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TIDA-00281: MOSFET selection

Part Number: TIDA-00281
Other Parts Discussed in Thread: CSD19535KTT, CSD19536KTT, LM5060-Q1


      Thanks for publishing TIDA-00281. It was well written and informative.

      I have a question on MOSFET section. The MOSFETs selected for the 48 V application to control the BLDC motor is CSD19535KTT. I am concerned whether it is suitable for 30 A continuous current as designed. CSD19535KTT data sheet says that R_DS(ON) = 3.4 mOhms at 25 degree C and more then 7 mOhms at 175 degree C. Thus, the worst case of the conduction loss could be as high as P_cond = I^2 * R_DS(ON) = 30^2 * 0.007 = 6.3 W. Assuming a typical automotive application with ambient temperature of 100 degree C, the thermal resistance from junction to ambient = (175 - 100)/ (30 x 30 x 0.007) = 12 C/W, which is well below the thermal resistance specified in the data sheet. Could you please tell how the MOSFET was selected?

      In addition, The TIDA-00281 design reference (page3 23~ 24) says two CSD19536KTT are needed in 48 V reverse polarity protection to reduce the power dissipation, in turn, reduces the temperature rise. So the question is why CSD19536KTT cannot be used alone, but a similar CSD19535KTT can be used to control BLDC with the same 30 A current?

     Thanks and I look forward to hearing from you,



  • Hi John, 

    I tried to reach the author of the reference design. Just briefly checking the design, here are some thoughts:

    • 30 A is the maximum input DC current. Power per phase is Iin/sqrt(3) = 17.32 A
    • The motor drive is rated for 1 kW, this is about 21 A of the input current for 100% efficiency.

    Regarding the second part of your question:

    - Q2 and Q10 see the higher DC current, not the phase current. 

    Note: If you design your 48-V application according to VDA320, the LM5060-Q1 does not have sufficient input voltage range. You may consider LM5050-Q1 instead.

    I hope this helps.

    Best regards, Jiri

  • Hi Jiri,

          Thanks for your answer. This helps a lot.

          Best regards,