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LM5141: Why the MOSFET used for high side and low side using different part with different power?

Sam Chen
Prodigy 20 points
Part Number: LM5141

I see some reference design for the Synchronous Buck Controller like LM5141 using different MOSFET for high side and low side. I want to know the design principle for this design.

Below is a snapshot of a design of LM5141 and the low side MOSFET using a larger power part than the high side part.

  • +1
    TI__Guru 51781 points

    Hello Sam 

    I think this training https://training.ti.com/node/1138903  helps you to understand how to select MOSFETs. Especially, please refer Part3 

    - Eric Lee

  • 0 in reply to EricLee
    Prodigy 20 points

    Hi Eric,

    Thanks for quick response. But I still can't understand why the low side MOSFET will consumer more power. According to the datasheet of LM5141 the power loss of the low side may a bit higher but I think that should not the main reason?

  • 0 in reply to Sam Chen
    TI__Guru 51781 points

    Hello Sam 

    I think you are talking about the power losses in typical high step-down & high current applications. For example, if VIN=20V and VOUT=5V, then duty cycle is 25%. Which means 25% of inductor current will flow through the high-side MOFET and 75% of inductor current will flow through the low-side MOSFET. In this case, the conduction loss of the low-side MOSFET will be more than the high-side MOSFET's.  

    - Eric Lee