Heat disipation required to keep below a temp threshold

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Discussion Overview

The discussion revolves around the heat dissipation requirements for a silicon microprocessor that consumes 100W of power, with the goal of keeping its temperature below 60°C in an environment with surrounding air at 22°C. Participants explore the relevant formulas and relationships involved in thermal management, particularly in steady-state conditions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that in steady-state conditions, the microprocessor must dissipate all 100W of power to maintain its temperature below 60°C.
  • One participant proposes a more precise question regarding the thermal resistance (θja) needed to keep the IC junction temperature below 60°C.
  • Another participant emphasizes the importance of understanding how long the microprocessor must stay below 60°C when considering heat dissipation.
  • It is noted that heat transfer will primarily occur through convection, and the convective heat transfer coefficient (hc) can be estimated based on conditions such as surface shape and airflow.
  • Participants discuss the necessity of heat sinks and fans to increase the surface area and enhance convective heat transfer, suggesting that the required surface area for heat dissipation may exceed that of the microprocessor itself.

Areas of Agreement / Disagreement

Participants generally agree that the microprocessor must dissipate 100W of heat to maintain the desired temperature, but there are varying views on the specifics of thermal resistance and the factors influencing heat transfer efficiency.

Contextual Notes

Some limitations include the dependence on specific conditions such as the shape and position of the microprocessor's surface, as well as the nature of convection (induced vs. forced). The discussion does not resolve the exact calculations needed for thermal resistance or surface area.

CraigH
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This question is easier to explain with an example.

A silicon microprocessor consumes 100W of real power. If the surrounding air has a temperature of 22°C, and the temperature of the microprocessor must stay below 60°C, how much heat must the microprocessor dissipate?

This isn’t a homework or coursework question. I just want to know the formulas and relationships involved.

Thanks!
 
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CraigH said:
This question is easier to explain with an example.

A silicon microprocessor consumes 100W of real power. If the surrounding air has a temperature of 22°C, and the temperature of the microprocessor must stay below 60°C, how much heat must the microprocessor dissipate?

This isn’t a homework or coursework question. I just want to know the formulas and relationships involved.

Thanks!

Seems like in the steady state, it would have to dissipate all 100W to keep from heating up more...

A more precise question would be what the thermal resistance θja would have to be to keep the IC junction temperature below 60C. Do you know how to calculate that?
 
berkeman said:
Seems like in the steady state, it would have to dissipate all 100W to keep from heating up more...

Ahh of course. Thanks. I'm going to delete this question and ask a better one in the morning.
 
CraigH said:
Ahh of course. Thanks. I'm going to delete this question and ask a better one in the morning.

No, please don't delete or alter your OP. That is against the PF rules. Just add your new question later here, or you could start a new thread if the question is different enough. :-)
 
CraigH said:
A silicon microprocessor consumes 100W of real power. If the surrounding air has a temperature of 22°C, and the temperature of the microprocessor must stay below 60°C, how much heat must the microprocessor dissipate?

You probably want to ask how much heat should it dissipate if it has to stay below 60°C for a certain amount of time.
 
As berkeman pointed out, the heat that the processor has to dissipate is 100W. Due to the temperatures involved, almost all the heat transfer will be through convection:

Q=hc·A·ΔT

You know the power you have to dissipate (Q). Depending on the conditions (mainly the shape and position of the surface and whether it is induced or forced convection) you can estimate the convective heat transfer coefficient (hc).

If the heat is directly dissipated through the microprocessor's surface, and we can consider it to be at 60°C, we would already know the difference of temperatures, therefore we could calculate the required surface you need (A) in order to get rid of the heat.

But I'm afraid you will get a much bigger surface than the microprocessor's, that's why they put heat sinks (the aluminium plates with fins) so there is a much bigger surface. And also they put fans on top of that: forced conductivity with a significant air velocity through the fins means the convective heat transfer coefficient (hc) is much higher.
 

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