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CSD19534KCS: CSD19534KCS - Parallel mosfet

Mohan M
Prodigy 70 points
Part Number: CSD19534KCS
Other Parts Discussed in Thread: CSD19536KCS, , CSD18541F5

Dear Team,

We are considering parallel operation of mosfet for one of our application.

We came across a design (TIDA-00364) where we were able to find thermal model of parallel SMD mosfets in metal clad.

We have considered the same calculation for through hole mosfet as well. [Rtotal = ((Rjc+Rcs)/n)+Rsa)]

Is this the right way of computing the combined thermal resistance for through hole mosfet as well?

*Note: We have neglected other connecting thermal resistance for present calculation.TI Query.xlsx

  • 0
    TI__Mastermind 38390 points

    Hi Mohan,

    Thanks for the inquiry. I think this is a valid approach assuming all of the FETs are attached to the same heatsink, at approximately the same temperature and sharing current equally. However, I believe there is a mistake in your power dissipation calculation. You referenced the CSD19534KCS which has max rds(on) = 16.5mOhm at VGS = 10V but your calculation is using 3.3mOhm. Did you mean CSD19536KCS which is a much lower rds(on) FET? Please check the part number and review the datasheet to use the correct value of rds(on) for your conduction loss calculation. Secondly, please refer to Figure 8 in the datasheet. The FET rds(on) has a positive temperature coefficient meaning it is higher at elevated temperature. Figure 8 will allow you to estimate rds(on) at your operating junction temperature. You need to use this value for conduction loss calculations. Lastly, I do not know your application details. If the FETs are used in a hot swap, OR'ing or load switch application, then there will only be conduction losses. If this is a switch mode application, such as power supply or motor drive, where the FETs are switching at some kHz frequency, you will also need to estimate the switching losses in addition to the conduction loss.

  • 0 in reply to John Wallace1
    TI__Mastermind 38390 points

    Mohan,

    I forgot to mention some additional resources. The links below are to a blog on how TI tests and specs MOSFET thermal impedance, load switch FET selection and we have a load switch FET selection tool that will allow you to calculate conduction loss in a FET at elevated temperature.

    How to Select a MOSFET – Load Switching
    Other Parts Discussed in Post: CSD18541F5 “Two roads diverged in a yellow wood, marked FemtoFET and SOT-23, So I chose the path of FemtoFET to shrink my PCB” – Robert Frost (I think) MOSFETs are…
    By in Technical articles > Power management

  • 0 in reply to John Wallace1
    Prodigy 70 points

    Hi John,

    Thanks for the detailed explanation.

    Mistake was on my end. I did not consider the right Rds on value while posting.

    However, since the approach of thermal model is same, we will continue on the same.

    Our end outcome is a 48V, 5kW inverter.

    Thanks for the support.