prodigy 80 points

    Dear Kedar,

    Would like to know your opinion and suggestions on the following changes in the schematic. 

    1. Changing D2 from 1N5819 to 1N4148.

    2. Adding a series resistor to the Gate Driver of both the MOSFETs

    Do let us know if any of these changes would help.

    Thanks,

    Rajesh

  • 0 in reply to Rajesh Prasad
    TI__Mastermind 24075 points

    Hi Rajesh,

       Adding a gate resistor will increase gate driver loss, as it slows down both turn and turn off time of the FET. This technique is primarily used when EMI is a concern, to reduce ringing on rising edge of switch node waveform.

    Only when battery is connected, will charger start charging. BQ24650 employs battery absent detection and will detect when battery is plugged in. I do not see any D2 in your schematic. Issue is layout related. Please review some layout instructions in section 11. of the datasheet.