Judgment 24V DC Cooling Fan chips need to increase the cooling measures

The first step: to collect "24V DC Cooling Fan" thermal parameters of the chip. There are: P, Rja, Rjc, Tj, etc.
Step Two: Calculate Tc-max: Tc-max = Tj- Rjc * P
The third step: Calculate required to achieve the target Rca: Rca = (Tc-max-Ta) / P
Step four: computing "24V DC Cooling Fan" chip itself Rca ': Rca' = Rja-Rjc
If more than Rca Rca ', explained the need for additional cooling measures.
If less than Rca Rca ', explained the need for additional cooling measures. Such as increasing the radiator, fan, etc. increase.
Rja not be used to accurately calculate the temperature of the "24V DC Cooling Fan" of the chip, so this method can only be used for simple judgments. And not for a final basis. The following give a simple example: