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BQ4050: CHG is OK but DSG MOSFETs not turning on

Harald Schilke15
Prodigy 50 points
Part Number: BQ4050

Hello

I have designed my own PCB based on the application in datasheet (4s1p --> 14.4V). The DSG and CHG FETs are changed to PSMN1R7-30YL.

Fuse is not used --> FUSE-Pin ist connected to Vss.

The FET-EN is set to HIGH.

Charging works properly but discharging dosn't work.

In charging mode the CHG-Pin is about 25V and this FET is ON. --> OK!

But in discharging mode: The DSG-output ist about 0 to 1.5V.

Another strange value is the PACK-voltage. This is only 4.6V insead of 15V battery voltage. At the resistor R12: PACK+ = 15V and chip-PACK-pin (pin 27) is only 4.6V????? 

In this Screen shot the charging mode is shown:

The same in charging mode: The DSG-FET is not switched ON!?!?!? The charging current flows only via the body diode of this FET --> high power dissipation! 

I work on this for hours :-(

Can anyone help?

Thanks...

  • 0
    TI__Guru* 76230 points
    Hi Harald,
    I believe something is wrong with your schematics. You need to review your schematic and carefully compare it to the reference schematic in the EVM user guide. I say this because the dsg flag is getting set meaning the gauge is sending the right signal something on your board is preventing the dsg pin from actually been controlled.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Prodigy 50 points
    Hi Onyx
    Thank you for your answer.
    I checked the schematic and the pcb. The only differences between my design and the schematic of the EVM are the NC-Pins (Pin7, 14,29) and the Fuse-pin.
    The NC-pins are not connected in my design. (NC = not connected). Now I checked the datasheet and there it is recommended to connect the NC-pins to Vss. Could that be a Problem if this pins are open ?????? I can not believe. Because of the smal package and pins it is impossible to connect this pins by hand. Therefore I can't check it. I try to connect one of These NC-pins with a fine needle. But nothing changes.
    The Fuse pin is connected to Vss via 10kOhm. I tried to connect it directly to Vss --> nothing changes.
    I tried a second board and all the same disaster.
    I have no idea what the Problem is!

    DSG FET doesn't switch ON!

    Have you any idea????
    Many thanks
    Harald
  • 0 in reply to Harald Schilke15
    Prodigy 50 points
    Hi Onyx
    the Problem is solved! Yeah!!!
    The reason for this was a false footprint of the Q4 transistor. Gate and Source of this transistor was swapped. Than the gate of Q3 (DSG-FET) is pulled to Vss via its body diode.
    The Pack-Voltage is now OK too!

    All is fine.
    Sorry for your time wasting and many thanks...
    Harald