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# CSD25402Q3A: Looking for the right MOSFET for my application.

Part Number: CSD25402Q3A
Other Parts Discussed in Thread: CSD25404Q3, CSD16570Q5B, CSD17570Q5B

Hi Team,
I am designing a battery-operated system whose input charging current is about 10A maximum and the discharge current is up to 40A maximum.
The 40A current will only last for a few minutes(10 minutes maximum) but all the other time the expected current is 10-15A(1-hour maximum).
I'll need both P-channel as well as N-channel MOSFET in my application.
The CSD25402Q3A has a maximum current of 76A.
But how a small device like this can handle such a high current without damaging itself?.
My required drain voltage is 12.8V
Also, how can I design a thermal heatsink to transfer heat effectively for such SMD components?.

• Hi Shibin,

Thanks for your interest in TI FETs. Please refer to the blog in the link below. The maximum continuous current rating of 76A in the FET datasheet is a calculated value and assumes that the case of the device can be held at 25C. This implies an "infinite" heatsink. From a practical standpoint, the maximum current the device can conduct is limited by power dissipation and maximum junction temperature. For this 3x3mm SON package, it can dissipate about 2.5W maximum depending on PCB layout and stackup. There is another link below to a technical article which provides "rule of thumb" power dissipation by package for TI FETs. Any pulse width greater than about 100ms is considered continuous and certainly, 40A for 10 minutes is continuous. The CSD25402Q3A power dissipation at 40A at TJ = 150C, would be nearly 21W which is not possible. For 40A you would need to parallel 3 to 4 of the P-channel FETs. We have another lower on resistance device, CSD25404Q3, but you would still need to parallel at least 3 FETs. Also, keep in mind operating with 12.8V gate-source exceeds the an absolute maximum VGS rating of +/-12V. TI does not guarantee reliable operation if the abs max ratings in the datasheet are exceeded.

The primary path to remove heat from these SMT packages is thru the thermal pad on the bottom and into the PCB. Thermal vias are used to spread the heat into internal layers and the opposite side of the PCB. Applying a heatsink is possible but not always practical. The thermal impedance thru the top of the plastic package is much higher and heatsinking it is not very effective.

Can you use N-channel devices for the charge and discharge FETs in your application? The CSD16570Q5B (25V) or CSD17570Q5B (30V) are the recommended FETs. However, you would need to provide a gate drive voltage at least 4.5V higher than the input voltage to drive the FET.

Please review and let me know if you have any questions.