"The Natural Philosopher"  wrote in message
news:q3e8ur$ms2$1@dont-email.me...
> On 06/02/2019 09:13, NY wrote:
>> I'm always amazed at how small switched-mode power supplies are these
>> days: the ones that Amazon supply for the Kindle fit the mains
>> transformer, the diodes and the PSU control circuitry into the size of a
>> normal-pin plug.
>
> Dont be.
>
> Understand that a transformer is, for the purposes of power transfer, able
> to store so much energy in its magnetic feild per cycle of the frequency
> it is operated at.
>
> A 5W transformer at 50Hz needs to be 1000 times the volume (or ten times
> thh (liner) size) of one operated at 50KHz...and as power FETs get better
> and better at HF switching so the frequency goes up.


I presume the switching is done at mains voltage (to avoid the need for a
step-down transformer at 50 Hz) and then  the 50 kHz switched, regulated
voltage is then stepped down to 5 V where it is smoothed.

When I made a SMPSU as a university project, we did the stepping down first,
so we had an enormous transformer. This was mainly because the regulator IC
that we used needed low voltage (probably TTL 5V). If you can drive the
regulator circuit from rectified and crudely smoothed mains, it avoid the
needs for such a big transformer.

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