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BQ24650: No charging current

Fabio
Prodigy 10 points

Part Number: BQ24650

Tool/software:

Hello,

We have designed a solar charging circuit using the BQ24650 for a single cell Li-ion 3x18650 battery pack (nominal 3.7V).

We have tested using various panels at 9V and 12V and adjusted MPPSET divider accordingly. Also tested with current limited power supply and the result is always the same, no charge current and 0V between SRP and SRN.

As soon as Vin is applied STAT1 is ON and STAT2 is OFF and they stay like this no matter whether there is a battery connected or not. This is not the expected behaviour, which seem to indicate that the battery absent detection is not working properly. 

More details:

The VFB divider is made out of 2x 100K resistors. The charge current is set to 0.667A by using a sense resistor of 60 mOhm. The inductor is 22uH (Bourns SRN8040-220M). The MOSFETs are a Dual N-Channel 40 V (SI7288DP-T1-GE3).

We followed the reference design from the datasheet and the EVM (www.ti.com/.../sluu444a.pdf) with some small modifications as suggested here: www.ti.com/.../slva829.pdf. In particular, we removed D1 but we decided not to mount the Schottky at the output. Output capacitors (after SRN) are 0.1uF in parallel to 10uF then 0Ohm in series and a 1uF in parallel at the output.

See below voltages:


VREF= 3.3V
VTS = 1.953V
VREGN = 5.96V
VMPPSET = 1.20V
STAT1 = ON
STAT2 = OFF
Vin is about 12.5V (tried various voltages between 9 and 15V and the other voltages don't change significantly)
VCC is about 12.5V
Vterm_en = VREF (tried also =GND and the result is exactly the same)
MPPT set to 12V by using divider made up of 324K and 36K resistors.

I should ideally be using a scope the measure the below but I am afraid I don't currently have access to it so I am sharing multimeter measurements (the values read are stable). Values are approximated but should hopefully give you an idea on what is going on. 

When battery connected:
VBTST= 8.94V
VSRN = VSRP = 3.6V (= battery voltage)
VHIDRV= 3.8V
VLODRV = 0V
VFB = 1.788V
No current drawn from the panel/power supplier

When battery disconnected
VBTST= 7V
VHIDRV= 1.2V
VLODRV = 0V
VFB = 380mV
VSRN = 0.75V
No current drawn from the panel/power supplier

I'd very much appreciate your help with this. Happy to share the full schematic if I can do that privately.

Best Regards,
Fabio

  • 0
    TI__Mastermind 47978 points

    Hello Fabio,

    Yes, Can you provide your schematic? Here is my email c-moyer@ti.com

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 10 points

    Thanks, Christian. Schematic sent.

  • 0 in reply to Fabio
    TI__Mastermind 47978 points

    Hello Fabio,

    I'm reviewing your schematic, I will provide an update after I gather more information.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    TI__Mastermind 47978 points

    Hello Fabio,

    Fabio said:
    we removed D1 but we decided not to mount the Schottky at the output.

    This is required in the design. The diode is used to block reverse current from the battery back to the input, and provides reverse voltage protection for the VCC pin. Can you add this diode?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 10 points

    Hi Christian,

    I have added a diode at the output (before the 1uF capacitor - where R72 was) and not much has changed. I have use what I had on hand that would more or less fit, i.e. PMEG2010ER (https://assets.nexperia.com/documents/data-sheet/PMEG2010ER.pdf). It is probably not the best choice for the application but I thought it should do. Please let me know your thoughts on this and advise.

    Apart from the fact that now SRN and SRP are not at the same voltage as the battery (I measure about 650mV when the battery is connected) all else is basically the same. To be honest, I am not clear on how this diode missing would explain the no charging current issue. Please elaborate.

    Do you see an issue happening with the battery absent detection?

    What else do you advise? 

    I am looking forward to hearing from you.

    Best Regards,

    Fabio

  • 0 in reply to Fabio
    Prodigy 10 points

    Hi Christian,

    I have done some more testing over the weekend and I have got some slightly more encouraging information to report.

    While poking and probing the board I got the charging started, or at least that it's how it looks like. First noticed this when I measuring VMPPSET test point (through hole pin header) with the multimeter. It may well be some dodgy contact but if so I have not found it yet. I have not been able to properly replicate this behaviour, funnily enough currently only some seemingly random noise on some of the charger pins seem to get the charge started.

    Please note that for simplicity I have tested the above using a power supply with current limit at around 0.6A, which should be pretty close to the max current 0.667A set by the 60m Ohm sense resistor. Occasionally tried with a solar panel too and didn't observe any difference.

    1) Any idea whether some noise and/or some charging/discharging capacitor could explain this behaviour?

    In any case, when the charge starts, the current drawn is stuck at around 30mA and SRP-SRN is constant at 4.6mV. Left this going for a while (well over 30 minutes) and all stays the same.

    STAT1 is always ON and STAT2 always OFF, before and after the charge starts.

    VMPPSET is 1.20V. The battery is at about 3.6V.

    I observed exactly the same behaviour explained above for both TERM_EN=GND and TERM_EN=VREF.

    2) How can the fixed current and sense voltage be explained? By reading the datasheet it feels like the 4.6mV value is significant as the charger could be stuck in a pre-charge or perhaps in under-current (please note that same behaviour is observed when termination is enabled or disabled)? Any suggestions?

    I am looking forward to hearing from you.

    Best Regards,
    Fabio

  • 0 in reply to Fabio
    TI__Mastermind 47978 points

    Hello Fabio,

    I'm reviewing this information, I will provide update after I gather more information.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    TI__Mastermind 47978 points

    Hello Fabio,

    Yes, it seems that the device is in precharge mode. What is the maximum charge voltage?

    Can you try this test with a battery 1-2V less then the maximum charge voltage.

    Best Regards,

    Christian