DN Staff

August 16, 2004

21 Min Read
Online Power Supply Supply Design Tools--In-depth Evaluation

Web-based SMPS Design Programs Evaluation:
Fairchild Semiconductor

Marty Brown
Sierra Energy Management Systems, LLC

Key Items for Evaluating the Web Design Tools:

I assumed that the majority of users would be relative novices in the field of switching power supply design. I noted the difficulties I had encountered during the process of evaluating these sites.

Some of the factors I looked for are:

1. Ease of navigation.
2. Understandability of terminology used.
3. Usefulness of information presented.
4. Detailed documentation of results -paper copies.
5. User computer revisions of SW needed to interface to the Web application
6. Ease of building first prototype
7. Comparing the design with classic approaches and to other simulation programs.

Fairchild Website tools and training available:

The system I ran was: 800 MHz P2, w/ explorer 6.0 & latest JAVA.

Minimum System Requirements are: MAC System OS X with JAVA version 1.4.2.

Earlier systems are not supported by Apple and Sun Microsystems. PC: System 98, ME, XP, and 2000.

Must download most recent JAVA tools to your system (free).


From The Home Page: http://www.fairchildsemi.com

Finding the tools is somewhat obscure. Select "the Power Franchise." Selecting this option presents block diagrams of the three types of basic switching power supplies (a good beginning). Clicking on the block diagrams results in a list of parts usually incorporated within that functional block.

Click on "technical Information." Then select "FETbench (online)"

First section "Device Analysis."

This section aids in the selection of the proper MOSFET for the application. The first function is a parametric selection tool. You inter your package, polarity (N-CH or P-CH), RDS(ON) range, and VDS range. As with most parametric search tools, if the information does not coincide with the actual product offering, a series of "No Matches" will result with no guidance as to where one can yield a good series of selections. One of website's solution is to download a 211-page selector guide where the MOSFETs are separated by package.

It might be more useful to select the RDS(ON) range and the maximum VDS range and then show the packages that support that size of die or those ratings. Obviously (to us in the field) an SC70 package cannot support a die that is less than 4 ohms and is greater than 100 V. So why allow this excessive level of flexibility in the sorting process - it only produces an extraordinary amount of "no matches" and the frustration associated with it. (This is a comment from a former design engineer (16 yrs)). [Point for all semiconductor companies.]

Once a device is found, re-running the curves that the semiconductor device engineers have run serves little or no benefit to the end user. The overwhelming majority of the design engineers have never touched a curve tracer and do not understand many of the plots.

Besides, all of the plots are pop-up screens and cannot be printed. I am concerned about the amount that Fairchild is willing to stand behind these resulting graphs. If they are mirrored from the datasheet that is fine. BUT, if they are generated on-line, then it may be a matter of "I verbally tell you this truth today, but tomorrow you cannot prove I said it."

In short, a parametric sort, of the possible selections, and a printout of the datasheet would be more informative than going through this exercise.

Dynamic Characteristics:

This section is more useful, if the user understands his or her circuit.

Body Diode simulation:

They present the general test schematic and describe the test conditions, which can be changed.

It would be nice to do a calculation of the uJ of dissipation during the reverse recovery given the forward current and reverse voltage. This loss happens every time the MOSFET turns off. This result times the frequency of operation results in the turn-off switching loss (in W) of the system. (Just a suggested improvement).

Otherwise it is good to see a reverse recovery plot for the device. The user still cannot print this window out because it is a pop-up where no print utilities are available.

Application Analysis:

The next selection is "application analysis" then press "start". The viewer has a choice of 5 basic applications. The "advanced" selection asks more of the operational parameters of the applications. This includes input voltage range, output voltage and current, device derating, drive levels, and ambient temperature and cooling methods.

After answering questions about the application, the next screen asks about the importance of each of the listed factors, such as PCB area, cost, parts count, etc. It then gives you a list of appropriate MOSFETs with their associated expected loss (W) in the application.

Like the Intersil web-based tool, when one selects "View Schematic, one cannot print out the entire schematic. The window is in landscape mode, but the print button only produces portrait -style prints for the first page only. The second page is not printed out. Unless your print driver has an orientation button on the "print" window, the printer cannot render the whole schematic.

Selecting the "run Simulation" button, produces an occasional JAVA error, even when the latest JAVA is loaded. To reduce the errors, one should go back to the beginning of the FETbench program and re-enter the data. In this way, the fields seem to reinitialize themselves and the error disappear.

The simulations are informative. You can select switching waveforms (VDS of each FET, branch currents), gate drive current and voltage waveforms, and output waveforms (output Vripple, inductor current and output current). Each display can be magnified, but the final resolution is not enough to view the close-in transition periods (deadtime, rise and fall times, etc). But the displays are informative to novice engineers as to the operation of that particular topology. It also informs the user of information where he or she might want to change the value of the output inductor or capacitor.

