# Estimating Max Junction Temperature Power MOSFET Help

#### christopher_88

Joined Aug 22, 2022
8
Hi,
Firstly thank you for answering my thread. I have a circuit I want to design and want to verify my values. I have a MOSFET that when turned on will pass 12.5A to the load at Vgs = 10V. According to the datasheet, Rds(on) = 0.0011 for Vgs = 10V. Therefore, the power dissipated is P_diss = I_drain*I_drain*Rds(on) = 0.172W. This a typical operating environment and I think is correct. What I want to do now is determine the junction temperature of the MOSFET at the max ambient temperature of 33C (circuit will be enclosed) and confirm it is operable at that Ta. Attached is the datasheet for the mosfet I am using.

This is what I have calculated using values from the datasheet I think are appropriate (this is where I have trouble understanding).
The following values are from the datasheet and used to make my calculations.
Max Junction Temperature = 150C
Max Thermal Resistance RthJA = 33 C/W
Max Thermal Resistance RthJC = 2.4 C/W
Using On-Resistance vs Junction Temperature Graph Max Rds_ON (Vgs = 10V) = Normalized_Value_Rds_On * Max_Ambient_Temp_Rds_On = 1.125*2.7 = 4.16 ohms

What I have calculated

Max Power Dissipated = 12.5*12.5*4.16 = 650.5W
Estimated Max Junction Temperature = Ta + (Max_Power_Dissipated*RthJC) = 1593.9C

I am having trouble justifying this and doesn't seem to make sense. I used theRthJA instead and got 21495.9C. Which is way crazy! Not sure what I am missing here.

Thank you.

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#### crutschow

Joined Mar 14, 2008
33,983
Using On-Resistance vs Junction Temperature Graph Max Rds_ON (Vgs = 10V) = Normalized_Value_Rds_On * Max_Ambient_Temp_Rds_On = 1.125*2.7 = 4.16 ohms
I'm confused.
Where does the value 2.7 come from?
That on-resistance at temperature should be 1.125 * 1.1mΩ = 1.23mΩ.

No wonder your power calculations are off the wall.

#### LowQCab

Joined Nov 6, 2012
3,929
Where are the specs for your Heat-Sink ?
You can probably get-away-with not using one, but I wouldn't try it.
You can't do any calculations without knowing the Heat-Dissipation characteristics
of your Circuit-Board, along with the other calculated Heat-Sources on the Board.
.
.
.

#### christopher_88

Joined Aug 22, 2022
8
I'm confused.
Where does the value 2.7 come from?
That on-resistance at temperature should be 1.125 * 1.1mΩ = 1.23mΩ.

No wonder your power calculations are off the wall.
Hi! thank you. This helps. I actually meant to write 3.7W which is still wrong as I got my units wrong. Using 1.1mΩ (Max Rds_On at Tj = 25C) makes sense.