Conventional power equipment, such as motors, generators, and dry-type transformers, are cooled primarily by air circulated inside either by natural convection or by forced air from cooling fans. In power electronics devices, the heat is generated way inside a small semiconducting device having small wafer-thin volume around the junction area. This results in a high thermal gradient from the heat-generating junction to the ambient air where the heat is finally dissipated. Therefore, limiting the semiconductor junction temperature is much more challenging. It generally requires a heat sink (finned metal) with a large surface area around the semiconducting device to keep the junction temperature below the allowable limit. The heat sinks, commonly made of aluminum, remove heat from the inside junction to the outside surface by conduction and then to the ambient air by convection and radiation. Figure 17.1 shows a small power electronics subassembly mounted on a heat sink.