+ - 15 volt supply

[harrybissell at prodigy.net]

It was common for older SMPS (switchmode power supplies) to use linear post-regulation (78XX). This works well but the linear reg losses hurt the overall efficiency.
They are almost never used today. The switcher does the regulation by feedback on one output (usually the main output). The auxilliary outputs just go along for the ride.

Switchers have more high frequency "hash" noise than linears but they do regulate well. The problem is the ripple (50-250KHz typ.) is hard to filter out. It can go (usually) right through (actually around...) a linear post regulator. There can be a real problem with RF circuits nearby (wireless mics...) and possibly things like high speed A/D circuits (can u say "sampler"?).

OTOH most linear synth circuits won't be affected at all by typical switcher noise.

Matching of voltage between positive and negative supplies is usually as good as you would get with fixed linear regulators. If you circuit requires tight matching you won't get good results from a switcher...

OTOH: If your designs are sensitive to voltage variation you have a weak design. Good techniques are to use "ratio" methods (like the 555 timer) or provide accurate references in the circuit (small linear regulators... zener diodes... etc). I never rely on the rails for a stable voltage and you probably shouldn't either... (especially in a modular where current draw changes a lot).

Today I'd consider the switcher as a viable supply. Careful attention to application is a must. I prefer the switchers that are enclosed in a chassis... they then to be of higher quality than the open frame or PC board only designs...

:^) Harry



 ---- On Dec 24 "Rob" <cyborg_0 at iquest.net> wrote: 
> I had always thought that a switcher just dealt with the front end of the
> signal, then, the switched signal is fed to a 78xx or ua723 where it is
> regulated.. Well, at that point, where does the issues that define the
> differences between a switcher and a regulated supply?
> 
> This has always confused me..
> 
> Rob
> 
> ----- Original Message -----
> From: <harrybissell at prodigy.net>
> To: Dave Halliday <daveh at microsoft.com>
> Cc: <synth-diy at mailhost.bpa.nl>
> Sent: Thursday, December 23, 1999 5:08 PM
> Subject: RE: + - 15 volt supply
> 
> 
> > Avoiding "switching supplies" is a good way to avoid
> > potential problems. Give it a try... if you keep the switcher isolated
> (metal box etc..) you might find there is no problem. Most switchers today
> run for 50-250KHz...
> > it might mess up a sensitive clock circuit... but it can work.
> >
> > :^) Harry (debugging the VCO4d's with a Lambda triple output switcher...
> no problems)
> >
> >
> >  ---- On Dec 23 Dave Halliday <daveh at microsoft.com> wrote:
> > > You might want to avoid switching supplies as they are very noisy.
> > > Linear regulation is the way to go.
> > >
> > >
> > > -----Original Message-----
> > > From: CHoaglin at aol.com [mailto:CHoaglin at aol.com]
> > > Sent: Tuesday, December 21, 1999 5:22 PM
> > > To: synth-diy at mailhost.bpa.nl
> > > Subject: Re: + - 15 volt supply
> > >
> > >
> > > What are the current requirements of the +/- 15V supply? I have a number
> of
> > > +/-15V switching supplies from a salvage job I did..they're very small
> and
> > > compact, and might be just right for the job...
> > >
> > > Chris
> >
> >
> >
> >
>