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BQ77915: Charge mosfet continuous toggle ON - OFF

Michele Giorato
Prodigy 30 points
Part Number: BQ77915 
Hi, I need help to understand what the BQ77915 is doing .
I have inserted it to protect 4 x 18650 3500mAh batteries.
What happens is that charging the batteries in constant current at 2A (both the charge and discharge mosfets are on) and everything seems to work regularly.
At a certain voltage of around 3.95 V per cell, the charge mosfet opens and charging stops. After perhaps 2 seconds the mosfet closes and continues to open
close the circuit until the charger gets angry and signals the fault.
At the beginning I thought it was cell balancing that interfered and then I disabled it but nothing changes.
t would seem an overvoltage but theoretically it occurs at 4.2V, it is not an overcurrent because I measured the voltage on Rsense and is 20 mV, well below the 60mV of overcurrent detection.

To be on the safe side, I replaced the thermistor with a 10Kohm resistor in order to disable the thermal protection but this also does not solve the problem.

The only thing I noticed tonight is that the chip ordered by digikey in order to mount these 2 prototypes brings back in the container bag BQ7791500 but the chip is marked as

BQ77915 
(Ti logo) 89K G4
C3LZ

I checked on the datasheets but I can't find a hypothetical version 89, only 01, 02, 03, 04

What version of chip am I using?

  • 0
    TI__Guru* 81034 points

    Hi Michele,

    I checked the lot trace code based on the chip marking and you are using the BQ7791500. If you are seeing both of the FETs turn off, then the fault table in the datasheet indicates the possibilities of open wire, over-current if you are charging. Temperature faults or over-current can turn off both FETs during discharge.

     If you want to send an image of your board schematic, we can help check your connections. Also, make sure the CCFG pin is set to 4-cell mode.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 30 points

    Thanks Matt for the quick reply.
    The confirmation that it is really a BQ7791500 comforts me.
    The mosfet which continues to change from on to off is only that of charge Q2. R17 is not mounted in order to disable the cell balancing, the thermistor TH1 is not mounted because there is R19 in its place.

    On pin 13 there are about 13.2V which continue to pulsate while on pin 12 there are constant 13.2V.

  • 0 in reply to Michele Giorato
    TI__Guru* 81034 points

    Hi Michele,

    I don't see anything incorrect in the schematic. Maybe check carefully to see if there is an open wire condition. Check whether the CHG FET stops toggling when you disconnect the charger. 

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 30 points

    Hi Matt,

    I paused and widened the view.

    The toggle of the mosfet is caused by an overvoltage which activates the protection by opening the charging mosfet.

    First I realized that I was checking the voltage across the batteries but the chip doesn't necessarily do the same.

    At present there are two parasitic resistors due to the connector and the wiring.
    The contribution is less on the negative pole because more contacts are used in parallel.

    What happens is simple. When charging at the ends of the battery I read 3,951 Volts but the chip "sees" also the voltage drops on the parasitic resistors therefore 3.95V + (100 mOhm x 2A) or 4.195V.
    The 4.2V open the charge mosfet and the contribution of the current passing through the parasitic resistors disappears and the chip "sees" 3.95V again.
    Once the delay time has elapsed, the chip leaves the protection and the 2A charge flows again but at this point the parasitic 200mV still reappears and then the overvoltage protection is reactivated.

    Obviously the phenomenon repeats itself until the battery charger realizes that something is not working as it should and is deactivated.

    Now everything makes sense, I just hope this experience can be useful to someone else.

    Thanks for the collaboration otherwise I would have continued to think that the chip was not the right one.

  • 0 in reply to Michele Giorato
    TI__Guru* 81034 points

    That's great news Michele! Thanks for sharing the details - I am sure this will help us and others to recognize what might be happening if this symptom is observed.

    Matt