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LM3478: MOSFET selection guide

Nam, Dino
Expert 6445 points
Part Number: LM3478

Dear team, 

My customer are trying to use LM3478, boost controller for their application. 

-       Input : typ24V , original application input spec : 10 ~29V

-       Output : 215V / under 50mA

How to select external power FET? please let me know regarding selection guide for external power FET(Vds, Id, Q, Rds and etc).   

Thank you. 

  • 0
    TI__Mastermind 31315 points

    Hi Dino,

    Please refer to section 8.2.1.2.7 of the LM3478 datasheet for selection of the power MOSFET. 

    http://www.ti.com/lit/ds/symlink/lm3478.pdf

    As a general guideline, you will want your FET and diode to be able to handle the maximum output voltage, with your peak currents equivalent to the sum of your average input current at minimal input voltage and half of its current ripple. 

    As for selection of rds_on, this is a matter of conductive losses, which is proportional to your (Irms_FET)^2*Rds_on*D, where D is your duty cycle. At lower frequencies, conductive losses make up a larger portion of your total MOSFET losses, compared to switching losses. This is the opposite at higher switching frequencies. At high switching frequencies, you should strive for low gate-drive losses, so Q should be lower. 

    This article is a useful resource: http://www.ti.com/lit/an/slyt664/slyt664.pdf (I would especially look at pages 23+24 for more information)

    Thanks,

    Richard