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BQ25730: Info on Max Power capability

Alberto Difrancesco
Prodigy 30 points
Part Number: BQ25730
Other Parts Discussed in Thread: BQ25731, BQ25723, , PMP

Dear all,

We are currently using the BQ25723 battery charger and would like to upgrade it to the newer BQ2573x (either BQ25730 or BQ25731) model to gain compatibility with 5S batteries and increase charge power.

The max output power that the new charging chip can deliver is critical to our application, so I would like to unambiguously understand the candidate chip limitations.

Below is my simple calculation of input and output power values using input/output voltage and current specs from the BQ25730 datasheet:

  • Ichg_max=16.256A, up to 5S (Vbat_max=21.75V)  -->  Pout_max=353W
  • Input: up to 26V – 10A  -->  Pin_max=260W

Assuming the output power cannot be higher than the input power, the input power becomes the limiting factor. This brings me to the following two questions:

  1. Am I correct in thinking that the maximum power delivered to the battery is limited to 260W in the best case?
  2. Whether yes or no, what is the highest practical (accounting for recommended safety margins, losses, etc) output power that the BQ2573x charger chips can deliver to the battery?

Thank you very much for your time in advance.

Kind regards,

  • 0
    TI__Mastermind 33525 points

    Hi Alberto,

    Theoretically, there is no power limiting for BQ25730. The charger detects the current through the sense resistor. The smaller the current sense, the higher the power.

    However, each component has thermal limiting. When the power level increases, the power loss increases proportionally. Eventually, the system could not dissipate the thermal. So, thermal is the limiting factor. 

    For example, below 5S power tool can reach 240W with a heatsink and 200LFM air flow, but is derated to 130W in natural convention. So you can see the power level depends on the components choices and many other factors. 

    www.ti.com/.../PMP22805

    The good news, you might put two chargers in parallel in some cases. There are a few scalable reference designs using BQ25731 on ti.com. 

    I would suggest you get a BQ25730EVM and try it out. 

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 150 points

    Hi Tiger,

    My name is Vasily, I work together with Alberto. Thanks a lot for your prompt response.

    I wonder how your comment on BQ25730 theoretically having no power limiting relates to the 10A max input current limit mentioned in table 9.1 of the chip datasheet. If I read this right, the input current into the charger caps at 10A. This threshold is backed up by the upper bound of a IIN_HOST() register value range of 10000 mA given in section 9.6 Register Map. If this input current cap is real, the module power limit should be at least 320W (32V max allowed V_in * 10A). Am I missing something?

    We will look into the scalable reference design, thanks for the interesting suggestion. What are the pros and cons of parallel charger configuration (disregarding BOM cost)? Are there any concerns for converter stability or anything else?

    Kind regards,

    Vasily

  • 0 in reply to Vasily Shmatalyuk
    TI__Mastermind 33525 points

    Hi Vasily,

    If you use 5-mohm current sense as the RAC, the input current is limited to 10A.  That is what TI suggests and most customers used. 

    Some customers trick the charger with smaller current sense, and hope to design for higher current. It is risky and not recommended. Sorry to confuse you.

    Dual charger has been used for a long time, in order to spread out the heat. See training video below.

    https://www.ti.com/video/5838266346001

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 150 points

    Hi Tiger,

    Thank you for the clarification and apologies for the time it took me to get back to you. I came up with several follow up questions in the meantime.

    (1) is it fair to assume the nominal power limit with a single BQ25730/31 design is thus 260W, based on the 10A input current limit with a 5-mOhm sense resistor and 26V as the highest recommended external voltage?

    (2) what is the recommended safety factor/margin that should be used for devising a real output power based on the nominal max input power? Looking at the BQ25731 reference design (product page, test report) advertised as rated for 240W, I see that the max output power never goes above 218W (Vin=24.012V, Ibus=8.863A @97.45% efficiency -> 24.012V * 8.86A + 5.418W loss = 218.2W). This implies a ~10% margin of nominal max in vs max real out and supposedly includes the power losses. Am I reading this right? If we were to use 26V in and thus 260W nominal max in, would it be safe to design for ~334W max real out?

    (3) can external voltage of 28V be safely used with BQ25730/31 or is it discouraged? The absolute max external voltage is 32V and it makes sense if 4V of margin is too low but I want to hear your opinion on this nonetheless.

    Kind regards.

    Vasily

  • 0 in reply to Vasily Shmatalyuk
    TI__Mastermind 33525 points

    Hi Vasily,

    1. Your understanding is correct. 

    2. The derating guideline depends on the applications. I don't read 10% margin in the PMP test report. I don't know how you could design for 334W. 

    3. Please follow our recommended operation range spec in the datasheet. Exceeding absolute max rating is not encourage, it violates the part spec and may impact long time reliability. 

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 150 points

    Hi Tiger,

    1. Thanks.

    2. I made a silly typo, sorry about that. I meant 234W calculated as 260W - 0.1*260W(10% margin) = 234W.

    3. Sorry for being annoying, but I want to remove all ambiguity from your response. You mentioned minding the recommended operation spec and cautioned against exceeding the absolute max spec, but these are too separate numbers. The recommended range is for the Vbus pin on BQ25730 is 0-26V, while the absolute max limit is 32V. Does exceeding the recommended 26V with 28V, while still staying below the absolute max of 32V have potential to impact long time reliability of charger?

    Thanks a lot for your time and patience!

    Kind regards,

    Vasily

  • 0 in reply to Vasily Shmatalyuk
    TI__Mastermind 33525 points

    3. Stresses beyond those listed under Absolute Maximum Rating may cause permanent damage to the device. These are stress ratings
    only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under
    Recommended Operating Condition. Exposure to absolute-maximum-rated conditions for extended periods may affect device
    reliability.