On Wed, 6 Feb 2019 10:54:45 -0000
"NY"  wrote:

> "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.

 The regulation is usually applied by measuring the 5V output to
derive the feedback to the switcher which does indeed run at rectified mains
voltage. High voltage switching transistors (FETs) are far smaller and
cheaper than low frequency transformers and efficiencies can run well into
the 90s mainly due to storing far less energy in the PSU which means
there's less to lose.

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