"Wall wart" charger used to be large because they contain an AC transformer.
Recently, cellphone and usb chargers have become smaller and lighter than a typical transformer, yet they usually work with world voltages (110 to 240). How do they do it?
I opened up a cheap no brand usb charger and found some discrete components (transistors, capacitors, resistors, ...), and a tiny transformer. I guess the AC input is first reduced to DC (perhaps with a zener diode to accomodate variable input voltages) to power a high frequency oscillator (much higher than 60Hz). This then passes through an isolation transformer (because of high frequency it is much smaller than an AC transformer), and then rectified and regulated into 5V DC output.
The charger for ipad2 is even smaller than those no brand usb charger and outputs 2A. Does it use a similiar design? I don't want to break it open to look (there is no screws on it for easy disassembly).
I googled for such schematic on the web and found some circuits that do not contain any transformer. This means the output is not really isolated from the AC input, which I think is a no-no for this type of application.