What technological advance allows modern AC adapters to be so small?
A modern AC adapter is roughly 2/3 the size and weight it would've been just a few years ago. Why is this? What has changed?
bwr, the evolution of smaller more efficient SMC's with greater capacities in smaller packages. Also, devices are not as power hungry as they once used to be (again most of this is caused by smaller, higher capacity components). Therefore you do not need the big bulky power supplies that were once needed. Hope this helps, good luck.
The adapter is so small because there is no longer a transformer inside.
The power voltage is rectified, filtered and then chopped (cut off) several 1000 times a second.
If you adjust the duty cycle (how long the voltage is switched on and off) you get the desired output voltage.
You have to filter the output voltage once again for a clean DC output voltage.
Because of the high frequency of the switching you don't need big capacitors in the filter.
Combined with the above mentioned SMD's (surface mounted devices) you get a much more compact power supply.
Hope this helped.