There is no single source or collection of thermal designs. However, the following three points can be cited as good fundamentals for starting thermal design.
Suppress the total power dissipation by derating.
Decrease the thermal resistance by radiating heat (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?"
This is how to calculate the thermal resistance of the following semiconductor device when the total power dissipation increases.
(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 the 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.
θj-a = 70°C/3 W = 23.3°C/W (In this example, since there is no point of high precision, 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.
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 we assume that all heat is radiated from the case without the temperature rising.
Please note that ASICs do not have a prescribed case temperature, so this kind of thermal design cannot be applied to them. If you have an ASIC that needs a strict thermal design, please contact a Renesas Electronics-related distributor.