Hi,

On some of these smaller power supplies there is no transformer at
all! I.E. no primary and secondary. They do have an inductor that is
used to step down the power and rely on a Thysistor (aka. electronic
switch) to turn on an off very fast. Usually 60 times a second (ac
mains frequency). The voltage regulation is dependent on the amount of
time the switch is on during the power cycle.


http://en.wikipedia.org/wiki/Switched_mode_power_supply

One nice thing about this is that they can adapt to many mains
voltages and mains frequency. Hence for laptop computers you only need
one "International" power brik like supply.

Warmest regards, Mike.