There is an impression that the capability of small form factor surface mount device (SMD) packages in handling current and heat is limited. This is not true, at least the limitation does not come from the package itself. For example, in an application note, Infineon shows that the continuous current rating of power MOSFET with QFN5x6 can reach 282A, and the limitation to a higher rated current is still not the package but the chip.
In theory, there is no difference in junction-to-case thermal resistance (Rth,jc) for SMD packages such as DPAK (TO-252) or through hole packages such as TO-220. But the junction-to-ambient thermal resistance (Rth,ja) can be different depending on how heat is dissipated (e.g. with or without heatsink, the size of Cu pad etc.). Taking On Semiconductor’s power MOSFET FDP120AN15A0/FDD120AN15A0 as an example, in the datasheet, the Rth,jc are the same for DPAK(TO-252) and TO-220, but the Rth,ja of TO-252 with minimum footprint is higher than TO-220. However, if a larger copper pad is used, the Rth,ja of TO-252 becomes lower than TO-220.
Similarly, for different SMD packages, the junction-to-ambient thermal resistance is highly dependent to the copper pad size under the package tab but not the form factor of packages. For example, with a proper copper pad, there is no difference in junction-to-PCB thermal resistance (Rth,j-PCB) for D2PAK (TO-263) and QFN5x6.
As experienced engineers would say “the best way to deal with heat is to not generate it in the first place”, the best way to deal with thermal problems is to choose the devices with as low as possible switching loss and conduction loss.
