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BQ25792EVM: BQ25792 efficiency at low charge current

Emilien Chappatte
Intellectual 355 points
Part Number: BQ25792EVM
Other Parts Discussed in Thread: BQSTUDIO

Hello,

I did some efficiency tests with the new BQ25792EVM board, this IC is interesting with FETs integration.

The configuration IS 2S, 10Ah lithium cells and input is 12V.
When charging current is set from 180mA to 2000mA, efficiency is very good, from 91% to 94%

When charge current is set at 170mA or below, efficiency is worst, around 80% in my tests.
I did some measurements with thermal imager and at about 21°C ambient temp, when IC is charging at 200mA it is at around 30°C.
If I set the charge current at 170mA, IC temperature rise to 32-33°C - I set lower charge current and IC is hotter ?

But one more annoying/strange behavior, once current is set at 170mA or below, if the current is set anywhere up to 800mA, efficiency is lower from about 10%, in the 80-90% range instead of above 90% !
To "restore" the efficiency, disable en enabling charger (from BQStudio) solve the issue, or using current more than 800mA solve the issue too - until we set a value at 170mA or below.

Can someone help me about this behavior ? Anything I set wrong in charger ?

Thanks and best regards,

  • 0
    TI__Guru**** 165886 points

    Hi Emilien,

    I am not aware of any issues at light load.  Can you provide the battery voltage at which you are testing?  Are you using an EVM and if so, what is its revision, E1, E2 or A? 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 355 points

    Hello Jeff,

    Thanks for the answer ! I am using the EVM board, labeled BQ25792EVM and the silkscreen just above is BMS034E2, so rev E2 I guess ?

    Here is my efficiency chart so you have more detailed information. Please note between 180 and 800mA the efficiency is good , but this was because after rising the current above 170mA, I disabled and enabled the charger, otherwise the efficiency would have been around 80-85%.

    Values are measured with DMM at terminals of BQ25972 EVM Board. Battery side is connected to BQ25972EVM and large batteries.

    Let me know what you find, if I have some settings to change ? Or if you can reproduce this behavior ?

    Thanks !

  • 0 in reply to Emilien Chappatte
    TI__Guru**** 165886 points

    Hi Emilien,

    The data above looks like what I would expect.

    To confirm you test setup, is a the DMM set to measure current in between the BAT pin and the battery or simulated battery?  If so, is it set to auto range?  DMM's measuring BAT current set to auto range can cause oscillation.  Better to set it to manual and change it as the current changes.   Best is use a source meter like a Keithley 2400 series. 

    I will test efficiency using a Keithley source meter.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Guru**** 165886 points

    Hi Emilien,

    I reproduced your efficiency results.  I have confirmed with design that this behavior is not unexpected.  In order to prevent oscillating between PFM and PWM modes over such a wide input voltage range, we set the peak inductor current trip point to 2A with a wide hysteresis range.  If the host ramps charge current down from high (PWM mode) to low current (PFM mode) you won't see the issue. But if you start low (without first toggling CE) then go high, you will stay in PFM mode until the inductor current hits the 2A peak.  If this is problem in your application, you can toggle /CE as you have seen or toggle PFM mode using REG12b4.

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Intellectual 355 points

    Hi Jeff,

    Many thanks for your explanation and doing the test again !
    Yes the data above are good, when efficiency is lower is not reported in this table.

    Ok I missed the PFM operation in the datasheet, now I better understand what is going on.

    I do some tests again very soon, and I post result if the PFM mode is what cause me the efficiency drop in the 200-800mA output current for my 2s setup (when starting with a small current).


    Regards,

    Emilien

  • 0 in reply to Emilien Chappatte
    Intellectual 355 points

    Hi Jeff,


    I did more testing and indeed PFM mode activation/deactivation is what affect efficiency in my setup !

    For example, when starting with a low current, then setting it to 300mA give me just above 80% efficiency. Disabling PFM mode at this same current make the efficiency rise above 90%, as in table above.

    However, disabling PFM at 100mA drop efficiency to about 65%, instead of almost 80%  when enabled.
    So I just have to know and handle this when starting at low current in our 2s application, to ensure best efficiency within our usage.

    Thanks for your great help !