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From: jeffw@midas.TEK.COM (Jeff Winslow)
Newsgroups: sci.electronics
Subject: Re: Choosing Cap Size in Power Supply
Message-ID: <1672@midas.TEK.COM>
Date: Thu, 30-Jul-87 20:36:07 EDT
Article-I.D.: midas.1672
Posted: Thu Jul 30 20:36:07 1987
Date-Received: Sat, 1-Aug-87 16:11:12 EDT
References: <5705@ut-ngp.UUCP> <1884@kitty.UUCP> <935@sdcc7.ucsd.EDU> <1666@midas.TEK.COM> <936@sdcc7.ucsd.EDU>
Reply-To: jeffw@midas.UUCP (Jeff Winslow)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 45

In article <936@sdcc7.ucsd.EDU> muller@sdcc7.ucsd.EDU (Keith Muller) writes:

>On good quality supplies, the RAW supply (before the regulator) does have
>large capacitors for what can very loosely be called an energy store.

Nothing loose about it. If no energy gets stored there, the regulator doesn't
have a chance.

>But the actual performance of the supply is solely the function of the closed
>loop feedback system that consists of the reference supply , difference
>amps and the regulator. 

As long as the regulator has enough input voltage to stay operating. All the
control system theory in the world won't help a saturated regulator. This is
where the capacitor vs. transformer response that I mentioned in my earlier
article becomes an issue. We aren't really disagreeing, are we?

>For example a typical low voltage small wattage
>supply would have something like 1800 uf between the bridge and the
>regulator. The output cap is on the order of 51 uf. The whole system 
>follows simple control system theory in terms of performance
>mesurements (which shows why the output cap has to be kept small).

There are many ways of designing regulators. What you want to do, usually,
is minimize the output impedance, over as wide a frequency range as
possible. Also, of course, the regulator loop has to be stable. There's
no intrinsic reason that a small output capacitor is necessary to satisfy
these requirements - you could have a small cap with a fast regulator (which
is usually the case in linear regulators) or a large, low-ESR cap with a
slow regulator (which is usually the case in switching regulators).
	
Wasn't the question which started this discussion referring to the capacitors
*behind* any regulator, that is, the energy storage elements I referred to
above?

>Take a look at a good supply circuit diagram and you can see (lambda, ac/dc,
>power designs, hp ...) what I mean.

I'm not sure if I'm supposed to read between the lines here, but I *have* 
designed several power supplies, both switching and linear, for Tektronix.

Once again, I would say these are quibbles for the fun of techincal discussion, 
rather than indicators of any basic disagreement.

		Jeff [but now I have to re-learn transmission lines] Winslow