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Measuring Efficiency of the bq25504

Nilson Yang
Prodigy 210 points
Other Parts Discussed in Thread: BQ25504

Hi Sir,

We tried to measure efficiency of the BQ25504 based on SLUA691. However, we can't get result which like fig.5 of specification.

We disabled the MPPT by VOC_SAMP connect to VSTOR.

We used 1 power supplier to add power in VIN_DC, and used 2 FLUKE-115 to measure input voltage and current.

Used one resistor 150OHM for output port loading (We also tried the PEL-2040 - Electronic Loads, the result is the same as resistor.) , and used 2 FLUKE-115 to measure output voltage and current.

The result is shown as below,

The efficiency is almost equal to 76.15% [(3.592x0.004)/(0.0993x0.019)].

Was our test wrong?

Does we need add any voltage source in output power (In SLUA691 SM2)? And why?

Thanks a lot.

  • 0
    TI__Mastermind 19430 points
    The low input resistance of the voltmeter inflates IIN measuremts. Please make sure you
    - set the output impedenct to HiZ if available.
    - Disconnect before measuring current. - Move to positive side of current meter
    - Add additional CM to measure leakage into voltmeter
    When you connect a current meter in series at the input, the pulsing input current of the hysteretic DCM switcher will be measured inaccurately.
    - You can add a low leakage cap to filter, such as COG, X7R low leakage ceramics.
    - Increase number of samples or average for measurement averaging.
    Please refer to App Note SLYT558 at ti.com for more detail on how to accurately measure efficiency of ultralow-Iq devices.
  • 0 in reply to Jing Zou
    Prodigy 210 points

    Hi Jing Zou,

    We had ever tried to remove all the Volt Meter, but we get the same result.

    And we also tried to add lager CAP on input power (CHVR in SLUA691 FIG1 which is shown as below).

    But we still confuse that why does we need to add one voltage source at SM2?

  • 0 in reply to Nilson Yang
    TI__Mastermind 19430 points
    SM2 is used to simulate a battery/storage element. Please see datasheet page 7 for the test conditions of the efficiency measurements.
    1. Set the VBAT_OV threshold to 5.5V. Disable MPPT.
    2. Connect source meter to VSTOR. Set the source meter to measure current and voltage source (VSRC) o 1.8V, 3.0V, and 5.5V.
    3. Connect source meter to VIN_DC. Set voltage source = 1V and step current compliance (ICOMP) FROM 0.01 to 100 mA
    The effiency shown in the datasheet is the efficiency when the IC is trying to charge a storage element.
  • 0 in reply to Jing Zou
    Prodigy 210 points

    Hi Jing Zou,

    Does it mean that we need to add battery to CBAT? Because if we set BAT_OV threshold to 5.5V, the VSTOR only Output 5.5V without battery. We think that we remove the battery to make sure which battery does not have any influence on our test (input power (VIN_DC) convert to output power (VSTOR))

    VSTORY is an output. Why does we need to add another external voltage source (VSRC) to VSTORY?

    I redraw out test block which is shown as below,

  • 0
    Prodigy 210 points
    Hi Jing Zou,

    Does it mean that we need to add battery to CBAT? Because if we set BAT_OV threshold to 5.5V, the VSTOR only Output 5.5V without battery. We think that we remove the battery to make sure which battery does not have any influence on our test (input power (VIN_DC) convert to output power (VSTOR))

    VSTORY is an output. Why does we need to add another external voltage source (VSRC) to VSTORY?


    Thanks.
  • 0 in reply to Nilson Yang
    TI__Guru*** 143830 points

    You must have something to hold the voltage at VSTOR at a fixed point long enough to take the efficiency measurement.  A large capacitor or battery would do that or you can attempt to place the exact resistive load that balances with the input power (i.e., efficiency = Po/Pi=(Vout*Iout)/(Vin*Iin)) at VBAT_OV, without collapsing.  The source meter simulates the battery.  As I am sure Jing has mentioned, the pulsing input currents at in and out are going to be difficult to measure with most meters unless they have averaging capabilities.  The source meter has all of these features and will sit at a voltage (Vout) and sink current (Iout).  If you only have 1 source meter, use it on the output and use a power supply with series resistor on the input.

  • 0 in reply to Jeff F
    Prodigy 210 points
    Hi Jeff,

    Therefore, we maybe need one source meter instead of Fluke 115.
    Sorry for so much re-post, but I can't delete them.

    Thanks.