BQ77915: Charge MOSFET doesn't turn

Stefano Marrazza

Part Number: BQ77915

Hello,
in my project I use two BQ7791500 6S LiPo battery BMS (3 cells on each BQ7791500).

During tests, I found an anomalous behavior of the BMS.

When the battery voltage is lower than 23.8 V and I put PRES pin high, both charge and discharge MOSFET turns on and I can charge and discharge the battery and everything works fine.

When the battery voltage is gretaer than 23.8 V and I put the PRES pin high, only the discharge MOSFET turns on while the charge MOSFET stay off: in this condition I can discharge the battery, but charge is interdicted. If I discharge the battery with a current greater than 1.2 A, the charge MOSFET turns on (and this behavior is ok as described in the datasheet) and it remains on also if I reduce the discharge current and so I can charge the battery. As soon as I put the PRES pin floating and high again (after 4.5 s in order to have a complete switch off of the MOSFETs) I have the same condition: discharge MOSFET turns on while the charge MOSFET stay off.

I measure the cells voltage and I can't find any problem (OV, UV, unbalanced cells) and also temperatures reading are correct.

Do you have any suggestions?

Thank you in advance

over 1 year ago

0 Max Verboncoeur over 1 year ago

TI__Mastermind 25980 points

Hi Stefano,

It may be that the device is triggering an OV, possibly due to a transient during turn on, and then the cell voltage isn't relaxing below the hysteresis threshold, so the OVP persists.

What could be happening with the discharge current is that the body diode protection turns the FET back on, and the cell voltage might also fall below the hysteresis threshold during the discharge.

If you aren't already, you should measure the cell voltages at the device's pins to get a more accurate idea of what the device is seeing.

Regards,

Max Verboncoeur

0 Stefano Marrazza over 1 year ago in reply to Max Verboncoeur

Prodigy 135 points

Hi Max, thank you for your reply.

I measured the cell voltages and any overvoltage or even unbalanced cells are present.

I measure with oscilloscope also during turn on but no spikes or overvoltage are present.

Furthermore, I tested 4 battery pack and I have the same behavior on each one.

Any suggestions?

Thank you for your support, regards

Stefano

0 Max Verboncoeur over 1 year ago in reply to Stefano Marrazza

TI__Mastermind 25980 points

Hi Stefano,

Here's what I found in Section 9.3.2.1 of the datasheet:

Entering and exiting HIBERNATE mode counts as a reset and will cause the device to enter an OV fault state until all of the cells fall below the hysteresis threshold (4V in your case).

Here's an old thread that discusses the same: (+) BQ77915: CHG drive output being disabled erroneously? - Power management forum - Power management - TI E2E support forums.

Regards,

Max Verboncoeur