BQ76940: Power MOSFET calculation

Mohammed Suffiyan

Part Number: BQ76940
Other Parts Discussed in Thread: TIDA-010030,

Hello Team,

I have one query regarding the number of POWER MOSFETs to be used in a BMS application.

I have gone through the design files of TIDA-010030 main schematic, it is designed for 30 A maximum discharge current as per TIDA-010030 design guide (TIDUEG7B). But when I calculated the current through a single MOSFET it is coming around 20 A. So, 2 MOSFETs in parallel is more than enough, why 3 MOSFETs in parallel has been used?

Pd=Tj-Ta/Rthja = 150-50/62 = 1.613 W

pd= I*I*RDSON @50 degree celsius

I=20A taking Normalized RDSON at 50 degree celsius

Am I missing something here, please assist me in understanding the reasoning of the design.

Thank you!

over 3 years ago

+1 Caleb Perez over 3 years ago

TI__Genius 11315 points

Hey Mohammed,

It looks like the Short Circuit in Discharge (SCD) threshold for this design is 56A. This is specified in Table 1 of the design guide. There would need to be enough MOSFETs in parallel to be able to handle the SCD current draw until the discharge MOSFETs can be turned off.

This functionality is explained in section 8.3.1.2.1 of the BQ76940 datasheet.

Thanks,
Caleb