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I have this MOSFET - MOSFET Datasheet

I want to calculate the maximum junction temperature of the MOSFET part. I have calculated the power dissipation.

In the MOSFET datasheet, there is only Thermal Resistance (junction to mounting base) parameter is only given. Junction to Ambient thermal resistance is absent.

How to approach or calculate the value for thermal resistance junction to ambient from junction to mounting base value?

I have tried to look in the manufacturers website but couldn't find better documents that can help me.

Supporting documents - LFPAK Thermal Design Guide Part Information

Please help to find the thermal resistance junction to ambient value

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    \$\begingroup\$ But for this MOSFET the PCB will be a heatsink. \$\endgroup\$ – G36 Jul 6 '20 at 15:33
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    \$\begingroup\$ I would assume the PCB would have its own data sheet which will have some info from which you can calculate the required value. I don't think you can take the value from the thermal design guide as such. That value may be quoted for a specific copper area. The pdf says (page 3/41) "Tj depends greatly on length “x” and thus copper area, the bigger the area the better thermal performance". There seems to be lot of online *PCB Thermal Calculators which may help you. \$\endgroup\$ – AJN Jul 6 '20 at 16:15
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    \$\begingroup\$ Numbers seem to be in the same range as shown in Fig 9 of the pdf i have linked to. Wait to see if anyone gives a definite answer. \$\endgroup\$ – AJN Jul 6 '20 at 16:42
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    \$\begingroup\$ That's a lot of power for such a small package. So take a look at page 25 or 28 page assets.nexperia.com/documents/application-note/AN90003.pdf and look at the coper area you need to dissipate such power. \$\endgroup\$ – G36 Jul 6 '20 at 18:27
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    \$\begingroup\$ 167W is only possible in a situation when you have an IDEAL heatsink at 20°C. Also due to high ambient temperature and the MOSFET will be mounted on PCB Tj cannot be large than 120°C the copper area should be larger than 12mm x 12mm. And you should perform the empirical test on the actual circuit board inside the enclosure. \$\endgroup\$ – G36 Jul 7 '20 at 15:58

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