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CSD18511Q5A: the rated power consumption and rated current

nnn sato
Prodigy 220 points
Part Number: CSD18511Q5A

Tool/software:

I have a question about the formulas in Fig. 1 and 2 on the following website.

Understanding MOSFET Data Sheets, Part 3 - Continuous Current Ratings
https://www.ti.com/document-viewer/lit/html/SSZTCP0

Formula Fig. 1
①When calculating MaxPDiss, if the ambient temperature changes, will the 25°C in the formula be that temperature⑤?
②If Rθjc in the formula changes to Rθja, can the same formula be used for calculation?

Formula Fig. 2
③If I substitute the MaxPDiss calculated above into MaxPDiss in this formula to calculate ID, will it be the ID at that temperature ⑤?

④Is it correct to understand that the rated power consumption and rated current of a MOSFET change (decreases) depending on the temperature?

  • 0
    TI__Mastermind 34165 points

    Hello Sato san,

    Thanks for your interest in TI FETs. The more general equation is: Max PDiss = (Max Tj - Tcase)/Rθjc OR Max PDiss = (Max Tj - Tamb)/Rθja. The max drain current is calculated as follws: Max ID =√( Max PDiss/Rds(on)). The datasheet values use the max Rds(on) at Tj = 150°C. See Figure 8 in the datasheet for normalized Rds(on) vs. temperature. For example:

    Max ID = √(3.125W/(2.3mΩ x 1.8)) = 27.4A using Rθja at Tamb = 25°C

    You can make the same calculations using Rθjc and Tcase.

    You are correct that the power dissipation and rated drain current decrease with temperature. See Figure 12 in the datasheet. Please let me know if you have any additional questions.

    Best Regards,

    John Wallace

    TI FET Applications

  • 0 in reply to John Wallace1
    Prodigy 220 points

    If I substitute any temperature to "tcase" and "tamb" of "max pdiss = (max tj -tcase)/RθJc" and "max pdiss = (max tj -tamb)/rθja".
    Does it match with the perception that the maximum power consumption of the that temperature is got?

  • +1 in reply to nnn sato
    TI__Mastermind 34165 points

    Hi Sato san,

    Yes, your interpretation is correct. Please let me know if I can be of any further assistance.

    Best Regards,

    John

  • 0 in reply to John Wallace1
    Prodigy 220 points

    Is the ON resistance value for the drain current in Fig. 2 must be the maximum value (Max TJ) regardless of the temperature?
    Or is "ON resistor  x  temperature coefficient (data sheet figure 8 (value of temperature at that time))" according to the temperature?

  • 0 in reply to nnn sato
    TI__Mastermind 34165 points

    Hi Sato san,

    I am off for the Thanksgiving Holiday this week and will get back to you when I return next Monday.

    Thanks,

    John

  • 0 in reply to John Wallace1
    TI__Mastermind 34165 points

    Hi Sato san,

    The equation uses max TJ. Most customers actually derate the max operating temperature < max TJ. For example. if max TJ = 150°C the max operating TJ is typically derated to 125°C for reliability purposes. In that case, you would use the temperature coefficient at 125°C.

    Thanks,

    John

  • 0 in reply to John Wallace1
    Prodigy 220 points

    Does "MaxTJ" in  Figure1 and Figure2 have to be the same temperature?
    For example, is it okay to calculate "MaxTJ" in Figure1 at 150°C and "MaxTJ" in Figure2 at 125°C, and then calculate ID using RDS(on) at temperature(125°C (Figure8. NormalizedOn-StateResistancevs Temperature))?

  • 0 in reply to nnn sato
    TI__Mastermind 34165 points

    Hi Sato san,

    Yes, you can use whatever MaxTJ that you want to use in your system.

    Thanks,

    John