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OPA2891: balancing supercapacitor, help with design

Emanuele Peruzzi
Intellectual 645 points
Part Number: OPA2891
Other Parts Discussed in Thread: OPA567

Tool/software:

Hello dear members of the team,

i need to design a supercapacitor balancing circuit, i have 8 supercaps in series and i would like to balance it with operational amplifiers.

I attach a design with opa2891 used to balance the caps, but this circuit is totally not working. I understand that this opa2891 needs 9V between supply pins to operate, so, in this design, it will not work until the voltage at the cap series is above 9V. But it isn't working anyway, even with series' voltage > 9V.

- I can't understand why. Do you have any suggestion? Is there a correction that should i do? 

- Can the OPA567 be a better option than OPA2891? 

Thanks if you have any hint,

Kind Regards

emazed

de1247b.pdf

  • 0
    TI__Genius 16605 points

    Hi Emazed,

    Can you share what you are seeing with the circuit? One issue is the stability concerns due to the capacitor directly at the output. One option would be to add an isolation resistor at the output of each amplifier and see if this helps stabilize the circuit. It is a very large capacitor, so I am not sure if this would ultimately make the circuit stable, but this would be something that would help. The other concern is potential input/output range violations for the last devices, as this resistor ladder is dropping voltages towards U4, the input voltage at that buffer might be violating your input common-mode range. As far as the choice of amplifier, I am not sure if a high-speed amplifier is required compared to a slower device like the OPA567. High speed amplifiers usually come with more stability considerations. For this application, if higher current drive is also needed the OPA567 would certainly help. For the OPA567, the supply range is up to 5.5V, which might not work if you require 20V.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 645 points

    Hello Ignacio,

    i have these voltages onto the 8 caps:

    V1 = 2.180. V2 = 1.560 V3 = 1.140. V4 = 0.84. V5 = 0.710. V6 = 0.475. V7 = 0.450. V8 = 1.005.

    I read these values when i charge the whole stack to near 10V. After i disconnect the supply voltages, i can see that no capacitor is still charging, so the balancer is not working at all.

    This balancer is too hard to fix. I give up. I will make some simpler with a voltage supervisor and a bjt. Thanks

    Kind Regards

    emazed

  • 0 in reply to Emanuele Peruzzi
    TI__Genius 16605 points

    Hi Emazed,

    I am sorry to hear that. The concern with the configuration is the stability of the devices as there is no isolation from the very large capacitive load. If you have any questions in the future with any of our devices, please feel free to reach out.

    Best Regards,

    Ignacio