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Original question:

BQ77915: BQ7791500 AFE chip DSG and CHG MOSFETs not turning ON.

BQ77915: BQ7791500

Bharath Balakrishnan
Prodigy 85 points
Part Number: BQ77915

Hi,

I'm using the AFE chip BQ7791500 for a 48V BMS system. There are 3 chips cascaded (as per the data sheet connection recommendations) to achieve this configuration. The cell voltages are reaching the ICs and are well within the required range. I ran a low current discharge to test the UV cutoff. When the cell voltages hit 2.9V, the DSG MOSFETs cutoff (CHG MOSFET still ON). However on load removal, the DSG MOSFETs did not turn back ON. Plus, when I connected the charger, the battery pack was not being charged even with CHG MOSFETs being ON for all the time. As far as I know, for charging the battery pack, DSG MOSFETs needn't be ON and CHG MOSFETs should be ON. Please let me know what could be the issue?

Thanks in advance!

Bharath B

  • 0
    TI__Guru 54715 points
    Hi Bharath,
    To recover from a UV, the bq7791500 must have both the load removal and the recovery of the voltages above the UV Hyst level. See the section 6 configuration and table 5 in the data sheet. The parts will pull down on LD, so they may recover from a load when it is disconnected. In some cases the charger must apply a voltage to the PACK terminals to bring the PACK- low to cause the PACK- and LD to go low.
    At low current the cells will not likely recover from the UV event by themselves, charging will be required. As you noted, CHG remains on which should turn on the charge FET if a sufficient voltage is applied to the PACK terminals. DSG is low so the discharge FET is off and will act as a diode to allow charging. The PACK- terminal is allowed to float, it may float up to the level of the CHG output depending on the resistors used.
    When your charger applies a voltage more than about 0.7V higher than the battery voltage, charge current should begin to flow. When the cell voltages recover above the UV + UVHyst level DSG should go on. Alternately if the charge current is high enough to trip the state comparator, the DSG pin comes on for body diode protection of the discharge FET.
    Check the battery voltage and charger voltages when no current is flowing. Also check the gate voltages of the FETs to be sure all resistors are connected and voltages are reaching the gates. Finally check that the FETs were not damaged in some way during assembly.
  • 0 in reply to WM5295
    Prodigy 85 points
    Hi,
    Thanks for the prompt reply. We tested the charging using a different charger. The battery started charging even under the condition DSG-OFF and CHG-ON. Apparently, the first charger we used has got a pack voltage measuring step before starting the charging and it doesn't provide the Pack voltage +0.7V unless it measures. Hence it couldn't charge.
    Your suggestion was helpful.
    Thank you,
    Bharath B