From: email@example.com (Bob Wilson)
Subject: Re: power supply pwm design help
Date: Sun, 29 Sep 2002 18:01:55 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article , firstname.lastname@example.org 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. 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