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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

  • So you want to use the MOSFET without PCB as a heatsink? – G36 Jul 06 '20 at 14:46
  • I want to use the MOSFET on the PCB. Just want calculate whether I require a heatsink on the part or not? –  Jul 06 '20 at 15:12
  • Mounting base to ambient value depends on the PCB or the heat sink. It may not be dependent on the MOSFET itself. In the Thermal design guide pdf you have linked to, *Fig 6* gives mounting base to ambient value for a *particular* configuration of board copper layout (30K/W). – AJN Jul 06 '20 at 15:27
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    But for this MOSFET the PCB will be a heatsink. – G36 Jul 06 '20 at 15:33
  • @AJN , So you are telling the thermal resistance (junction to mounting base) depends on the MOSFET and the thermal resistance (mounting base to ambient) depends on the surrounding PCB/heatsink? Am I correct? And I am planning to use a 6 layer PCB FR-4? So, how to get the value of the thermal resistance (mounting base to ambient) for the MOSFET if my PCB is 6 layers and I am not planning to use a heatsink? Or should I just take the thermal resistance (junction to ambient) as (0.9K/W + 30K/W) as mentioned in the datasheet and the thermal design guide? –  Jul 06 '20 at 16:04
  • @G36 , I am not able to understand your point. Could you please explain me what should I do? My PCB is 6-layers and I am not planning to use a heatsink for this MOSFET. So, how to arrive at the thermal resistance junction to ambient value in this case? –  Jul 06 '20 at 16:05
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    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. – AJN Jul 06 '20 at 16:15
  • Some google results https://www.onsemi.com/pub/Collateral/AND9596-D.PDF and https://www.ti.com/design-resources/design-tools-simulation/models-simulators/pcb-thermal-calculator.html – AJN Jul 06 '20 at 16:15
  • @AJN, thank you for the suggestion. Let me check out those links first –  Jul 06 '20 at 16:18
  • https://www.infineon.com/dgdl/Infineon--AN-v01_00-EN.pdf?fileId=db3a304313b8b5a60113d4249da10431 - I found this Application Note. In figure 2, the give the value of thermal resistance (mounting base to ambient) based on the copper pad size. Is this graph like a standard one? So, Can I take the value of Rthsa from this graph by calculating my copper pad area itself? or is it not correct? –  Jul 06 '20 at 16:32
  • What power do you want to dissipated across the MOSFET? – G36 Jul 06 '20 at 16:42
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    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. – AJN Jul 06 '20 at 16:42
  • @G36 , Actually, My drain voltage is 16V and my maximum drain current is 12A. Rds(on) is taken as 6.9mOhm. So, I am calculating only conduction loss here and this MOSFET does not switch ON/OFF very frequently. So no switching frequency here. So I2R Conduction loss = 12A * 6.9mOhms * 6.9mOhms = 0.99W. So, this is my total power dissipation and my extreme ambient temperature is 105degC. So, I need to calculate the junction temperature at this extreme ambient temperature on a 6 Layer PCB with No heatsink and at this maximum 0.99W Power Dissipation –  Jul 06 '20 at 17:38
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    That's a lot of power for such a small package. So take a look at page 25 or 28 page https://assets.nexperia.com/documents/application-note/AN90003.pdf and look at the coper area you need to dissipate such power. – G36 Jul 06 '20 at 18:27
  • @G36, Thank you. Could you please explain it as an answer. I am new to these thermal concepts with the packages. Could you help to write an answer calculating what the maximum junction temperature of the MOSFET will be? And also could you mention what is the acceptable power dissipation values of certain important packages as you told that my package is small and it dissipates a lot of power –  Jul 07 '20 at 04:33
  • And the maximum power dissipation of the package mentioned in the datasheet is 167W. But my package dissipation is 1W. How come 1W is very high power dissipation for this package? –  Jul 07 '20 at 05:06
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    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. – G36 Jul 07 '20 at 15:58

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