No that's not quite right. The supply you had is not especially easy
to recreate. It is cheaper to buy than design in this case.
The original supply was a standard offline switching supply that
outputs 20V at 3A. It was probably designed for low THD (a.k.a. high
power factor or power factor corrected).
The stages are rectify the incoming AC then regulate the DC output
with a flyback regulator. The AC voltage is first rectified; there is
no input transformer. There is probably some circuitry that doubles
the input voltage if it is between 150VAC to 95VAC and does not double
it if it is higher. The high voltage DC powers a flyback regulator
which regulates the output to 20V. It also provides isolation from the
mains to 2500V or so.
The tricky part is getting the right magnetics for the flyback
regulator and making everything stable.
You probably should not attempt this if you are not already familiar
with offline switching supplies. Try reading the section in The Art of
Electronics about off line switching supplies.
You can cobble something together that will work but you will need to
switch voltage ranges depending on whether you are operating from
120VAC or 230VAC.
--- In Electronics_101@..., "Bene Tam" <bene-tam@d...> wrote:
hi all
I am a firmware engineer, recently my power adapter for the laptop
is broken
Although I am going to buy a new one out there, i would like to find
out how to build a simple substitute.
There should be 3 stage, right?? AC-AC, then AC-DC, then DC-DC
stage 1, use transformer, say turn ratio 5:1 to lower the voltage to
18VAC-48VAC, right?
stage 2, use bridge rectifier to convert the AC to DC, the outcome
will be 24-68VDC, right??
but is there any combined alternative for stage 1 and 2?? seems some
sort of switching technique out there?!
stage 3, use some sort of IC, just those from national semi. or
other MOS switching circuit to convert that to exactly 20VDC3A output,
right?? this stage is the most difficult to me, anyone can tell me
more detail??
Thanks much
