LinearView - Product Data & Applications 1/LinearView - Product Data & Applications.iso/swcad/manual.txt

Text File | 1996-07-09 | 11.9 KB | 236 lines

SwitcherCAD Software Walk-Through

Introduction

SwitcherCad is a powerful design tool that significantly eases the
task of selecting topologies, calculating operating points, and
selecting component values and part numbers for dc-to-dc
converters. It can cut days off of the design cycle by eliminating the
process of wading through multiple data sheets, application notes,
and magazine cookbook articles searching for answers in a field
where the user may have little familiarity. As is the case with any
computer aided design software, SwitcherCAD is computer
aided design, not computer design. At the core of SwitcherCAD
there are as many as 300 design equations for each topology. The
equations quantify the operating point of each circuit component,
but they can not guarantee that once assembled the circuit will
meet  the user's every expectation. It is the responsibility of the
user to verify SwitcherCAD's work by actually building the circuit
and evaluating its performance, as well as measuring component
stresses under all expected operating conditions.

SwitcherCAD works on all PCs and compatibles with a minimum
of 640K memory, 1.2MB floppy drive, and EGA, CGA, VGA, or
Hercules monitor. A hard disk and mouse are recommended.

Loading the Software

SwitcherCAD can be run directly on its floppy by simply typing SW
(RETURN). To install on a hard disk, log into that disk and type

MKDIR SWCAD
COPY A:*.* C:\SWCAD

This creates a new directory SWCAD on the hard disk, and
copies the files on the SwitcherCAD disk (in drive A:) to this new
directory.

Software Overview

SwitcherCAD works with the basic power supply parameters:
input voltage, output voltage, and output current. From these
inputs, SwitcherCAD builds a list of feasible topologies and part
numbers from which the user can choose for his design. Once this
choice is made the circuit's operating conditions are calculated,
and a parts list for a first-cut design is generated. This serves as a
starting point for physically breadboarding and optimizing the final
design.

The software consists of several different windows or screens. The
first serves as a specification sheet for the design, and screen two
allows the user to pick a topology and IC. From here the schematic
can be viewed, a parts list printed, and the finished design can be
saved for future reference.

Running the Software

To start SwitcherCAD, simply type SW ENTER. SwitcherCAD will
respond with a title block and two menu bars. The program is
designed for use either with or without a mouse. Mouseless users
move around the program using arrow keys, SPACE bar ESCAPE,
TAB, and ENTER. SHIFT TAB moves the cursor backwards.
Function keys serve as shortcuts for major operations.  The menu
bar at the bottom of the screen is a reminder as to what the
function keys do:   access a help file (F1); exit the program (Alt-X);
save the current switcher design file (F2); open a new file (F3);
start a new design (F4); and enable the upper menu bar (F10).
F10 is necessary for mouseless users. Except for help (F1), all
function keys are disabled during the design process. Both menu
bars work with a mouse--just click on the desired item. ESCAPE
has the same effect as the Close button.

Screen 1.  A mouse click, SPACE bar, ENTER or ESCAPE will
close the title block and automatically open screen 1 of a default
design file. All input and output parameters are entered on this
screen. Enter the input/output parameters for your proposed
supply. Note that output ripple is expressed in millivolts. If the
supply has only one output, leave the values for outputs #2 and #3
as zero. Mouseless users use TAB to get from one data field to the
next, and SHIFT TAB to back up. To back out of screen 1 click on
the Close button or hit ESCAPE. You can then return to DOS by
typing Alt-X, or by clicking on Exit in the bottom menu bar, or by
selecting Exit in the File pull-down menu at the top of the screen.
To move from screen one to screen two, click (ENTER) on the Next
button.

Screen 2. SwitcherCAD has now narrowed the topology search to
the list Allowed topologies.  If there is more than one choice for a
topology a flyback is, more often than not, the alternative. Avoid
choosing flyback unless you require isolation, multiple outputs, or
where the input and output voltages differ by a wide margin.

When selecting a part, start with the lowest current device listed.
Switch current ratings (Im) are shown near the bottom of the
screen, along with the peak switch current and several device-
specific parameters. These parameters change as different parts
are selected. A package can also be specified; this information is
used in thermal calculations. Not all packages are available
(denoted by N/A) for all devices.

Schematics. You can branch off in any of several directions from
screen 2. A circuit diagram of the chosen topology can be viewed
by clicking on the Schematics button or typing S. Press any key to
return to screen 2.

Fastdesign. For a fast design, a partslist and a listing of
component operating conditions can be displayed by clicking on
the Fastdesign button or typing F. The partslist can be printed or
stored in a file for later use using the Print button. Click on Close or
type C to return to screen 2.

EnableAll. SwitcherCAD restricts the choice of topology and
device to those that make sense for standard applications. Expert
users may want to experiment with other combinations. To do so,
simply click EnableAll or type E. But beware: it is easy to get
nonsensical answers in this mode. Back up to screen 1 to reset
this function. In some cases SwitcherCAD may find a couple of
topologies, but no parts. If you select a topology, and then
EnableAll to force a part into the application, SwitcherCAD will
undo your topology selection. Don't forget to reselect the topology
after clicking EnableAll.

