Electronic – How do the tiny AC->USB power supplies work

acdcpower supplyschematics

The iPhone and Kindle both come with tiny power supplies that take in 120v AC and output around 5V at 0.85 to 1 amp. They seem to be too small and light to have a transformer, and generate very little heat. What is the electrical topology of these little power supplies?

Best Answer

This is not limited to these USB charger supplies. Most modern "wall wart" type power supplies are like this. They can be small because they are switchers. The line power is full wave rectified, then the result chopped at high frequency thru a transformer. The much higher frequency allows for a much smaller transformer to handle the same power. 5 Watts (5 V at 1 A) is easily doable in something that fits in the palm of your hand.

If you dig around, you can probably find a schematic for a small switcher like this. It is usually a full wave bridge, cap, and some sort of oscillator driving the primary of a small transformer at a few 100 kHz. The output of the transformer is rectified, filtered, and the result compared to the voltage setpoint. The over/under voltage indication is then transmitted back to the oscillator on the high voltage side via a opto-coupler.