power supply problem

[Goddard, Duncan]

>>>I'm still trying to get a grasp on the benefits of switching power
> supplies...Harry said transformers are smaller...my Xpander supply has one
> big transformer whose output is regulated & feeds the switching circuit,
> which drives a smaller transformer... and the first transformer is just as
> large as what's used in a linear supply.  So - is the difference that the
> big transformer in the switching supply somehow generates less heat than
> the one in the linear supply?<<<
> 
put *very* simply, a switched-mode psu rectifies the mains and uses the high
voltage d.c. to run a high frequency oscillator which is then stepped down
to the required voltage range, rectified and smoothed as normal. regulation
is achieved by feedback into the oscillator driver.

the benefits are: you can use a much smaller transformer at higher
frequencies and the smoothing capacitors don't need to be so big either.
quite a bit less power is thrown away warming up the iron core of the
transformer. so it's light and efficient.

problems: costs more to make, throws out h.f. and so needs shielding if it's
near audio, residual h.f. on the output rails may be more objectionable than
the equivalent 50/60Hz in a linear psu, use of "racehorse" transistors in
the oscillator/driver section- they are run very hard and die suddenly.
these psu's come in a huge variety of designs, all of them quite tricky for
the non-expert to fault-find and repair; the components aren't easy to find
either. but they're not that expensive to just replace.

I don't know what's going on in this xpander- I can't recall seeing a
configuration like that. most smpsu's will run over a wide range of input
voltage (like, 70-300) so pre-ranging/regulation seems a bit odd....

I've never had trouble with stray radiation or residual ripple; one of my
esi32 samplers has a shielded unit, the other has an unshielded unit.... my
only advice, especially if the psu is in gear that's to be gigged or hired
out, is to keep spares. bear in mind that most equipment is 12's and 5's, so
in an emergency, even a £50 p.c. power supply could get you out of a mess.

d.




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