OptimizeDsn. SwitcherCad arrives at a finished design through a
series of theoretical calculations, assumptions, and carefully
applied rules-of-thumb. Expert users may want to override a few of
the assumptions  and rules-of-thumb to optimize a particular
design. Click on OptimizeDsn or type O to enable this mode (call it
screen 3).

Screen 3 displays each step of the design process starting with
circuit specifications at the top, and finishing with circuit
performance at the bottom. In between, operating points and
component values are calculated, all in the form of a spreadsheet.
Just below this window is a single Input line where spreadsheet
values can be changed. Only I (input) lines can be changed. The
first letter of each line is either I or O, signifying either an input
variable or a calculated output. Input variables can be selected
and altered on the Input line just below the spreadsheet window.
When a new value is typed on the Input line, ENTER will put the
new value into the spreadsheet and initiate recalculation of all
other spreadsheet values.

Watch. Since the spreadsheet is necessarily long, Variable watch
list is included to simplify watching important variables in the
spreadsheet. Up to 6 spreadsheet lines can be placed on the
watch list, allowing widely-scattered output variables of interest to
be conveniently monitored while input variables are changed. To
add a variable to the watch list, click on that line in the
spreadsheet window, and then click on Watch or type W. If you
exceed 6 items, the last line in the list will fall off the bottom to
make room for the new line. Click on Close or type C to return to
screen 2. Note that all screen 3 entries are reset when returning to
screen 2; this serves as a convenient way to get back to where you
started if screen 3 changes drift too far from reality. However, any
changes made to the basic specifications (input and output
voltages, for example) will be carried back to screens 1 and 2,
which can potentially override the topology and device selection
process. Return to screen 1 to update the topology selector.

Print. The results of screen 3 changes can be saved to a file or
printed by clicking on the Print button. For an abbreviated partslist,
deselect Input_Eqs and Output_Eqs.

Saving your work. On page 3, enter the Print menu and save your
results as a text file. Files saved thusly have .SWO extensions, and
are saved only for the purposes of subsequent printing. To save
the input file for later work, back out of the program to the blank
menu screen and access SaveAs under the File pull-down
menu. Give the file a name, using a .SWI extension, and save.
During the save process, the path and file name are shown across
the bottom of the screen. You can move out into other directories
and disk drives by double clicking on ..\ where the various files
are listed.

Important Points for FAEs:

1)  Typing an ESCAPE on screen 3 will cause an immediate return
to page 2 and resetting of all inputs. This could be disconcerting--
especially if you have just spent 10 minutes making all sorts of
changes. Don't type ESCAPE unless you really want to dump all
of the tweeks you've made on screen 3. Save from inside the Print
menu first.

2)  There are two diode voltage drops shown on page 3.  One is
grouped with the IC characteristics, and the other is grouped with
diode operating conditions. The first instance is used for early
operating point calculations, and the second instance is used for
the diode╒s thermal calculations. Both can be modified by the user.

3)  You may find that the software chokes with certain
combinations of isolated converters, especially - to -, - to +, and +
to -. Pretend it's + to + and change your grounds later.

A similar situation is found with multiple output flyback converters.
The primary output must be positive.

4)  Some seemingly OK designs won't find a topology.  For
example, a 5 to 12 negative won't work because the LT1074
only works down to about 8V. If the program fails to find a
topology/part number, think about the supply voltage limits and
current limitations of the ICs. This usually explains why the
program draws a blank.

5)  SwitcherCAD includes inductor, diode, IC, and capacitor
databases. Component characteristics are extracted from the
databases when passing from screen 2 to screen 3. If changes are
made in screen 3, the database is NOT consulted. Instead, the
program advances to a customize mode where inductor, diode,
IC, and capacitor parameters are simply variables defined by the
user.

6)  If various specifications are entered on screen 1, they cannot
be save unless you move to screen 2. If you back out of screen 1
without making it to screen 2, screen 1 entries will be lost.

7)  Sometimes it is desireable to make several changes in parallel
on screen 3. However, each time a change is made SwitcherCAD
recomputes the entire spreadsheet. This process could take 10
seconds or more per change on a slow machine. As an expedient,
hit F9 immediately after ENTERing an input change. F9 will
capture the new value and change it in the spreadsheet, and then
halt execution. When you make your last change hit only ENTER,
and allow the program to recalculate all of the output values.

8)  If the rms current requirement in the capacitor is high, multiple
capacitors may be required. This is denoted by (2 ea.) etc.
appended to the part number. If the ripple requirement too
stringent SwitcherCad will pick the output capacitor based on rms
current rating, and suggest a filter to meet the ripple spec. We
have limited the capacitor database to a range of 10 to 100V.

9)  The suggest L value is calculated to give 25% ripple current in
continuous mode, and is also sandbagged on the high side. In a
practical circuit the inductance may be reduced if desired. In
flyback designs the inductance is picked to achieve the required
output power, with a 25uH minimum.