BQ76952: Charge MOSFET Damage when testing Over Discharge Current Protection

Naga Penmetsa

Part Number: BQ76952

Tool/software:

Hello TI Experts,

We have been conducting tests for Over Charge Protection and unfortunately, Our Charge MOSFET's are damaging (D-S Shorted) everytime the protection kicks-in. We have tried adjusting the Gate resistors from 100 to 3K, and same results. We are using battery charge/discharge testers for testing the circuit. Please see below for the circuit.

Our Current Limits, are set at 20 amps during charging. We have disabled the Over current protections in BQ76952 and our Host controller reads the CC2 Current and disabled both Charge and discharge MOSFT's if the current goes above 20 amps using I2C commands. We have collected oscilloscope pictures:

BLUE: BAT+ (sOURCE OF CHARGE FET'S)

PURPLE: (DRAIN OF CHARGE FETS)

Yellow: (Charge Current)

Unfortunately, i don't have a differential probe handy, so collected both Source and Dain voltages w.r.t BAT-

MOSFET's are rated at 100V, and 300A peak. Whats puzzling is that, the MOSFET;s turn OFF initially and Drain keeps ramping up and then the MOSFET current starts to go up again and MOSFET's are damaging.

Any thoughts ?

Matt Sunna

Thanks,

Naga Penmetsa

over 1 year ago

0 Thomas Rainey over 1 year ago

TI__Expert 4320 points

Hi Naga,

This could be caused by one of these four reason:

The FETs are not commanded off and are damaged
The FETs turn off too slowly and are damaged
The FETs turn off too quickly and the inductive spike damages the FETs or the BMS circuitry
Parallel FETs oscillate during turn off and are damaged.

For more information on these causes you can refer to: [FAQ] BQ769142: MOSFET short after short-circuit test-voltage now always on.

Best regards,

Thomas Rainey

0 Naga Penmetsa over 1 year ago in reply to Thomas Rainey

Prodigy 60 points

Hi Thomas,

I have reference that post multiple times and so we have played with the gate resistors from 100 ohm to 5.1k ohm to play with different turn OFF times, but still no luck.

As you can see in the picture I sent, the FETs are commanded Off and they initially turned OFF nicely. Not sure why drain saw that ramp as in the piccture.

We have used individual gate resistors and we have seen no oscillations as you can see in the picture.

We have also done Shoet circuit tests, and NO damage is observed in the short circuit tests. This happens only in charging tests.

0 Thomas Rainey over 1 year ago in reply to Naga Penmetsa

TI__Expert 4320 points

Hi Naga,

It looks like the charger is ramping the voltage of PACK+ and blows the charge FET.

Can you check the charger configuration to make sure the voltage is not ramping up? What charger IC are you using?

Best regards,

Thomas Rainey

0 Naga Penmetsa over 1 year ago in reply to Thomas Rainey

Prodigy 60 points

Hi Thomas,

We are using battery testers that are specifically designed to charge with CCCV charging profiles and also does discharging.

Something like this.

https://www.tmaxcn.com/100v-100a-lithium-battery-tester-grading-machine_p2517.html

We setup CCCV for charging test. I am thinking if this charger is going out of regulation because its charging in CC mode during test and BMS turns off charging FET suddenly because of Over chargee current. Then may be because of sudden loss of current in CC mode, the charger may be ramping up voltage to compensate.

I'll try to test with a different charger and see how that behaves and keep you posted.

Thanks,

Naga Penmetsa

0 Thomas Rainey over 1 year ago in reply to Naga Penmetsa

TI__Expert 4320 points

Hello Naga,

Sounds good, let us know what you find. Let me know if you have anymore questions.

Best regards,

Thomas Rainey

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BQ76952: Charge MOSFET Damage when testing Over Discharge Current Protection