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Junction Temperature Calculation

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Advanced Member level 4
Apr 6, 2011
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Hello ,

I was trying to calculate the junction temperature of a device MMDT5451 .

The datasheet states it can handle power up to 320mW , if proper PCB layout is taken care and provides an equivalent 𝑅𝑡ℎ𝐽A=390°C/W.

The power dissipation in application can come close to 195mW which is less than rated value of 320mW

But if i use the junction temperature equation Tj=Ta+ Pd x𝑅𝑡ℎ𝐽A ( with ambient around 100°C) Tj=100+0.195x390=176.05°C which will violate the Tjmax spec 150°C.

I understand there is an 𝑅𝑡ℎ𝐽C=140°C/W and the usage of 𝑅𝑡ℎ𝐽A=390°C/W alone in the equation may not be accurate.

I'm not using any heatsink , no exposed pad just the plastic case and soldered on to the PCB.

Is there any better way by modifying the Tj equation to result in more a realistic Tj estimate.

Your junction temperature depends on ambient. You can’t just say “I’ve got 100 degree ambient, but I want to dissipate more power”. You can’t “modify the equation” because you just don’t like the results. That IS a realistic estimate, what makes you think it’s not?? What makes you think RthJA is not accurate? If you’re not going to believe the data sheet, who ARE you going to believe, some strangers on the internet?

And 100 deg ambient is REALLY high. What’s your environment, the surface of Mercury?

Therefore max Industrial Temperature Range is 85 degree. You cannot make it to force that in 100 degree ambient temperature. Instead, search Military version that will work up to 105 degree but the price will be 50 times expensive. Or cool it appropriately.

You should have noticed that 390 K/W is achieved with 1 square inch (2 oz!) copper area. I guess your layout is nearer to 625 K/W. Means the intended power dissipation isn't feasible at elevated ambient temperatures.


as mentioned by @FvM, you should expect a higher thermal resistance as the standard copper thickness is usually 1oz. So you would have to pay extra/more for your pcb to get the lower thermal resistance. You may also include thermal VIAs in your layout to decrease the thermal resistance.


The bad idea is to run the small SOT363 transistor pair at this power level. I would consider transistor packages with lower thermal resistance. We however don't know the application and which transistor parameters matter for it.
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