''A switched-mode power supply ... is an electronic power supply unit (PSU) that incorporates a switching regulator.
...
The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency.'' [source]