switched-mode power supply (SMPS)

The whole self-contained box that turns wall AC into the clean DC your laptop needs, built around switching converters. It rectifies the 120/230 V mains to rough DC, then a transistor chops it at tens to hundreds of kHz through a small high-frequency transformer for isolation and voltage scaling, and finally rectifies and filters the result. Because it switches at high frequency, the transformer is the size of a walnut instead of a brick.

This is why a modern charger is small, cool, and 90%+ efficient where a 1970s linear supply with a heavy 60 Hz transformer was a hot doorstop. The flyback topology dominates phone chargers; the LLC resonant converter rules quiet, high-efficiency laptop and TV supplies. A feedback opto-coupler tells the primary-side controller to widen or narrow its pulses to hold the output.