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Could you show some cases of thermal design?

Latest Updated:02/01/2008

Question:

Could you show some cases of thermal design?

Answer:

There are no special documents with collections of examples.
The following three can be cited as points of thermal designing.

  • Suppress the total power dissipation by derating.
  • Decrease the thermal resistance by radiating heat (by using a heat sink, cooling fan, or water cooling).
  • Lower the temperature range in the operating environment.

For how to calculate the thermal resistance and junction temperature of transistors, see FAQ" How to calculate data sheet it not included in thermal resistance?" .

Here, how to calculate the thermal resistance of the following semiconductor device when the total power dissipation increases is explained.

(1) The maximum junction temperature is 150°C.
(2) A single device can consume up to 2 W at an ambient temperature of 80°C.
(3) Thermal resistance θj-c between the junction and case of the device is 5°C/W.




The thermal resistance θj-a of this device is 35°C/W (= (150°C - 80°C)/2 W) between the junction and atmosphere.
If the power consumption exceeds 2 W, the thermal resistance must be lower than that. For example, shall we consider a case where this device consumes 3 W at an ambient temperature of up to 80°C?

To transfer a heat generation of 3 W at a temperature difference of 70°C (= 150°C - 80°C), the value of θj-a must be as follows or less.

θj-a = 70°C/3 W = 23.3°C/W (In this example, since having many digits is meaningless, the thermal resistance is assumed to be simply 23°C/W or less.)


Condition (3) states that this device has θj-c = 5°C/W. Therefore, thermal resistance θc-a between the case and the atmosphere must be as follows or less.

θc-a = θj-a - θj-c = 23°C/W - 5°C/W = 18°C/W


Incidentally, where θc-a = 0°C/W, a temperature difference of 70°C is generated in θj-c. The maximum power consumption is therefore 14 W (= 70°C/5(°C/W)) when it is assumed that heat radiated from the case is 100% without temperature rise.



Note that ASICs do not have a prescribed case temperature, so this kind of thermal design cannot be applied. If you have an ASIC that needs a strict thermal design, please contact the Renesas Electronics-related distributor where you requested development.

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