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J/K Selection Method for MOSFET Selection

Bill Baker1
Prodigy 110 points
Other Parts Discussed in Thread: TPS40060, TPS40170, TPS40192

I'm looking at the datasheet for TPS40060/61 and calculating component values. There is a section on MOSFET Switch Selection that uses the J/K method with equations and provides results for a design example. Unfortunately the equations use some terms that are not defined and the example does not show values for terms, only a result. It is not obvious to me where, in the TPS40060/61 electrical  characteristics the numbers have come from for the undefined terms.

I don't see reference to an applications note and a search on the main TI web page for J/K does not return a result that answers my questions. 

Can any of you direct me to good reference material?

 Also, TI should fix an error in equation 20. The first two radicals have the 1/12 inside the Iripple^2 term. 

  • 0
    TI__Mastermind 35165 points

    Hi Bill,

    Would you happen to be looking at the TPS40170 datasheet and not the TPS40060/1 datasheet?

    I haven't used the J/K method before but will look for some reference material.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    TI__Mastermind 35165 points

    Here's an article written up on this method. The author is likely the one who did the example design done in the datasheet. Let me know if you have any questions.

    http://powerelectronics.com/power_semiconductors/power_modules/mosfet-selection-power-loss-201006/index.html

  • 0 in reply to Anthony F
    Prodigy 110 points

    Hi Anthony -

    Thank you for the response and catching my error. I actually have both datasheets on my desk.

    Yes, the reference I made to the J/K Method is in the TPS40170 datasheet. I was looking at the TPS40060/61 first for my application but need a chip that can operate at a lower input voltage.

    Cheers!

    - Bill

  • 0 in reply to Anthony F
    Prodigy 110 points

    Yeah, I saw this article and it does provide some information but I noticed a couple things made we want to look further.

    Equation 5 shows calculations for the high side FET using TPS40192. 

    1. 12V was selected for Vdrive but the datasheet shows absolute max voltage of 6V from HDRV to SW.

    2. The equation has the term (Vin * Iout)/(Idrive). The article defines Idrive as (VGATE - Vth) / (Rdrive + R gate). The equation with values shows 5.2V - 2.0V / 2.25 ohms. 

    a. Doing the math and looking at the result shows the author followed the article formula

    b. The author doesn't mention where the values for VGATE, Vth, Rdrive or R gate came from. 

    I was hoping that more detailed information would be available. 

    Saludos!

    - Bill

  • 0 in reply to Bill Baker1
    TI__Mastermind 35165 points

    Hi Bill,

    Below are the definitions for the variables which should help clarify on how to use the formulas. I think there was some simplification in the formula for the Idrive when used in the article which was not shown when the equation became 5.2V-2.0V/2.25Ω.

    Vdrive - Source voltage for the high-side FET which is VIN

    Vgate - Gate voltage for the FET which is approximately equal to the internal 5.3V typical regulator voltage

    Vth - Gate threshold voltage for the external FET

    Rdrive - High-side gate driver pull up resistance

    Rgate – External series resistance added to the gate drive plus the internal gate resistance of the MOSFET

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 110 points

    Hi Anthony -

    Thant answers my questions, thank you.

    Enjoy the weekend!

    - Bill