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BQ25504 boards in parallel, Load switching

Adri E
Prodigy 145 points
Other Parts Discussed in Thread: BQ25504

Hi,

I posted in http://e2e.ti.com/support/power_management/battery_management/f/179/t/301685.aspx about some problems with the BQ25504 in parallel operation.

I would like to share my further findings and experiments with the community and I have a next question concerning the parallel use.

 

 First the current system.

We prevented the situation that the load could bring the BQ25504 paralleled Vstor’s below Vbat_uv. After this the system is (still) up and running, and works ok now for more than two months continuously without problems.

This is in agreement with the answer of Yogesh (load issue) and with the datasheet and I think also I am understanding it now more or less. For my understanding I did some experiments on a single EVM on the combination of not using Vbat_ok for switching and the BQ25504 switching off at Vbat_uv. When in that case you do not have enough power from the source in cold-start mode you get a low Vstor on the load and the system will freeze. The only solutions I saw and have verified is to lower or switch off the load or provide enough power at the input. This works ok and of course the automatic Vbat_ok switching of the load also works ok.

So far so good.

 

We now are planning a further project and we would now like to implement the Vbat_ok switching for the load of (many) BQ25504’s with outputs in parallel (inputs all separate).

The basic set-up will be as the EVM schematic and we plan to have a super cap on each Vbat and Vbat_ok will be far above Vbat_uv.

I see at least three options:

  1. Each BQ25504 has its own FET switch on Vbat_ok and then paralleling these switched outputs to the load.

  2. All BQ25504 outputs in parallel and then switch one FET to the load on one of the Vbat_ok outputs.

  3. All BQ25504 outputs in parallel and then switch a FET on an AND function of the Vbat_ok’s of all of the BQ25504’s.

We would prefer (as the simplest and most cost effective solution) option 2 with a low (ground) side switching. I think that this works ok at least in normal operation when all Vstor to Vbat FETs are conducting.

Maybe option 1 has some advantage when there is a failure in one of BQ25504 circuits or the inputs.

 

Please your opinion on this or another possibility.

 

Thanks and regards,

 

Adri

  • 0
    TI__Guru**** 166176 points

    Keep in mind that  VBAT_OK actually monitors VSTOR.  Also, I am assuming you are tying only the VSTOR outputs in parallel.

    If the VSTOR pins of each are tied directly together (i.e. no isolating diode following each before connecting to the load), then option 2 is the simplest since the output of each 504 is helping the others keep the voltage up anyway.  The VBAT_OK of one will essentially be the same for all since the outputs are tied together.  But this means that if 1 of the 504's inputs collapses, the others will have to pick up the burden of keeping that  504's supercap charged. 

    Another option would be to connect the VSTOR outputs in parallel following Schottky diodes on the output of each.  Then if one collapses, it does not burden the others.  To remove the load, you would use one PFET switch with a pullup resistor and a pull down NFET.  The gate of the NFET would be driven by parallel Schottky's from each 504's VBAT_OK outputs.

     

     

  • 0 in reply to Jeff F
    Prodigy 145 points

    Hi Jeff,

    Thanks for the very fast answer in the weekend.

    Yes, I found out with the experiments about the Vbat_ok monitoring.

    Thanks for the suggestions, both  are useful options and the price of the schottky's is no big issue.

    The wiring will in our case probably be the most important factor of choice for a solution.

    Thank you very much and best regards,

    Adri

  • 0 in reply to Adri E
    Prodigy 145 points

    Yet another question,

    How about paralleling BQ25504's with some equipped with a re-chargeable battery and some with super-caps on Vbat?

    Of course with the appropriate values for Vbat_ov, Vbat_ok and Vbat_Uv.

    I do not see problems except for the thermal dissipation of the internal FET in the BQ25504 at this moment: Is this OK?

    Thanks,

    Adri

  • 0 in reply to Adri E
    TI__Guru**** 166176 points

    As long as the VBAT_UV levels are such that a depleted super cap is isolated from the batteries and therefore doesn't drain them, then I don't see a problem.  Also, keep in mind that the internal reference voltage (VBIAS) against which the all the thersholds are compared will be slightly different for each 504; therefore, you will not have exact load balancing between the 504's.

  • 0 in reply to Jeff F
    Prodigy 145 points

    Thank you very much Jeff.

    I think we will run some tests with this setup when we have time for it.

    Best regards,

    Adri