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There appear to be two kinds of power supply adapters in market.

The first/older kinds were such that they were reasonably heavy and had a small transformer inside the adapter itself. The newer ones in market (those that come with a mobile phone for instance) are surprisingly lightweight; I find myself wondering whether there is even a transformer inside.

How do these new-fangled adapters achieve step-down?

Everyone
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1 Answers1

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These are called switched-mode or switching power supplies.

Yes, they still have a transformer inside. That's necessary, otherwise the low-voltage side wouldn't be touch-safe.
The transformer is kept very small. That's possible because it works at a very high frequency; the mains voltage is rectified and a high-frequency switcher feeds the transformer. The higher the frequency the better the energy transfer between primary and secondary are. Remember that transformers only work on variations in current/magnetic field, so a higher frequency makes for more changes.

Power supplies like this are not only recognizable by their light weight, they usually accept a wide range of input voltages as well, like 100V to 240V.

edit
This is a clever method which allows using a small transformer:

enter image description here

Here the high frequency comes from two spikes per 50/60Hz period, where a capacitor discharges into the transformer when a small thyristor switches the voltage near its maximum. Due to the brief transfers (once per 10/8.33ms) this is only suitable for low power applications. The schematic shows a 150mW supply.

stevenvh
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  • Lost me there; do you mean the mains voltage is rectified first, and then stepped down? Could you point me at a link please? – Everyone Sep 21 '11 at 08:52
  • Yes, rectified first and then stepped down. If I can find a link I'll add it to my answer. – stevenvh Sep 21 '11 at 08:54
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    actually rectified first, converted to high-frequency AC, stepped down, and then *rectified again*. – tcrosley Sep 21 '11 at 10:36
  • Uh. Here on the sub-continent supply lines are typically 240V. Wouldn't a 1KW diode (assuming 240V 5A point) be pretty large to fit into the charger plug? – Everyone Sep 21 '11 at 10:45
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    @Everyone - Yes, it would, but you don't have a 5A diode because you can't draw 1kW from the adapter. You may have up to 20W (e.g. 4A @ 5V), but that will translate to less than 100mA @ 240V. – stevenvh Sep 21 '11 at 11:00
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    @tcrosley, why do they do it that way instead of rectify and then DC-DC (buck converter)? – Matt B. Sep 21 '11 at 16:48
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    @Matt B. You need isolation for safety. Bucking the rectified AC is fine to make a rail that's not touch-safe (i.e. an internal housekeeping supply for a microwave or washing machine) but if the end user can touch it, it has to meet safety requirements (including isolation). – Adam Lawrence Sep 21 '11 at 17:19
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    @Everyone: There's an application note that shows in great detail how one could implement these types of switched power supplies: http://www.powerint.com/sites/default/files/PDFFiles/der265.pdf (Disclaimer: I do not work for Power Integrations, I just happened to stumble across it). It's consistent with tcrosley's comment: the high voltage end is rectified, converted to HF AC, then rectified and filtered. – In silico Sep 21 '11 at 17:20
  • @Insilico: Sweet! Great link! +1 – Everyone Sep 21 '11 at 18:17