Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|improve this question
FWIW, newer laptop power adapters tend to be much less bulky too. My current adapter weighs about a fifth of one I got five years ago. – leftaroundabout Feb 6 at 11:27
Mine weighs more, but it's also more powerful 105W vs 60W – Jasen Feb 7 at 11:58

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).

share|improve this answer

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.

share|improve this answer
but both power supplies do use an transformer, they just operate at a higher frequency than 50/60 Hz. The transformer is used to provide isolation between line voltage and output voltage. – markrages Feb 6 at 16:55
@markrages Well, some do have isolating transformers & some don't...that's a matter of cost/quality & I've seen plenty of 'usb chargers' that fell off the bottom end of the cost/quality slope! I guess that means you brought up another valid point though: the need to protect your laptop with an isolating xfmr can add a bit of weight too (as can the weight-plates I've found in a few usb chargers...reminiscent of desk-phones from a few decades ago, lol. – Robherc KV5ROB Feb 6 at 17:08
In my experiance even the really shitty ones have a transformer (though it's sometimes a rather poorly made transfromer). Do you have a link to a teardown of one that doesn't. – Peter Green Feb 7 at 4:56
At less than 100W they're mostly flyback converters so it's actually a coupled inductor, not a transformer, but it does use coupled magnetic flux to transfer energy while providing isolation. so it's like a transformer. – Jasen Feb 7 at 11:56
@jasen what is the difference between a transformer and a coupled inductor? – markrages Feb 23 at 1:31

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.