Subject: Re: power supply pwm design help
Date: 30 Sep 2002 05:18:14 -0500
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On Sun, 29 Sep 2002 18:01:55 -0000, firstname.lastname@example.org
(Bob Wilson) wrote:
>In article , email@example.com says...
>>> > Get yourself a copy of SwitcherCAD III from www.linear.com/software.
>>> > It will synthesize a SMPS for you from the above description. Use
>>> > the menu item "File=>Switch Selector Guide" I just tried it and it
>>> > will design a SEPIC topology converter that's perfectly stable over
>>> > input voltage range.
>>> Yes, you too can design a switchmode power supply without
>>> understanding anything about it!
>>I really don't understand this comment. I wrote the syntheszier in
>>SwCADIII and just verified that it makes a completely practical
>>circuit for his application. BTW, the topology is called SEPIC, not
>>"standard Buck-Boost topology" as you wrote it your other post.
>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. What it does is first connect the inductor from Vin to
>ground. Then after energy is build up in the inductor, it the end that was
>connected to Vin is switched to ground, and the end perviously grounded is
>switched to be connected to the output.
This would seem to be a cascade of the standard buck converter
(http://www.boostbuck.com/BuckConverters.html) and the standard boost
converter (http://www.boostbuck.com/BoostConverters.html) with the two
inductors merged into one. I've never heard this called a buck-boost
converter, but rather, a buck-boost cascade. There is another
converter which I've heard referred to as a buck-boost converter
(http://www.boostbuck.com/BuckboostConverters.html). What do you call
this converter, Bob?
The converter you have described has the disadvantage you mentioned of
more semiconductors in the current paths; it also has pulsating
current on both input and output. If you rearrange the cascade as
shown in (http://www.boostbuck.com/TheFourTopologies.html), the input
and output currents become non-pulsating at the cost of an additional
inductor. The converter you described would, in practice, probably
need both input and output filters, requiring TWO more inductors.
It is a very simple circuit that is
>part of the flyback family. In continuous mode operation, Vout = [(Vin x
>D)/(1-D)], for an output voltage that can seamlessly range from far below
>Vin to well above. It main disadvantage is that since current passes through
>2 devices (a diode and a transistor or FET), its efficiency is a little
>lower than for a separate Buck or Boost. But this is offset by its
>simplicity and ease of design, which is far less inscruitable tha a SEPIC.
>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. 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
>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) 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