# Encyclopedia Magnetica™

### Site Tools

thermal_resistance

# Thermal resistance

 Stan Zurek, Thermal resistance, Encyclopedia Magnetica, http://e-magnetica.pl/doku.php/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 Rth with the units of K/W (kelvin per watt)1) or °C/W (degree Celsius per watt).

## Importance

Thermal resistance is an important parameter used for correct design of most electronic power transformers. For instance, enamelled wire has a maximum rated operating temperature, which should not be exceeded.9) Depending on the cooling conditions and size, each transformer will have a specific value of thermal resistance.10)

Hence, knowing the highest expected ambient temperature (e.g. Tambient = 50°C), the highest allowed operating temperature (e.g. Tmax = 155°C), and the thermal resistance of transformer (e.g. 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 loss and core loss) must be kept below this value.

## Calculator of maximum power loss from thermal resistance

If thermal resistance is known for given device and cooling conditions then a maximum loss dissipation can be calculated from the following equation.

 $$P_{max} = \frac{T_{max} - T_{ambient}}{R_{th}}$$ (W)

Tmax (°C) =       Tambient (°C) =       Rth (°C/W) =

Pmax (W) =

Note: the maximum temperature must be greater than the ambient value.