Thermal resistance



Thermal resistance (Rth) - a parameter describing steady state temperature rise versus dissipated power within a given device.

Thermal resistance is often denoted as $$R_{th}$$ with the units of K/W (kelvin per watt) or C/W (degree Celsius per watt).

Thermal resistance depends mostly on geometry and materials involved, and not on operating conditions (e.g. shape of current waveform, or frequency). For ferrite transformers it is found to be independent on operating frequency.

For example, if thermal resistance of a given device is Rth=8 C/W, then dissipation of 5 W will make the device to run at a temperature 8 C/W * 5W = 40C higher than its ambient temperature.

A similar concept is thermal impedance (Zth), which is used if the power is applied not continuously, but intermittently. The thermal impedance is used for defining the thermal properties under dynamic (transient) conditions. Thermal resistance applies to the state of equilibrium (steady state).

Ways of deriving


The value of thermal resistance for a given device depends on the ratio of its total surface area to its volume, as well as thermal conductivity of the case, access to coolant (e.g. fresh air), emissivity of the surface, etc. Therefore, the only certain way of deriving the value of thermal resistance is to empirically measure it under nominal operating conditions, for instance on a prototype sample.

There are several empirical equations linking the size and shape of a given class of devices. For example for EE, EI, ETD and EC ferrite transformers the following formula could be used:

$$R_{th} = 53 \cdot (V_{core})^{-0.54} \ \ \ \ (^o C/W)$$

where: Vcore - core volume in cm (i.e. this the volume of the ferrite core itself as specified by the manufacturer, and not volume of the whole transformer).

So for instance for ETD44 the core volume is 17.8 cm gives a value of 11.2 C/W. And the values given in various sources of thermal resistance of ferrite transformers are between 11-12 C/W, so there is a reasonably good agreement. An experimental measurement for a particular winding configuration and type of bobbin) give a value of 10.2 K/W (see also the graph), which is also quite close.

Empirical data given by Epcos also suggest that the thermal resistance is roughly proportional to the reciprocal of square root of the ferrite core volume, which is in agreement with the previous equation:

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

Importance
Thermal resistance is an important parameter used for correct design. For instance, enamelled wire has a maximum rated operating temperature. So during operation the transformer must not exceed that temperature. Depending on the cooling conditions and its size, each transformer will have a specific thermal resistance. Hence, knowing the highest expected ambient temperature (e.g. Tambient=50C), the highest operating temperature (e.g. Tmax=155C), and the thermal resistance of transformer (8 K/W) it is possible to calculate the maximum losses allowed in such transformer, which with the example values given above is 13.1 W. So the total loss (e.g. the sum of copper and cores loss) must be kept below this value.