Cell phone charger
i broke one open to answer the same question. it used a diode bridge to convert 120vAC to 240VDC then a HV step-down to buck 240v to 5v.
it's light because there is no transformer, but it is not simple to implement 240v buck converter.
one - it's lethal if you screw up
two - 300v chips are hard to find
three - 300v caps, etc are expensive
purifier - a buck converter switches on and off at a few hundred kHz - the width of each pulse can vary from 0% to 100%, giving an output voltage of 0-VIN. For this 240v-5v application, the converter is running at about 2% duty cycle. and it's not that there are NO magnetics, there is just no transformer ,so no heavy iron core.
all DC-DC converter switch into an inductor, but they switch so fast (kHz-MHz) the inductor can be tiny, so they are small and light -> giving a total system weight made up mostly of the plastic shell for the AC wall wart.