Some of the major components and FET parasitic elements can be changed in this screen. The simulation must be re-run to view the results of any changes.

The user cannot print the simulation windows because they are in a pop-up window where the print utilities are not accessible.

Thermal Analysis Section:

The first screen encountered is one that roughly describes the physical PCB such as the PCB area, distance to the next PCB (if in a wrack mount system), and number of PCB layers. The units can be in English or metric.

The next screen describes the characteristics of each layer, such as copper thickness and percent of area is copper, the thickness of the fiberglass layer to the next layer (from top to bottom). The thicknesses are in inches or mm, but a conversion table is given at the bottom of the screen for converting "1oz" or "2oz" copper to inches or mm. This is very handy.

The next screen is the airflow estimations. For convection-type airflow, it will not allow less than 20 (CFM and LFM (English)) air flows. Totally enclosed applications is a manual exercise. No good estimates were given for the rates of the convection airflow with vent openings in the case. Admittedly, this area of the design is highly dictated by the physical enclosure, PCB orientation and the vent openings and placement. This is probably expecting a lot from a universal simulation program.

The next screen is where on the previously dimensioned PCB, the power MOSFETs are placed. The first step is to place the devices at their anticipated locations. This can be done by Click-and-drag or by entering the exact X-Y locations.

The resulting thermal diagram was very informative as to the temperature of the PCB just under the device(s) and its (their) effect on the temperature of the rest of the PCB. This is useful for the considerations of the other temperature sensitive parts on the PCB.

Changes can be made in the placement and PCB heatsink areas by going back to the relevant screens and re-running the simulation. One of the amazing benefits of this simulation is that one can place "dummy loads" onto the PCB to simulate the other major heat-producing components within the system (ICs and other "non-MOSFETs), and simulate a more complete PCB Thermal system. I consider this above and beyond the call of duty for this Fairchild web-based simulator.

The Thermal analysis is located on a base browser page so all of the print utilities are available. This means that the thermal image can be printed out (orientaion, scaling, etc.) for documenting the results. (I love it).

In summary:

I love this set of web-based tools! There are some bugs in the programs with regards to printing of the schematic and the electrical simulations, which should be addressed for user documentation purposes.

Although there needs to be some basic electrical and thermal knowledge on the part of the user, the tools in general seem to be very flexible and straightforward. It is a shame that the analog group and the discrete groups within Fairchild don't work closer together to develop a power supply system design package which would include the analog controllers, waveform analysis, feedback loop analysis and thermal analysis. That would be an incredible tool available on the web for power supply development.

Web-based SMPS Design Programs Evaluation: National Semiconductor

Marty Brown
Sierra Energy Management Systems, LLC

Key Items for Evaluating the Web Design Tools:

I assumed that the majority of users would be relative novices in the field of switching power supply design. I noted the difficulties I had encountered during the process of evaluating these sites.

Some of the factors I looked for are:

8. Ease of navigation.
9. Understandability of terminology used.
10. Usefulness of information presented.
11. Detailed documentation of results -paper copies.
12. User computer revisions of SW needed to interface to the Web application
13. Ease of building first prototype
14. Comparing the design with classic approaches and to other simulation programs.


My System Ran:
System 1: MAC OS 8.5 & 9.1, Internet Explorer vers 5.1,
PC 800 MHz Pentium II, Explorer 6.0.

The range of the test software are:

MAC: MAC OS 8.0 Thru X
PC: System 98. ME, XP, and 2000.
Explorer version 5.1 or later
Netscape version 4.7 and above

The Home Page URL is: http://www.national.com

Choose "WEBENCH Tool" - "Power"

There are step-by step instructions as to how to start and proceed through the design.

Press "Start Here."

Type in your input and output requirements (Vin, Vout1, Vout2 ..., I out 1, Iout2 ...),

Ancillary functions such as ON/OFF, fail (error) signal, sync pin.

If you do not have a particular part in mind, the program will then select the best-fit part with the closest set of features (ON/OFF, synchronization, etc.) to the minimally-described application). It will also present a list of less-than-the-best appropriate ICs for the described application. These additional parts have larger drain currents and other additional features. Many of the additional parts are "super-sets" of the application.

Click on "create design" on either the recommended best part or one of the other parts that may also work in the application.

The next screen presents a Bill-of-Materials based upon the IC and the conditions inputted from the last screen. Each of the passive components can be reselected from a limited list of vendors (2 each) and values (1 - 3X). Then the new values are used in the following simulations.

