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TL431-Q1: Supercapacitor balancing circuit

Jessica Riley
Intellectual 480 points
Part Number: TL431-Q1
Other Parts Discussed in Thread: TL431

Tool/software:

Is it possible to use the TL431-Q1 shunt regulator as a supercapacitor balancer?

I have a 12V supply with the output current limited to 100mA .  If I understand this correctly, I should be able to use the TL431-Q1 to clamp the supercapacitor voltages to 2.5V (less than the 2.7V rating) while they are charging up.  Once they are fully charged, they should be balanced to ~2.4V, at which point the TL431-Q1 should only conduct the quiescent current.  Effectively this is using the TL431-Q1 as a zener diode replacement for more precise balancing voltage clamping.

  • 0
    TI__Expert 3940 points

    Hi Jessica,

    Thanks for your question!

    I can try to run some tests in the lab tomorrow to confirm, but in the meantime, here are some thoughts to consider:

    1. A 2.5V drop across five TL431 devices results in a 12.5V drop in total (greater than V1)

    2. Once all the capacitors are charged, all five TL431 devices will still be turned on and draining current.

    Best Regards,

    Andrew Li

  • 0 in reply to Andrew Li
    Intellectual 480 points

    I appreciate the help!

    Supposing the supercapacitors are perfectly balanced in charging, I would not expect the TL431 devices to turn on at all (since 12V / 5 = 2.4V which is less than VREF=2.5V).  The TL431 are only needed to clamp the supercapacitor voltage in case they do not charge up equally (worst case being one supercap charges up much faster than the others).  

    When the supercapacitors are balanced (~2.4V), I would expect the current into the TL431 to be the sum of the reference pin current (4uA max) + off-state cathode current (1uA max) = 5uA total "on" current.

  • 0 in reply to Jessica Riley
    TI__Expert 3940 points

    Hi Jessica,

    Even if 2.4V is applied to the reference of the TL431, there will be some Ika as a result as shown in this chart below:

    Additionally, I just conducted a test with two TL431 devices and they can be stacked as shown in your diagram. However, they will remain active and drain current according to V1 and R1 as found in your diagram even after the capacitors are charged. Their behavior was as expected with about 2.5V drop across each TL431 until GND.

    Please let me know if you have anymore questions, and I hope this helps!

    Best Regards,

    Andrew Li

  • 0 in reply to Andrew Li
    Intellectual 480 points

    Thank you for doing that test!

    So long as the current is <1mA, I am not concerned about the current draw through the TL431 while the capacitors are fully charged (it's negligible compared to my other system loads). 

    Given this, it looks like this is a good solution for clamping the capacitor voltages while charging up, so long as I can ensure the charging current is current limited to less than the current limit of the TL431 (currently I have it limited to 100mA constant current charge).

  • +1 in reply to Jessica Riley
    TI__Expert 3940 points

    Hi Jessica,

    I'm happy to hear that this will work for your application!

    Since there are no more questions, I will close this thread. To reopen this thread, simply reply to this message.

    Best Regards,

    Andrew Li