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 +...
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