# How does an iPhone car adapter work?

I know that the car cigarette adapter provides 12 V and around 10 A, while USB is ~6 V and x mA (I'm assuming it draws a small amount of current). How then does a car USB adapter work to recharge an iPhone? Some are so small, it's hard to imagine that there is a transformer inside there.

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USB works with 5V. The charger do not use a transformer, a transformer is used only with AC voltage, usually they use a step-down DC regulator. –  Bruno Ferreira Aug 22 '12 at 14:33

USB is 5 V. A difference of 7 V at for instance 1 A would cause a power dissipation of 7 W if a common linear regulator would be used, which would melt the device, so they use a switching regulator, aka SMPS (Switch Mode Power Supply), aka "switcher". There's no transformer, since you don't need isolation, but there's an inductor.

A linear regulator acts as a variable series resistor, whose resistance is regulated so that the output voltage remains constant. But that means that all the current which is consumed by the load also goes through the regulator, and that's why it's so inefficient, especially when the difference input-output voltage becomes large.

A switcher will use input current to build up a magnetic field, and then use that magnetic field to create the output current. The name switcher refers to the high frequency at which this occurs. So it pumps a small amount of energy in the coil, which releases it again, again energy in the coil, etc. The control circuit will vary the duty cycle of the switching to keep the output voltage constant.

Since there's no series resistor a switcher can be made much more efficient than a series regulator. Depending on parameters like input and output voltage efficiencies of more than 90 % are possible, that means that 1 A out at 5 V will only cause 500 mW dissipation. (Remember, the linear regulator was 7 W.)

Most of those adapters will use an MC34063 switcher, mainly because it's dirt cheap, although it doesn't perform as well as more recent switchers.

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They use switching regulators. Here is a picture of a generic car charger from ifixit:

It uses an MC34063 Switching regulator in Buck (Step-Down) mode. It only requires a few resistors, a pair of capacitors, and an inductor.

Apple of course is all about small and sexy. While they do not produce a car charger, they do make (or source) home chargers. So even when they use transformers as in their flyback switching wall wart supply, everything is made very small and carefully engineered to tetris together into the smallest possible form factor (while maintaining clear safety standards, unlike cheap clones and knockoffs).

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