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BQ24650: Application Report SLVA829

Jose
Intellectual 315 points
Part Number: BQ24650

Hi,

After reading your application report SLVA829-September 2016, we were very clear about the problem, and the suggested solutions .

As for the diode solution in figure 8, it's very clear, except that for an 8A charger the circuit will look like a toaster and not a solar charger.

It may be a good solution for a circuit with a low charging current, but not for 8A maximum charge current as is the case here.

Do you have any additional documentation or suggestions for solving the problem in hardware without high thermal dissipation, e.g. using Mosfets?

Thanks.

regards,

Jose

  • 0
    TI__Mastermind 30332 points

    Hi Jose, 

    Thank you for reaching out!

    Could you please clarify what your requirements are for the solar charger so that I can provide some suggestions?

    Best Regards, 

    Aya Khedr 

  • 0 in reply to Aya Khedr
    Intellectual 315 points

    Hello Aya,

    Thank you for your reply.

    We have already designed several solar chargers with the BQ24650, and they work adequately, albeit with a lower maximum charge current.

    Right now we're designing a version with a maximum charging current of 4A, and the entire circuit will be encapsulated in epoxy resin, so thermal performance is a concern.

    We want to implement a reverse current blocking circuit to prevent the battery from discharging.

    The main concern is not when the solar panel voltage is between Vmp and Vbat, but we need to prevent a possible short circuit at the solar panel terminals, and the discharge of the battery with a high current in these conditions. This is our main concern.

    That's why using a 5A diode isn't viable, due to the reduction in efficiency.

    Even ignoring the loss in efficiency, the thermal issue in the above described conditions is a concern for us

    I hope this data is enough for us to have your suggestions.

    regards

    Jose

  • 0 in reply to Jose
    TI__Mastermind 30332 points

    Hi Jose, 

    Thank you for clarifying!

    I will look into this and provide some feedback by Thursday. 

    Best Regards, 

    Aya Khedr 

  • 0 in reply to Aya Khedr
    Intellectual 315 points

    Hi Aya,

    Do you already have an alternative to replace the suggested diode?

    In the meantime, we've carried out tests with voltage and current measurements, and thermal camera analysis, and it's absolutely impossible to use a diode with currents above 2A due to the high thermal dissipation this causes.
    Even a diode with a DO-214AB-2 package is unacceptable

    So TI suggestion of the SLVA829 report is interesting in terms of concept or for low charge currents, but unfeasible for higher currents.

    regards

    Jose

  • 0 in reply to Jose
    TI__Guru**** 166626 points

    Hi Jose,

    You can try multiple diodes in parallel but they will not share the current equally and you will need at least 3, 3A diodes in parallel.  The only other alternative I see is an ideal diode like https://www.ti.com/lit/ds/symlink/sm74611.pdf or controller with external FET like https://www.ti.com/product/LM74670-Q1. 

    Regards,

    Jeff 

  • 0 in reply to Jeff F
    Intellectual 315 points

    Hello Jeff,

    Thank you in advance for your reply, and the information provided.

    As we already have the TPS259474ARPWR in the BOM, and in this project the maximum charge current is 4A, we were evaluating its use as an ideal diode.

    Is there anything about the TPS259474ARPWR that we're missing that makes it impossible to use?

    I'd appreciate your comments.

    regards

    Jose

  • 0 in reply to Jose
    TI__Mastermind 30332 points

    Hi Jose, 

    I will look into this and get back to you with feedback about its use as an ideal diode.

    Best Regards, 

    Aya Khedr 

  • +1 in reply to Aya Khedr
    TI__Mastermind 30332 points

    Hi Jose, 

    Jose said:

    Is there anything about the TPS259474ARPWR that we're missing that makes it impossible to use?

    I don't see anything obvious here but I would suggest testing this implementation in your design. 

    Please let me know if you have additional questions/concerns. 

    Best Regards, 

    Aya Khedr