Since "220V Cooling Fan" rear radiator installed will greatly reduce the thermal resistance, and the heat always tend to flow to the direction of least resistance, so when the power supply module mounted on the heat sink, can be considered, the power generated by the module basically through the radiator and the heat dissipated in. Only a few (less than 10%) of the heat from the enclosure floor "220V Cooling Fan" of the power module and the side walls and dissipated by heat exchange. From the previous sections we can find a formula consumed by the power module and the module housing and Pd heat the surrounding fluid (air) temperature difference △ T. Such "220V Cooling Fan" heatsink thermal resistance Rth required for the following task is to check the radiator catalog or manual to find out the size of the power supply module substrate rather, in a suitable environment for temperature and natural convection and radiation thermal resistance of the heat sink can be smaller than Rth.