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BQ25792: Device heats up when transitioning from CC to CV

Masashi Mineda
Expert 1175 points
Part Number: BQ25792

Tool/software:

Dear Sirs and Madams,

While evaluating the charging characteristics of the BQ25792, we found that the device heats up when switching from constant current (CC) to constant voltage (CV).

 - Input voltage : 15V, Charge current : 1.1A, Battery 4S,16.4V

The BQ25792 performing the charging process is IC200 (green), and the temperature reaches 43.6°C before switching to CV.

 - Input voltage : 12V, Charge current : 1.1A, 4S,16.4V

The BQ25792 performing the charging process is IC200 (green), and the temperature reaches about 43°C before switching to CV.

I thought that the PWM operation at light load was affecting this issue, It noticed no change in this heat generation with either the PWM or PFM settings.

Do you know what causes this phenomenon?

Regards,

Masashi

  • 0
    TI__Guru**** 166626 points

    HI Masashi,

    The charger is in CV mode when battery voltage is flat = VREG and charge current is tapering down.  The efficiency decreases at light load when the charger transitions from CCM PWM to DCM PWM or to PFM, especially if OOA is enabled.  You can monitor SW1 and SW2 to see the transition.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1175 points

    Hello Jeff,

    We don't understand why the heat generation of the BQ25792 increases even though the CC current has decreased after switching to CV charging.

    Have you ever observed a similar phenomenon in your company?

    I have investigated the terminals through which the device sinks current, but 4 FETs of buck-boost are built into the device, so we believe it is unlikely that shoot-through is occurring.

    Regards,

    Masashi

  • 0 in reply to Masashi Mineda
    TI__Guru**** 166626 points

    Hi Masashi,

    I have not observed this.  There are some conditions where the charger turns off the asynchronous FET for brief time.  Can you monitor the SW1 nodes to determine if that is occurring?  

    Regards,

    Jeff

  • +1 in reply to Jeff F
    TI__Guru**** 166626 points

    Hi Masashi,

    I just spoke to the designer and he reminded me that at light load, the charger's converter turns on a 30mA pull down current at SYS.  This additional load current keeps the converter stable.  This is the likely cause of the die temp spike.  I have lab time later today and will test to confirm.

    Regards,

    Jeff

  • +1 in reply to Jeff F
    TI__Guru**** 166626 points

    Hi Masashi,

    I tested my EVM on the bench and only see an large increase in die temp if I turn off PFM.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 1175 points

    Hello Jeff,

    We can see that the power at 30mA pull-down and 16.4V(battery 4s) is 492mW.

    The package thermal resistance of the BQ25792 is approximately 44.2°C/W, so a temperature rise of 21.8°C is expected.

    The measured waveform shows a temperature rise of nearly 10°C, and we understand that this is due to the 30mA pull-down.

    Regards,

    Masashi