BQ76930: Shutdown from Short-Circuit

Alex N

Part Number: BQ76930
Other Parts Discussed in Thread: BQ78350,

Hello,

I am working on a BQ78350 design using a BQ76930 analog front end. I am having trouble during some high short-circuit pulses.

The pulses are around 260A for 400us.

The problem is with the power input to the BQ76930. I have a separate connection to the cell for the input, but during a big pulse, the voltage will dip enough to shut the BMS down.

I tried putting a diode into the input rail, and this solution worked for the pulses; however it created a different potential between the VC10 input and bat input. This caused unstable cell readings.

My question is, what purpose does the diode between VC10 and BAT serve and could i remove it?

I have attached a [picture of this schematic section for reference.

over 1 year ago

0 Matt Sunna over 1 year ago

TI__Guru* 77353 points

Hi Alex,

The purpose of this diode is described in Section 3 of this application report: https://www.ti.com/lit/pdf/slua749

It should not cause unstable cell readings, so there could be a different issue.

Best regards,

Matt

0 Alex N over 1 year ago in reply to Matt Sunna

Prodigy 185 points

My apologies for the confusion.

I caused the unstable voltage readings by placing a diode into the BAT (Pin 15) power rail to keep a reverse current situation from happening during large short-circuit pulses. I would like to keep this modification to the recommended schematic if possible.

This modification caused a difference of 0.5V potential between pin 15 and 16 which caused unstable readings.

I am not using the chipset based balancing, so my question is, can i removed the diode (D8 in my schematic) between pin 15 and 16 to disconnect the pins from each other without causing issues during normal operations?

0 Matt Sunna over 1 year ago in reply to Alex N

TI__Guru* 77353 points

Hi Alex,

After looking at this again, I notice the 10uF capacitor for the BAT pin filter is on the wrong side of R78. I think this could maybe be causing the unstable readings.

Regards,

Matt

0 Alex N over 1 year ago in reply to Matt Sunna

Prodigy 185 points

Hello Matt,

I re-worked my board to place the capacitor on the other side of R78 and there was no change to the readings. The Cell 8 voltage still fluctuates around 100mV every reading,

0 Matt Sunna over 1 year ago in reply to Alex N

TI__Guru* 77353 points

Hi Alex,

The diode between pin 15 and pin 16 is recommended and should not cause unstable readings, so I am not sure what might be causing this.

+1 Alex N over 1 year ago in reply to Matt Sunna

Prodigy 185 points

Hello Matt,

After some more experimenting, removing the extra diode (D15) that i put in place and relocating that capacitor to the other side of R78; everything is running stable.

I believe I caused the unstable readings by adding D15 to the input of the AFE which was not in the recommended application schematic

0 Matt Sunna over 1 year ago in reply to Alex N

TI__Guru* 77353 points

Hi Alex,

I am glad to hear the issue appears to be resolved.

Best regards,

Matt

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BQ76930: Shutdown from Short-Circuit