Thermal resistance of ferrite transformers



The values of thermal resistance of main core shapes for ferrite transformers, cooled by convection in air are given in the tables below.

Empirical data given by Epcos (see image) suggest that the thermal resistance is proportional to the reciprocal of square root of the ferrite core volume:

$$R_{th} \propto \frac{1}{\sqrt{V_{core}}}$$

Some authors suggest using such equation as a first approximation of thermal resistance. If the value of the core volume is used in cm then the thermal resistance can be calculated as shown below.