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BQ2980EVM-883: Recommendations for source+sink power supplies

Christopher Buckley
Intellectual 330 points
Part Number: BQ2980EVM-883
Other Parts Discussed in Thread: BQ2980

Hello,

In the user guide it states I need two power supplies and one "with current source and sink to simulate a charging and discharging event."

Further down in the guide (Step 2.4.5) it claims I should have this ability to source and sink from both supplies from 100mA to 200 mA.

In various sections of the guide there are recommendations that, "When using a power supply for evaluation, be sure the power supply can accept or is protected from any reverse current such as with a blocking diode if another supply is used in the system."

I have three questions surrounding these requirements:

1.) If I have an external reverse current blocking diode on each supply, do I still need the supplies to handle sinking any current? These two recommendations (adding a diode and having sinking abilities) seem to be conflicting or maybe I am misunderstanding something.

2.) If I do in fact need a supply with sinking ability even with protection diodes, what is an example or recommended DC power supply for this? I'm not familiar with sinking abilities from supplies nor do I see any such spec on all bench supplies I've looked at.

3.) Why are the sink abilities only from 100mA to 200mA? Shouldn't we expect up to 8A as per the maximum of the board?

Best regards,
Christopher

  • 0
    TI__Mastermind 42060 points

    Hello Christopher,

    You'd add the diode on power supplies you do not want any current to flow into. Most power supplies are not able to sink a current, so you'd have to look for one that is able to if you want to test a charging current. If you only a single power supply that is able to sink current, you could use this one as the cell connections and add a diode in the second one for the charger simulator in order to not allow any current to flow into the power supply you do not want to get damaged.

    You could also use a source meter in order to conduct tests relating to current.

    The sink abilities are that low as this is only a test and not intended to simulate real scenarios. For real testing I'd recommend using real cells and a source meter or real charger.

    The 8-A is the maximum allowed by the PCB layout design, if higher, the PCB itself could be damaged. Different PCB designs could have larger current capabilities.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 330 points

    Hello Luis,

    Thank you for your reply.

    Can you please provide an example of a supply that has sink abilities? I'm unable to find any such filter on digikey or the like, and from opening several datasheets I'm seeing nothing related to this. I understand if TI cannot endorse other makers/etc., but just an example of one such that I can look for similar devices knowing better how to spot this in a datasheet or supplier filter/search.

    I should have been more specific about my #3 question (Why are the sink abilities only from 100mA to 200mA?). It appears the "Electronic Load" is not specified (i.e. it could be 1Ohm or 1MOhm), so why is this current specified at 100mA - 200mA? Is there some internal resistance to the eval board that even if I short the pack terminals will only draw this amount? Otherwise from steps 3 and 4 it appears we'd have 3.6V at the Bat terminal and 4.0V at the Pack terminal. Assuming I have diodes on each supply, that would mean Bat voltage is off, and I've got 4.0V over some undefined load. So I guess this implies the load should be 20-40ohms? What happens if someone evaluates this without diodes and uses a 10ohm or 1ohm load and forces their supplies to sink 400mA or 4A? The lack of specifics here seems a little disappointing for an eval board.

    Also I am unfamiliar with this kind of test setup and placing two power supplies against each other just sounds unsafe to me. Assuming no diodes used and two supplies with this sink ability, what is the expected current in/out from each supply in the case of a 1ohm "Electronic Load"?

    Best regards,
    Christopher

  • +1 in reply to Christopher Buckley
    TI__Mastermind 42060 points

    Hello Christopher.

    I cannot really say. From my understanding, there are some power supplies that are able to sink current safely. However this is not often the case from what I've seen. I believe that a source meter may be a better tool in order to conduct these types of testing, it should be able to provide and sink current. If using a power supply that cannot sink current, make sure to use protective measures, like the diodes.

    These particular steps in the EVM are only meant to be used a simple demonstration of how the IC works. Power supplies can limit the amount of current it can output, so even if one PSU is 3.6-Vand the other one is 4.0-V, it would be limited by the current you set it as; you could set it higher if desired. The EVM is able to pass current of up-to 8-A reliably. As I mentioned, the second supply is only needed if you wanted to conduct a charging current test demonstration, else you can just use a single power supply and an electronic load to adjust how much current you want to draw, as shown in Figure 5 of the EVM guide, where OCD protection is demonstrated.

    You can see the specific performance summary of the EVM board in Section 1.4 bq2980 Circuit Module Performance Specification Summary.

    The charging test set-up requires more complicated set-up in order to conduct safely. Most power supplies could possibly receive some damage if connected directly together. Hence why the need for a power supply or source meter that is able to sink current is recommended. I'd recommend to test the charging with a real cell.

    If I missed anything or you have any further questions let us know. Also, if you want to continue from your other thread into this one, that is okay.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 330 points

    Hi Luis,

    After blowing a few components and piecing together the info from here and the guide I was able to get the majority of the evaluation steps to work. That being said, I'd highly encourage TI to revise this eval board+guide to include the specifics discussed in these threads. There are just too many things to easily overlook or misinterpret.

    Luis H. S. said:
    Power supplies can limit the amount of current it can output,

    This is not true for any power supply I have, and as such Figure 1 shows a very dangerous configuration. Yes there are warnings in surrounding text, but I'd recommend at least adding the optional diodes or other warnings to the figure caption.

    Luis H. S. said:
    The charging test set-up requires more complicated set-up in order to conduct safely. Most power supplies could possibly receive some damage if connected directly together. Hence why the need for a power supply or source meter that is able to sink current is recommended. I'd recommend to test the charging with a real cell.

    Don't you think the details of this "complicated set-up" or recommendations to use a cell, should be included in the guide?

    Anyways, I appreciate your help, and despite still not understanding how to find a supply with sinking capabilities I eventually got the setup working with some diodes and higher wattage resistors.