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How do they make cellphone adapters, especially "travel adapters", so small?

I once peeked inside an old Nintendo adapter, and it had (among other circuitry) a rather-bulky step-down transformer to bring the voltage down from 220V mains to 12V, and a diode-based rectifier to convert AC to DC. This is my limited knowledge of how power adapters work.

I'm really curious how they manage to fit a transformer inside such the tiny travel adapter - so I tried to google it, and found this disassembly of an Apple adapter. I'm not sure if the thing with the yellow tape around it (towards the end of the vid) was a transformer - is it possible to make transformers so tiny these days?

Or is there some other way to reduce the voltage (apart from using a transformer) that is used in travel adapters?

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    \$\begingroup\$ The wide use switch mode power supplies is what's changed because now the controllers can be made so small, take a look into how those work. \$\endgroup\$
    – PeterJ
    Commented Apr 17, 2013 at 5:05
  • \$\begingroup\$ Without looking at the pictures, 'the thing with the yellow tape around it' was quite probably a transformer. Not sure why most SMPS transformers have this typical yellow color for the insulation. \$\endgroup\$
    – jippie
    Commented Apr 17, 2013 at 6:53

2 Answers 2

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The smallest travel chargers / adapters available these days do not contain a magnetic transformer.

Instead, isolation and AC coupling is achieved by using piezoelectric transformers: These are really tiny, and flat, in comparison to the conventional magnetic coil based transformer, and work at greater efficiency as well. The frequencies involved are not AC power line frequencies, they are much higher.

See the image below:

Travel Charger with no magnetic transformer

The design is the same as described in the answer by Nick Alexeev (switched mode), with power line AC being rectified to DC, then high frequency AC generated, isolated via the Piezoelectric transformer, and rectified again for DC.

The use of high frequencies also reduces the required size of capacitors for smoothing the DC voltage, another key factor contributing to the bulk of older chargers / wall warts.

For additional information about such tiny travel chargers, see this answer.

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  • \$\begingroup\$ While the development of piezoelectric transformers is fascinating and impressive, I've never come across one in practice in a power supply. I suspect the vast majority of small chargers currently on the market still employ (high frequency) magnetic transformers. Including the Apple charger linked by the OP. \$\endgroup\$
    – marcelm
    Commented Feb 20, 2019 at 20:27
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The transformer in the old adapters operated at 60Hz line frequency.

In the modern wall adapters, first AC is rectified to high voltage DC. This happens before the transformer. To step down the voltage and provide isolation, a flyback topology is typically used. The flyback transformer operates at 100kHz to 300kHz. The increase in frequency allows to make the transformer mechanically small.

Here's another tear-down report for a miniature 5W wall adapter.

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  • \$\begingroup\$ So it essentially has a rectifier, then an inverter, then a transformer, then another rectifier? Since there is a loss of energy in every one of those steps, that sounds very wasteful. \$\endgroup\$
    – Sponge Bob
    Commented Apr 17, 2013 at 5:36
  • \$\begingroup\$ It might sound wasteful, but it's still a switchmode power supply! These are far better than linear power supplies(the old bulky type) in terms of efficiency. \$\endgroup\$
    – Christoph B
    Commented Apr 17, 2013 at 5:43
  • \$\begingroup\$ @KeeganMcCarthy Small flyback AC-DC adapters like that have about 80% to 90% efficiency. \$\endgroup\$
    – Nick Alexeev
    Commented Apr 17, 2013 at 5:53
  • \$\begingroup\$ The tear-down report is fantastic. \$\endgroup\$
    – nonbeing
    Commented Apr 17, 2013 at 13:30

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