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LM117HVQML: Maximum Power dissipation for LM117HRLQMLV (5962R9951705VXA)

Bruno Simard67
Prodigy 240 points
Part Number: LM117HVQML

Tool/software:

Hello,

I would like to validate my understanding of the maximum power dissipation for the voltage regulator LM117HVQML. Based on the DSCC specification (https://landandmaritimeapps.dla.mil/Downloads/MilSpec/Smd/99517.pdf) , the absolute maximum power dissipation for my package (device type 05) is 2W. I understand that this 2W is likely calculated using the formula: PDmax=(TJ-TA)/thetaJA=(150-25)/64 =~1.95W

This calculation assumes a 500 LFPM air flow.

However, if I use a thermal adhesive between the PCB and the bottom of the TO-39 package, am I still limited to the 2W maximum power dissipation (which also needs to be derated)? For example, assuming: Pd=(TJ-T_PCB)/theta_PCB= (150-30)/40=3 watts. And you can assume that PCB temperature (T_PCB) is well mainted at 30degC in all time.

So in that case Do I still have to use 2W as the maximum power disspation to be derated or I can start with 3W?

Thank you for your answer

Best Regards

  • 0
    TI__Genius 9150 points

    Hello Bruno,

    Adding a thermal adhesive between the PCB and the bottom of the TO-39 package reduces the thermal resistance between junction and ambient in which case, a higher power dissipation can be achieved. So, you can start with 3W pre-derating however, please confirm where you got the 40°C/W figure for the thermal resistance in this condition.

    Best Regards

    Ishaan

  • 0 in reply to Ishaan Shetye
    Prodigy 240 points

    Hello, Ishann, the value of thermal resistance of 40°C/W was taken arbitrary for my question. But now,  I can give you a real example using one of the best thermal adhesive that I found/got (SCV1-2599, 1.6°C/W) recently. So assuming a surface contact of about 6mm, the TO-39 diameter, I got a surface contact of about (pi()*(d/2)^2=2.82E-5m^2. Considering a surface contact coverage of about 80%, we got a thermal resistance of about 22.1°C/W assuming a height of about 1mm from the PCB surface. So now, overall, we got a total thetajb=thetajc+thetacb=21+22.1=43.1°C/W. So at the end, Pdmax can be evaluated as PDmax=(TJ-Tpcb)/thetacb=(150-25)/43.1 = 2.9W.

    In conclusion, can you confirm me that I can work with the 2.9W based on the calculation above

    Thank you

    Best Regards

     

    Bruno

  • +1 in reply to Bruno Simard67
    TI__Genius 9150 points

    Yes, Bruno. I agree with the calculation and you can proceed with the 2.9W dissipation pre-deration