I was wondering why a laptop power adapter is so huge. Most laptops that I have seen use a ~19V power supply. Using transformer equation, and considering 100 turns in the primary(just an assumption), and a 220V power supply, I calculated that there should be about 8 turns in the secondary. Using the same equation for a mobile phone charger(5V) and considering 100 turns in the primary, there should be about 3 turns in the secondary. So there shouldn't be much size difference between the transformer used in a cell phone charger and a laptop charger. So why are laptop charger adapters so big while a cell phone charger adapter is small?
Laptops and cell phone both use switching power supplies so the adapters are not simple transformers.
For a given technology there is a relationship between power capability (measured in watts) and size (volume, specifically). So a cellphone that needs 2.1A at 5V (about 10W) can use an AC adapter that is much smaller and lighter than that for a notebook computer that requires 19V at 4.62A (about 90W).
Actually, neither laptops or cell phones use a transformer, per se.
What they use, instead, is called a "Switched-Mode Power Supply" that rectifies the 110 or 220V AC input into a DC capacitor, then uses a multi-KHz switching microcontroller to pulse that through an inductor to "convert" the voltage down. This requires far less space than a 50Hz transformer on a big, heavy core, and is usually more efficient to.
As for why the laptop converter is generally so much larger than the usb chargers for cell phones/tablets/etc. That's a matter of power handling. Due to the higher voltage & current demandex by the laptop, its power supply needs thicker wires, a larger inductor, and higher-power switching components. Also, with more power going through it, there's more heat to get rid of.
Because of the need for bigger, heavier components, and more heat dissipation, the lappy charger simply must be bigger, so long as you aren't willing to pay many times more money for rare & expensive materials.