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From: "Harry Dellamano"
Subject: Re: power supply pwm design help
Date: Sun, 29 Sep 2002 20:22:46 -0700
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Mike Engelhardt" wrote in message
> Bob wrote:
> > First, the topology I was referring to is *NOT* a SEIPC. What
> > *I* was referring to is indeed called a Buck-Boost. This
> > topology has 2 transistors and two diodes..
> Oh, I've always heard that called a 4-switch. A SEPIC is
> a better choice here. It also has the advantage that there
> only one switch drop.
> > My comment was not directed at you personally, but rather the apparent
> > suggestion one can substitute actual undersanding of the the underlying
> > principles of SMPS design, by having the software do the work.
> But the software does give an excellent solution for the original
> poster's problem. My impression was that you only have a
> problem what you assume the problem is and what you assume
> people are saying.
> > When the design doesn't work for some reason, the newbie
> > (to this field of design) is left without a clue how to
> > make it work because he has no understanding of
> > the basics.
> In general, the synthesizer just gives an initial design. Then the
> user is free to modify it an re-simulate it before bence
> verification. In the case of the original poster's problem, the
> synthesizer's solution is a good one.
> > My opinion of Switchercad is that it is a nice piece of
> > sales material intended to lock you into using Linear
> > Tech's devices. Sure, P-Spice (upon which it is based)
> SwitcherCAD III is not based in P-SPICE but LTspice. There
> shouldn't be a need here to comment on things one doesn't
> understand. It is not intended to lock you into using
> Linear parts, as the license allows the user to perform
> general circuit simulation simulation and allows the
> importation of 3rd party models. Of course, it does help
> people to use LT products, as it includes about 650 models
> of LT products.
> > is a good tool, but not for an amateur who doesn't
> > understand the principles. I am reminded of new engineers
> > at a former job who spent weeks (even months) "designing"
> > RF devices with their nose glued to the computer monitor.
> > Once the design was built, it generally did not work as
> > expected (or often, at all). The comments generally ran
> > along the line of "Well, there's NOTHING wrong with the
> > design, because it worked in SPICE!". They were generally
> > told something like "Welcome to the REAL world".
> But the same thing could happen to someone who spent months
> doing analytical analysis of the problem. So what? I've
> always thought that the best understanding of electronics
> comes from a combination of bench work, analytical
> analysis, and simulation.
> The original posters would still be well advised to punch
> his problem into SwCADIII and get a practical solution.
> LT does sell a one chip four-switch solution that I
> think works here, but the SEPIC is, in my opinion, a
> better choice.
The SEPIC topology my not be the best choice here. It uses two inductors
compared to one for the buck-boost or as Mike calls it, the four switch
topology. The SEPIC also has a right plane pole and the buck boost does not.
It's interesting that SC did not select the buck-boost topology and the
LTC3440 which would be a perfect fit. Maybe SC does not like the buck- boost
topology? Have you brainwashed it to think this way Mike?
More info is needed before the optimum topology can be selected.
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