Why Q30 272Amps is used for 30A BMS design? And what is the use of C68 and C69?
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Why Q30 272Amps is used for 30A BMS design? And what is the use of C68 and C69?
Hello Rajib,
Thanks for your interest in TI FETs. The maximum continuous current published in the datasheet is a calculated quantity assuming Tcase = 25°C & Tj = Tjmax = 175°C. For more details, please see the link below to a blog explaining maximum continuous current specs in TI datasheets.
Holding Tcase = 25°C would require an "infinite" heatsink to dissipate the heat. In the real application, the current thru the FETs is limited by the maximum power dissipation which keeps Tj < Tjmax (+ margin). This reference design is using 3 x devices in parallel for the charge/discharge FETs to reduce conduction (I2R) loss and spread the heat to multiple packages in over a larger surface area to cool the FETs. For 3 parallel FETs conducting 30A (10A each), the I2R loss at Tj = 150°C is about 0.5W per FET. The D2PAK is capable of about 4W maximum power dissipation depending on the PCB and ambient conditions.
The CSD19536KTT is our lowest on resistance/highest cost 100V FET in D2PAK. We may have more cost effective FETs in the same or smaller (5x6mm SON) packages depending on your requirements. The second link is to a TI FET selector for load switch applications. Just remember to divide the current by the number of FETs in parallel.
I am going to forward this to the BMS applications team to answer your question regarding the capacitors.
https://e2e.ti.com/blogs_/b/powerhouse/posts/understanding-mosfet-data-sheets-part-3
https://www.ti.com/tool/LOAD-SWITCH-FET-LOSS-CALC
Best Regards,
John Wallace
Hi Rajib,
The capacitors are for ESD protection. This application report explains the use of caps between BAT+ and PACK+ more: https://www.ti.com/lit/an/slua368/slua368.pdf
Many systems use 2 capacitors for redundancy in case one capacitor is shorted, the other will prevent BAT+ and PACK+ from shorting.
Best regards,
Matt