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TPS23754EVM-383: Junction Temperature of MOSFET(Q6)

Part Number: TPS23754EVM-383
Other Parts Discussed in Thread: PMP9563

Dear Specialists,

My customer has designed POE circuit based on TPS23754EVM-383.

The Switching MOSFET is used Si4848DY, it is same as TPS23754EVM-383.

The questions are listed below, could you please advise.


(1)They measured FET loss with calculating VDS and ID measured by oscilloscope at the condition of Vinmax(57V) and Imax(2.5A).  

The average power dissipation(MOSFET:Q6) is 3.67W and theta(j-a) max is 82deg/W, therefore junction temperature is 154.4deg that means overs Absolute maximum rating(150deg).

Could it possible to think Si4848 cannot use for maximum input voltage and maximum current.

(2)The customer'd like to confirm the switching waveform of TPS23754EVM-383, could you please provide. 

Their circuit has a power dissipation due to spike noise, so they'd like to confirm the EVM's waveform.


I appreciate your great help in advance.

Best regards,


  • Hello Shinichi, 

    The PMP9563 is the re-designed, optimized version of the TPS23754EVM-383. 

    We use the FDS86242 MOSFET for Q6. This package is better supported for this power level/topology. 

    I do not have the switching waveform data for the EVM at this time. The EVM was released in 2009, so the data could be as old as 2008. 

    I can try to see if I have a working EVM on in my lab. However, if you can provide the waveform I can provide guidance. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.




    Michael P.

    Applications Engineer

    Texas Instruments 

  • Hi Michael,

    Thank you for your reply.

    I understand Si4848DY is not suitable for worst case input/output condition.

    It is necessary to consider changing layout for better PCB thermal resistance or changing better thermal resistance FET.  

    I'll share this information with the customer.

    I appreciate your great help and cooperation.

    Best regards,