Once all of the component values have been selected, one can choose the "Webtherm" utility. This offers a choice between two different heatsink copper areas. These illustrated copper areas are also the layouts offered as the demo PCBs layouts. You can also vary the airflow around the PCB. Each change to the components or the PCB selection, the thermal simulation must be re-run. The simulation, depending upon the user-loading of the server may require some time to run the simulation. The server will notify you via an email when the simulation is complete and will provide the direct URL where the results can be found. Although you cannot change to location of the components on the layouts represented, you can certainly see which components you need to move in order to reduce the PCB "Hotspots".

The heat plot and the PCB shown in the thermal analysis use the demo PCB that the user can later order. Although the layout may not be optimum in the user's end product and may not be an optimum noise-sensitive layout, it does give the user a very good idea as to how the final circuit will operate. The power dissipations of each of the components are given separately in a table at the bottom of the screen. This information and the package thermal information allows the user to manually calculate the heatsink requirements of the end PCB.

Electrical Simulation: "Click to start your electrical simulation"

Here the simulator provides a modeled schematic. This schematic can easily be printed by using the browser's print utility. This schematic changes with the type of simulation being done, which may include independent and dependent voltage or current sources, etc. The components may be changed by returning to the previous screen, which contains the BOM, and then re-running the simulation.

The types of simulations that can be done are: Steady State (Vout, Vripple), Bode Plots (closed-loop gain and phase), Input transient (change on I/P voltage), Load transient (change in load current, the start-up transitions. The electrical simulations are dependent upon the most recent values placed into the BOM. These simulations are useful in demonstrating the operating characteristics of the circuit, but much of the control is within the IC and not under the control of the user's design control. These simple switcher designs are pre-canned and have very few (if any) programming components outside of the IC.

The Bode phase plots do not include the 180-degree error amplifier contribution to the phase. So add -180 degrees to the bode plot and be sure to check the phase difference between the phase lag caused by the output filter (at 5 to 10 times the output filter pole frequency).

The compensation is internal with many of the Simple Switcher ICs and the control is mostly out of the hands of the user. Caution must be practiced with the simple switchers in the choice and values and types of the output capacitor(s). Avoid using very inexpensive and small ceramic capacitors, since the compensation used within the National ICs may not be stable and will oscillate. This type of compensation is very sensitive the capacitor value and the ESR of the output capacitor. Use of ceramic output capacitors may cause the power supply to oscillate at certain line and load conditions. Remember that the software is aimed at the "middle-of-the-road" applications.

If selected, input and output transient simulations are also presented. Since both responses are dependent upon the feedback loop compensation, and this is not under the control of the designer (user), these simulations are just for information.

The start-up simulation is also just for information. The soft-start circuit is contained within the IC and the circuit's response is dependent upon the load impedance, the output inductor and capacitor.

The steady-state simulation basically only shows the output voltage ripple of the output voltage. This is mainly a function of the value and ESR of the output capacitor and depends less on the value of the output inductor.


There are many helpful utilities included within the WEBENCH program, such as small animated demonstration programs that help the user walk through a typical design process, etc.

Comparing the results of design simulations to other Simulators:

Since the compensation of the simple switchers is internal to the IC, and many different approaches can be done using this method, it is more appropriate to compare the final closed loop responses of the finished systems (classic vs. simple switcher).

In Summary:

The National Semiconductor simple switcher WEBENCH tool is very useful for entry-level engineers that are not expert in switching power supply design. These are board-level, non-isolated supply types where a local regulator is needed. National includes the feedback loop compensation within the ICs, which makes the user's task much simpler. The design program pre-selects the remaining components such as the input and output capacitors to yield not only the proper operation of the IC, but also provides commonly accepted levels of output ripple voltage.

Since the ICs usually have fixed output voltages, some of toil of calculating resistor divider resistor values is eliminated. Providing the initial value of the remaining major components is a great aide to the new designer, who can (with caution) modify the part numbers to their AVL (approved vendors list).

The amount of information provided, allows the designer to select higher-rated ICs, to lower the PCB operating temperatures, if the engineer is knowledgeable in these area. No external switch mode power supply (SMPS) design literature is needed to complete a satisfactory SMPS design using this tool. It would be helpful to show the simulated voltage signal on the switched node and other relevant points, so that the engineer can see what to expect at the major nodes when he or she works with the prototype.

In short the WEBENCH tool is very useful for the purpose it is designed.

Web-based SMPS Design Programs Evaluation:

Marty Brown
Sierra Energy Management Systems, LLC

Key Items for Evaluating the Web Design Tools:

I assumed that the majority of users would be relative novices in the field of switching power supply design. I noted the difficulties I had encountered during the process of evaluating these sites.

Some of the factors I looked for are:

15. Ease of navigation.
16. Understandability of terminology used.
17. Usefulness of information presented.
18. Detailed documentation of results -paper copies.
19. User computer revisions of SW needed to interface to the Web application
20. Ease of building first prototype
21. Comparing the design with classic approaches and to other simulation programs.

