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Charging super capacitance only with constant current

Wendy Wang1
Genius 11320 points
Other Parts Discussed in Thread: TPS92515-Q1

Hi,

Can i just use one phase: constant current to charge super capacitance? I want to know the difference between this kind of method and the typical charging method like the image below. Does the charging profile have to be first constant current and then current voltage? Or these two methods have their own advantages and disadvantages? Thank you!

Best Regards

  • 0
    TI__Guru*** 143950 points

    Wenxiu,

    Since a discharged capacitor looks like a short to ground to a power supply, it is best to use a constant current source or current limited power supply to charge the capacitor and then clamp with constant voltage when the capacitor reaches its maximum voltage.

    What is your capacitance rated voltage and charging current?  What is the input power supply to the charging IC?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 11320 points

    Hi Jeff,

    The two super-caps are in series and each one is 25F/2.7V. The charging current is 1A. And the block diagram is attached below. Our customer requires all the sockets to be qualified for Automotive applications. So we intend to choose TPS92515-Q1 to be the charger.


    Thanks & Best regards,

    Wenxiu

  • 0 in reply to Wendy Wang1
    TI__Guru*** 143950 points

    Wenxiu,

    That should work!

    Best regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 11320 points

    Hi Jeff,

     

    I have three more questions.

    1. Why in CV phase, the current will gradually taper down? Which part in the block diagram controls the current in this phase?

    2.  In CV phase, the super-cap has already been charged to the charge regulation voltage, however there still is power transferred to the super-cap since current is not zero. Is it ok ?

    3. If we just use a constant current source to charge the super-cap, when the voltage reaches the regulation voltage, we will stop the source from charging. And in this way, the current will suddenly be zero without tapering down. Is it ok for the super-cap?

     

    Thanks&Best Regards,

    Wenxiu

  • 0 in reply to Wendy Wang1
    TI__Guru*** 143950 points

    Hi Wenxiu,

    Regarding 1, the connection from the output regulation pin to the battery/cap and the battery/cap itself has some resistance. Therefore, the voltage sensed at the IC output regulation pin will be the battery cell/ideal capacitance + the charge current times that resistance.  As the battery cell/ideal capacitance continues to charge closer to the CV regulation (clamp) point, the current naturally tapers down. This is desired with a battery because batteries need time to relax down to their actual voltage.  In short, the charge IC doesn't control the taper, it only clamps the voltage to prevent overcharge.

    Regarding 2, the super-cap likely has some leakage current in its spec.  If small, that is probably okay.  If large, I suggest further investigation.

    Regarding 3, yes.

    Regards,

    Jeff