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20. The LM317 has a maximum operating temperature of 125°C. The TO-220 case version shows a...

20. The LM317 has a maximum operating temperature of 125°C. The TO-220 case version shows a thermal resistance of 4 C°/W, junction to case. It also shows 50 C°/W, junction to ambient (no heat sink used). Assuming an ambient temperature of 50°C. What is the maximum allowable power dissipation for each setup? Assume that the first version uses a 15 C°/W heat sink with a 2 C°/W case to heat sink interconnection.

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Answer #1

Maximum operating temperature =125⁰C

Version 1

Thermal resistance junction to case Rthjc = 40C /w

Thermal resistance case to heat sink thcH20C/w

Thermal resistance Heat sink to ambient KthHa15°C/w

Total thermal resistance from junction to ambientRehtotal - Rehjc RehcRehHA 21c thCH

Ambient temperature Ža 50°C

Maximum allowable power dissipationTy-TA- Rthtotal P = = 3.57W 351W

Version 2

Thermal resistance junction to ambient without heat sink 50°c/w

Maximum allowable power dissipation P = 125-50= 1.5W

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