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CSD87350Q5D: Conduction loss of MOSFET for Buck Converter

Biswajit Singh
Prodigy 30 points
Part Number: CSD87350Q5D

I am trying understand below formula for MOSFET conduction loss in Sync Buck Converter given in below app notes

https://www.ti.com/lit/an/slvaeq9/slvaeq9.pdf?ts=1710920873849&ref_url=https%253A%252F%252Fwww.google.com%252F

Why duty cycle multiplied with RMS current of switch ? why it is not simply I^2(RMS) * Rds(on) ?

It would really great if someone could help. Thanks in advanced.

  • 0
    TI__Mastermind 31435 points

    Hello Biswajit,

    Thanks for your interest in TI FETs. In the conduction loss equations, the inductor RMS current is multiplied by D, duty cycle, for the high side FET and multiplied by (1 - D) for the low side FET. This is because the high side FET conducts during Ton and the low side FET conducts during Toff. The inductor RMS current is common to both conduction loss equations and can be expressed as:

    ILrms = √(Io² x (1 + 1/12 x (Δilpp / Io)²))

    The high side & low side FET RMS currents:

    IHSrms = D x ILrms

    ILSrms = (1 - D) x ILrms

    TI has several Excel-based FET selection tools that allow the user to compare up to 3 different TI FET solutions based on power loss, 1ku price or package. The sync buck tool can be found at the link below. I'm also including a link to an app note that takes into account the common source inductance in the switching loss equations which was used in the FET selection tool. Please let me know if you have any questions.

    https://www.ti.com/tool/SYNC-BUCK-FET-LOSS-CALC

    https://www.ti.com/lit/pdf/slpa009

    Best Regards,

    John Wallace

    TI FET Applications

  • 0 in reply to John Wallace1
    Prodigy 30 points

    Hello John,

    Thanks for your response ,I have one further question in below app note there is formula of HS RMS current 

    I am not able to build the below relationship using above formula 

    IHSrms = D x ILrms

    ILSrms = (1 - D) x ILrms

    I am getting  IHSrms = SQRT(D ) x ILrms

    I am confused where I am making mistake

  • 0 in reply to Biswajit Singh
    TI__Mastermind 31435 points

    Hi Biswajit,

    Yes, you are correct and I mistyped. It should be the square root of the D and 1 - D factors. In the equations you included in your first post, the expression from Io² and following is the squared rms current. Taking the square root of those expressions, you can see that it would be square root of D in the first equation and square root of (1 - D) in the second equation. Apologies for the confusion.

    Best Regards,

    John