INTERSIL Web-based Design Program:

The system I ran was: 800 MHz P2, w/ explorer 6.0 & latest JAVA.

Minimum System Requirements are: MAC System OS X with JAVA version 1.4.2.

Earlier systems are not supported by Apple and Sun Microsystems.

PC: System 98, ME, XP, and 2000.

Must download most recent JAVA tools to your system (free).


The Intersil Web-based tools were obviously designed by only IT people. There is no latitude for older operating systems or JAVA tools. This is a PC tool only. Although MAC OSX is somewhat supported, it is an Apple-only supported JAVA plug-in.


From The Home Page: http://www.intersil.com.

There are many choices that look like the choice you may want to choose for the simulation program. But only one choice leads you to the actual simulation tools. Select "Design iSIM Design Simulation Tool" or go directly to: http://www.intersil.com/isim/

Here you are presented with several 2 to 4 Phase VRM buck converters that are meant for the Pentium or Power PC applications. However, there are five choices that involve other ICs and topologies.

Select the power topology desired: Click on button "iS" adjacent to the desired design.

You are presented with a list of suggested component values that can be changed.

IF you were to select "view schematic", it presents a schematic that can be blown-up to fit the screen. BUT, if one wants to print the entire schematic, it cannot be done. The INTERSIL pop-up page set-up is for portrait, but their typical schematic should be printed in landscape-mode. From schematic window, there is no way to select the "page Set-up". Unless the "print" window of your particular print driver includes the print orientation buttons for the printer. Otherwise, there is no way to print the right part of the schematic. Reducing the scale will make no change in the printout content. (apparently a bug or an oversight in the program).

I would suggest printing the datasheet out before this point, which may include this schematic (with the same component designation numbers).

Select "Run Simulation":

This produces two Bode plots of the "open-loop" (control-to-output) and the closed loop Bode plots of the final circuit elements. The units are in the proper units but difficult to understand because of the presentation. They do not include the -180 degrees of phase lag added by the inverting (negative feedback) amplifier. So the user must add this -180-degree phase lag to the closed-loop phase plot to view the actual point where the closed loop approaches -360 degrees.

There is no print utility included on Bode plot screens, so it cannot be printed or saved to the local computer for future design documentation. Only the "print screen" utility can be used in this option.

Comparing the simulation results against other methods:

When comparing the component values presented in the feedback loop compensation against those derived from my book (Power Supply Cookbook) and from Ridley Engineering's POWER456, the values were within 10 percent of each other. Therefore, the INTERSIL simulation program uses the straightforward method of determining open-loop poles and zeros and for placing the compensating poles and zeros for making the relative optimum closed-loop compensation.

In summary:

The INTERSIL ISIM tools cover less than 30 percent of the power management parts in INTERSIL's product portfolio. I would recommend that the user selects the part that he wants to use first, print-out the datasheet, implement the design using the datasheet guidelines, then proceed into the iSim utility. The designs offered by the simulation program are for one output voltage, output current and input power system (not directly changeable). It is good for verifying stability, but not as a design tool. The user must still design the end-circuit manually.

The presented designs only present the individual component values. So to change the operation of the selected switching power supply, such as the output voltage, frequency of operation, overcurrent settings, etc, the user must still manually perform the design equations or refer to the datasheet graphs for determining the new values.

The feedback compensation presents the straightforward approach of assigning the locations of the compensating poles and zeros within the error amplifier. If you want to change the location of these poles and zeros, the user must almost start from the beginning in determining the open-loop pole/zero characteristics. (you can get the main output filter pole from the open-loop plot ("power train transfer function)). The zero related to the output filter capacitor cannot be determined from the open-loop plot. Then the user must relocate the pole/zero frequencies, then run the classic equations for the actual compensation component values, which can then be placed within the simulator.

In short, if the user desires any changes to what was initially presented, the process is completely manual. Without the needed literary references on-hand, the effort would be a blind shot in the dark and be very iterative.

It is also very inconvenient because the user cannot properly print-out the schematic and feedback simulator results. This makes it very inconvenient to document the design for the user's internal design reviews, if the part were to be included in an actual product.

I encountered some JAVA errors when running the simulations that had to be re-run to finally get the desired results. This was more a factor of the server loading than not having the actual required JAVA within my desktop computer.

This web tool is for the seasoned power engineer since the design must be completed before the user can derive any useful information from the simulation tools. Also an accompanying design resource, such as the "Power Supply Cookbook" might be in order, to quickly refer to the various design areas within a switching power supply.

There needs to be some additional work on these tools. They are not very useful for the occasional power supply designer. Perhaps having a design team that not only includes the IT department, but also some savvy applications engineers might be in order.

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