Electronic Design

Versatile Design Tool Enhances Visualization And Extends Its Range

Many analog semiconductor companies have online or downloadable tools that automate circuit design for customers. One of the most mature, National Semiconductor’s Webench, is an online tool that stretches across multiple product lines. It also offers some recent enhancements that are significant.

For example, a new visualizer for dc-dc power supplies graphically presents design tradeoffs for all possible design configurations that meet a user’s parametric requirements. Tradeoffs include efficiency, footprint, and bill-of-materials (BOM) cost. Engineers designing a circuit work their way down through multiple competing designs that meet broad tradeoff criteria to isolate the best candidate.

Interestingly, the BOM pricing is based on feeds from multiple distributors that represent current pricing for all the third-party passives and magnetics in the various designs. The database is updated several times a day.

About two months after the latest enhancements to the user interface were introduced, National introduced an extension of its “Simple Switcher” line of power-supply ICs. Simple switchers now include modules that incorporate passives, magnetics, and FET switches, and these devices were included in the Webench tool when they were announced.

Finally, at about the time the earlier improvements were made public, National extended Webench to include design tools for LED lighting designers. There is an interesting syneregy here because the driver elements of LED lighting design have parallels with dc-dc converter design. All of these updates, of course, serve designers in addition to Webench’s established capabilities in mixed-signal designs.

STEP BY STEP

The new front end for simple switchers draws from 25 different switching power-supply architectures and a parts library of some 21,000 components. It creates 50 to 70 designs, based on input criteria. That’s the easier part. The tough part is presenting the results so engineers can choose among them.

The first step is to “dial in” the minimum acceptable conditions for footprint, BOM cost, and efficiency and to key in input and output voltage, load current, and other factors. Possibilities include VIN from 1 to 100 V, VOUT from 0.6 to 300 V, power to 300 W, efficiency to 96%, frequency to 3 MHz, and footprint down to 14 by 14 mm. A second visualizer control panel makes it possible to adjust design options for voltage, current, and temperature. After that, the designs are updated.

For evaluation, the software creates multiple designs and arranges them in a target sector (Fig. 1) as an array of circles. The Y axis of the target sector represents footprint, the X axis represents efficiency, and circle size represents BOM cost. Mousing over any circle brings up specific data for that design. You can get finer-grain detail on a narrow range of possible designs by drawing a box around them, which then expands that particular part of the target sector.

If your tradeoff criteria are different—for instance, if switching frequency is more important than footprint—you can change the axes and the circles will rearrange themselves. If there are a lot of choices, the software will have picked a safe, efficient design that best balances your input criteria and highlighted it in green.

Once a design is selected, engineers can further tune and optimize the design electrically and thermally. Select the “Build It!” feature, and National will ship a custom power-supply prototype kit within 24 hours. A hands-on demo is available online.

DESIGNING WITH MODULARIZED DC-DC CONVERTERS

National’s new Simple Switcher modules add a new dimension to dc-dc design with the Webench tool. The LMZ series of power modules with internal inductor and switching FETs reduces parts count, of course. These modules also shield electromagnetic interference (EMI), thanks to a novel packaging technology. Heat transfer through a large exposed-bottom pad lets them typically operate as much as 10°C cooler than comparable modules. Thermal resistance (theta-JA) is 20°C/W.

The first family members support 3.3-, 5-, 12-, and 24-V input voltage rails and offer load currents up to 4 A. For design scaling, they all come in the same 10.16- by 13.77- by 4.57-mm packages with seven leads plus the pad. The package was chosen to allow for easy and quick prototyping and at the same time to enable the use of the same pick-and-place manufacturing employed with standard TO-263 packages.

LED DESIGNS TOO

The Webench program handles both colored and white LEDs. It also accommodate LEDs’  variability in output in lumens and efficiency in terms of lumens/W, along with high-brightness white LED ranges of color temperature.

Dealing with all that variability is where Webench starts to show its utility. In most lighting applications, to get enough light, multiple LEDs must be combined. And that opens the first design consideration: should the LEDs be connected in parallel or in series?

A parallel connection keeps the total forward voltage drop low, which accommodates a buck driver topology. But the Vf of each LED will vary, so some LEDs may draw higher currents, making them brighter and raising their junction temperature. A series connection exhibits no problem with differences in current and thus brightness. The combined forward drops of the LEDs in a string add up, though, and that may require a driver with a boost topology.

Then there’s dynamic resistance, rD. Forward drop divided by current isn’t a linear function, and rD is typically five to 10 times lower than the result of simply dividing Vf-TYP by If-TYP. So when one is designing and simulating the driver’s control loop, it is vital to have an expression for rD, as it affects stability, LED ripple current, and the selection of the driver’s output capacitance.

With Webench, users start off by selecting an LED based on color, current, and luminous flux. In Webench, users make these sorts of selections with sliders. To refine the choices, selection can also be based on other parameters such as footprint, angle, and price. After the LED has been selected, the next step is to enter the LED array information and the input supply parameters. At this point, users can vary the desired operating current and view the effect on the LED forward voltage and dynamic resistance.

After the choices are refined, the next step is to actually select an LED. The tool offers complete parametric specs on all manufacturers’ LEDs. A table and a scatterplot present all candidates that meet the selection criteria so far. Users then choose a National LED driver from a tabular display of topology, efficiency, footprint, BOM cost, frequency, and feature set.

Webench then develops a design with a BOM that includes recommended switching FETs and passive components. It also provides an array of analysis tools (Fig. 2), including a graphical simulation and tabulations of losses. The core design technology and data are the same as that used for the dc-dc converter version of Webench.

National has a special dimming LED driver, the LM3445, which is designed to work with legacy triac dimmers for room lighting (see “High-Brightness White LEDs Light The Way To Greener Illumination”). With Webench, designers can build a lighting system around the LM3445 and run a simulation that demonstrates the no-flicker dimming range.

CLOUD CONSIDERATIONS

Some companies base their design and simulation tools on Matlab or their own simulators and make them downloadable. With its dynamically updated database of parts and prices from multiple distributors, Webench is an Internet “Software as a Server” (SAS) application.

Webench requires registration. Also, designs are stored on National servers, if designers need to store them. That raises questions of execution speed and privacy. Yet all the demonstrations I have seen execute extremely quickly, and while I registered with Webench several years ago and played with the tool from time to time, I have never received a solicitation that I suspected was based on my registration.

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