In this applet the issues associated with the thermal design are observed. Since the power devices generate power loss, this heat must be removed from the device otherwise its maximum junction temperature would be exceeded at relatively low current levels. This heat must flow from the junction of the device through the case, to the heatsink and finally to the ambient air. For each material and boundary through which the heat must flow there is a thermal resistance which must be considered. As the heat flows through this thermal resistance there is a temperature differential across the material. Different materials have different thermal resistances and therefore can influence the operating junction temperature for the semiconductor device. The junction temperature is also dependent on whether the forced air cooling is used, if thermal grease is applied and the type of ceramic material used in the device’s construction.

• Examine the various temperature drops from the device junction to the ambient air temperature.
• Change the power loss in the device by changing the red arrow, and observe the change in temperature distribution and junction temperature.
• Click on ‘Ceramic material’ to change the material used in the power device and observe the influence on the junction temperature
• Click on ‘Fan’ to turn the fan on and off. Observe the influence the fan has on the temperature profile.
• Click on ‘Thermal grease’ to apply or remove thermal grease from between the heatsink and the case. In which case will the junction temperature be minimised?