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LM27403: What's optimizing logic or methodology of LM27403 for high/low side MOSFET in tool of DC/DC Power Designs

Part Number: LM27403
Other Parts Discussed in Thread: CSD17573Q5B, , CSD17309Q3, TPS40425

Hi Teams,
In WEBENCH@ POWER DESIGNER, I select LM27403 (buck controller with discrete NMOSs) in DC/DC Power Designs. TPS40425 is a single phase controller combined with discrete NMOSs. In DC/DC Power Designs, I set the condition with VIN=12V, VOUT=1V, IOUT=40A an Ta=30℃. The high side MOSFET is generated to CSD17309Q3 (30V, RON=4.9mΩ@VGS=4.5V, PKG=3.3mm x 3.3mm, θJA=45℃/W) x1 and the low side MOSFET is generated to CSD17573Q5B (30V, RON=1.19mΩ@VGS=4.5V, PKG=5mm x 6mm, θJA=40℃/W) x2. In my awareness, low side MOSFET is unreasonable in output current 40A design. What's optimizing logic or methodology in this tool? Is it related to RON, PKG and θJA? Is the key limitation is θJA? But, θJA is based on PCB conditions. It seems to difficult to design it.

  • Webench helps calculate the power dissipated image below. 

      I believe the power dissipation and junction temperature are the key limitations .   

    Ron and ThetaJA are used to estimate power dissipation and estimate the junction temperature of the fets. 

    The ThetaJA used is published in the datasheet with criteria used to determine the thermal impedance.

    There is a an excel sheet on the product folder

    If you want a simpler solution consider one of these dc/dc converters link