As a second part of my bad question about voltage converters .

Well, I was asking about a regulator to get me from 220V AC/DC, into 12V DC . A regulator can't do that , but today I got the Apple charger for the iPhone , that takes my 220v into a 5v , and its very small.

I think if you take out the plastic you get a tiny transformer.

Is this a switching power supply ?

So , my question now is, what kind of transformer Apple uses in its iPhone chargers, is it some kind of regulator, or just a small transformer with coils ?

My needs are very small voltage conversion, from 220V AC (or DC with diodes), into about 12V-24V DC, with a very small current consumption (MCU), that is in one small enclosure box, and relatively cheap.

I was looking for it everywhere, but couldn't find such a small box to do that.

(By small I mean to be on a board, not more than 2-3 cm)

  • \$\begingroup\$ They use a thing called flyback converter. It indeed uses a small transformer. en.wikipedia.org/wiki/Flyback_converter \$\endgroup\$
    – Dzarda
    Nov 25, 2014 at 10:46
  • \$\begingroup\$ How much current/power you need for that MCU? What kind of device is this? \$\endgroup\$
    – Kamil
    Nov 25, 2014 at 12:20
  • \$\begingroup\$ Does your MCU/project need galvanic isolation from mains? It is quite simple to get 12v DC form 220v AC, but only if you're willing to completely isolate your circuit or kill yourself. \$\endgroup\$
    – RJR
    Nov 25, 2014 at 23:37

3 Answers 3


A full iPhone charger teardown is given by Ken Shirriff. Making something that small that is efficient (doesnt produce much heat) while also meeting the EMC and safety constraints is a very difficult task, and is only cheap because of the extremely high volumes that companies like Apple can produce.

Apart from the size of the major components such as transformer and transistors, one of the factors that stops these chargers getting much smaller is the mandatory spacings for safety agency approval. Pads/pins/tracks with high voltages between them have to comply with creepage and clearance distances so that they don't electrocute the users.

Since your priority is for a small size PSU, you have to compromise on the cost. The absolute cheapest power supply may not be the smallest, and as Ken shows, may compromise safety or performance. The Apple design seems to cost about 5 USD for the parts, which is cheap for such a high quality design. Their selling price, as with much consumer electronics, may not directly reflect the cost of the parts.

  • 4
    \$\begingroup\$ I wouldn't call those apple-made ones cheap.. \$\endgroup\$ Nov 25, 2014 at 18:02

Almost certainly it's an off-line flyback converter. Here's a typical circuit using a fairchild device: -

enter image description here

Just google for "off-line flyback converter" and you'll get dozens of hits.

So , my question now , what kind of transformer Apple uses in its iPhone chargers , is it some kind of regulator, or just a small transformer with coils ?

It's a small transformer (provides isolation and safety) made using ferrite cores.

Here's what wiki says.

I can vouch for a couple of the following designs in Premier Magnetics range (here). If you click on the link for UL-CSA recognized - Top switch transformers you can get to many designs for off-line flyback switching converters on sht 2 of the pdf.

  • \$\begingroup\$ i have just read its a switching power supply ..? \$\endgroup\$
    – Curnelious
    Nov 25, 2014 at 10:50
  • \$\begingroup\$ Yes, it is a switching power supply using flyback topology \$\endgroup\$
    – Andy aka
    Nov 25, 2014 at 10:54
  • 1
    \$\begingroup\$ @Curnelious A 50/60 Hz transformer is necessarily large to be able to support the volt-seconds applied by the AC line. So by using a high frequency switching topology like the flyback, you can get away with a much smaller transformer. Not to mention getting good regulation. \$\endgroup\$
    – John D
    Nov 25, 2014 at 18:19

If this is just for you (not for commercial product) - you can modify phone battery charger.

For example - here is Nokia AC-15E charger 5V -> 12V modification.

Here is YouTube video about this: link

And here is schematic with changes:

enter image description here

(source: video linked above)

  1. Modify R2 resistor value.

  2. Change C01 capacitor to 470uF/16V

  3. Remove output zener diode

I would suggest to put zener diode (12.1V or 12.5V depending on required voltage tolerance) instead of original (5.1V?) zener after you set R2 value.

  • 1
    \$\begingroup\$ Shouldn't the zener diode be somewhat higher than the output (say 12.1 or 12.5 or so depending on your over-voltage tolerance)? \$\endgroup\$ Nov 25, 2014 at 18:04
  • 1
    \$\begingroup\$ @user2813274 Good idea. I changed it in my answer. \$\endgroup\$
    – Kamil
    Nov 25, 2014 at 18:10

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