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TPS54240 power dissipation difference WEBENCH VS Data sheet equations

Kenji Itoh
Intellectual 420 points
Other Parts Discussed in Thread: TPS54240

 I found a difference  between power   dissipation  WEBENCH and calculation by data sheet.  The difference  is about  200%.

Please inform power dissipation equations  of  WEBENCH  in TPS54240.

  Result  of my calculate  and  WEBENCH are below.  And my conditions for the application are; VIN=42 V, VO=15V, IO=0.5A, fsw=1000KHz

             WEBENCH                   0.80W

              Datasheet calculate   0.37W 

     Then equations of  datasheet  are as  below and  Rds(on) =0.2

                    

             

  • 0
    TI__Expert 3085 points

    Kenji-San,

    We will get a response to you soon.

    Thanks,

  • 0
    TI__Mastermind 25430 points

    Hi Kenji-san,

     

    The datasheet equations have not considered a few other losses. Also, WEBENCH® considers temperature rise and updates Rdson according to the temperature coefficient.

    Below are the equations used in WEBENCH for this device:

    Psw  = 0.5 * Vin *Iout_Usr *(Trise+Tfall) *fsw (~345mW for your design specs)

    Pgd = fsw*Qg*Vin; (~420mW for your design specs)

    Pcond = I_fetrms*I_fetrms* Rdson (higher than datasheet loss because Rdson temperature effects are also considered - about 29mW for your design specs)

    Piq = Iq* Vin; (~6mW - 138uA is the value of Iq used)

    The accuracy of these equations with this device is around 3% for high load and about 5% for light load conditions.

     

    Regards,

    Amod

     

  • 0 in reply to Amod Vaze
    Intellectual 420 points

    　Amod-san

     Thank s for your quick response .  I have confirmed items  you noticed equations.

    1. Trise+Tfall  ( I see their mean are temperature deltacoefficient.)  And  what unit is  there's.

    2. I_ferms

     And  are these equations used other Switcher pro device  in WEBENCH.

     Regards,

     Kenji  Itoh 

  • 0 in reply to Kenji Itoh
    TI__Mastermind 25430 points

    Kenji-san,

    1. Trise and Tfall are the rise and fall time for the FET internal to the chip and the unit's are typically in nanoseconds(ns).

    2. I_fetrms is the RMS current through the internal FET and is Iout* Dutycycle^0.5 (Ampere). Duty cycle calculations take into consideration the losses as well.

    As this device is now supported in WEBENCH®, I provided the equations used in WEBENCH.

    Regards,

    Amod

  • 0 in reply to Amod Vaze
    Intellectual 420 points

    Amod-san

     Thanks. 

     Kenji  Itoh