Take the Pain out of Power Design - ADIsimPower/ADIsimPE

Let’s talk a little bit about power. Well, maybe more than just a little bit as this is a fairly long post.

Every circuit requires power to function, and from my experience as an applications engineer, power is usually one of the last things to be addressed in the design phase of product development. While commonly viewed as a commodity these days, the truth is that there are many subtle nuances in most power designs that must be accounted for to allow an application circuit to reach its full potential. And more and more, design engineers are not experienced in power or just don’t have the time to devote to it. Many books have been written on power design, but very few people have the time or scope in their jobs to fully understand and work with it. At the same time, data rates, bit counts, and noise floors have made power supply requirements more stringent than ever. But how do we design a power supply to meet these ever-increasing specifications with some confidence?

It takes about 3-5 hours for an experienced application engineer to perform a switching power rail design manually. Most of the external components need significant adjustment of their effective values to account for effects of DC bias, saturation, thermals, and many other factors. Unless these items are taken into account, the power rail will not meet the desired specifications and might even be unstable. Furthermore, most companies want not only a design, but also a simulation model that verifies that the design works as desired to reduce risk of board spins. This simulation model must also have the effective values of the components to provide accurate results.

ADI’s power applications team handles many requests for these designs. We developed a set of design tools that we use internally to assist in making sure all component and system effects are addressed, design time is minimized, and that a design will indeed meet the specifications required when built. It wasn’t too long before we realized that such a design tool would be a huge benefit for customers to use as well.

ADIsimPower and ADIsimPE is the result. Although not widely known, this tool set is groundbreaking in many ways. Using a very simple 3-step process, anyone, regardless of power experience, can get a schematic, Bill of Materials (BOM), starting layout and in many cases an exact simulation model in just a few minutes instead of hours or even days. Designs can be optimized for efficiency, cost, area, or parts count as needed. Advanced users can do far more to tweak a design (read the datasheet first!) such as change the major components and fine-tune the specifications. When built to the BOM, an eval board will meet the specifications completely, and the tool predictions and simulation results will match the measurements in the lab when built. Seven distinct topologies are fully supported and even combinations, including SEPIC, Cuk, and the inverting buck-boost. Many of our multi-rail devices are also supported.

Let me illustrate with an example just how fast and easy it is to complete a complicated power design from end to end. A very common application is providing bipolar power for a signal chain. Let’s assume that we have a 12V input rail with a tolerance of +/-5% (i.e. 10.8-13.2V), and that we need to deliver +5V and -3.3V at 200mA each. The maximum ambient temperature will be 55C. Let’s further assume that we need ripple of less than 5mV in this application and that lowest cost will be the design goal for optimization. Here goes:

Step 1: Part Selection

The first step is to use the rail specifications and choose the best part from ADI’s portfolio. To do this, we need to run the selector guide with the five basic parameters that every power rail needs specified. The tool is located at www.analog.com/adisimpower or can also be found by following the links from the Design Tools section of ADI’s website.

Now this selector guide does not operate like all the others out there which simply match the Vin, Vout, and max load currents with datasheet limits. Instead, ADIsimPower computes a partial design for every single power part that ADI manufactures. It determines the entire set of ADI parts that will meet the specified parameters in all of the topologies and displays them with a considerable amount of useful information for making decisions. The selector guide is a little bit conservative and will not suggest a part unless it is sure that the part will work to spec. More on that in a little while.

Let’s enter the positive rail first and see what comes up:

After entering the design parameters, we click the “Find Solutions” button and after a few seconds the following list comes up:

Hmm, lots here. Note each part has an estimate of the cost, area, parts count, and efficiency. While not exact (it is a partial design calculation), these numbers are surprisingly accurate and certainly very helpful to rank our parts by what is important as a design goal. Each of the informational columns can be sorted-on (click the header), and best in class is highlighted in bold. Furthermore, each applicable topology is addressed. There are many additional features on this page, including a very powerful filtering function. Clicking on “Quick Filters” will bring up a rather complicated dialog box that lets you select or exclude important features such as Tracking, Light-load efficiency, Clock synchronization, etc. These allow the user to very rapidly narrow down the list of possible parts and to choose the best candidate with the desired optimization. I’ll save a detailed description of these for another post.

For our purposes, we know that we need a negative rail too, and we can see that the ADP5070 or the ADP5071 in a SEPIC topology can do the positive rail and has a negative converter too. So let’s enter the -3.3V rail and check if those two parts can work for that solution as well:

Upon entering these design parameters, we get the following results:

Looks like we should target the ADP5071. So far, we have quickly narrowed the list of candidates from 39 parts to just one in about one minute. Try that searching through a standard selector guide!

The next step is to download the ADP507x tool by clicking the “Download Tool” button. A zip file will be made available, and after downloading it, you can extract the Excel spreadsheet inside (please extract the tool—Windows won’t allow saving the sheet back to the zip file) and begin the detailed design process.

Step 2: Detailed design and optimization

After downloading the tool, you will be asked to enable macros when you open it. This step is needed because most of the intelligence in the tools is contained in VBA code. The tools are safe, digitally signed and certified, so there is nothing to worry about here.

Now we enter the inputs again, this time both rails simultaneously. We use the basic inputs dialog which displays on startup or when the “Enter Inputs” button is clicked. Here is what we need:

We have one more piece of design information to enter to ask the tool to keep the ripple at or below 5mV. To do this, we need to access the “Advanced Settings” dialog. I don’t want to go too much into all of the possibilities of the advanced settings, but here is what we need to set things to for our design:

Note that I have set the “Vout Ripple” on both rails to 0.1%, with design specifications of 5mV and 3.3mV. I have also checked the boxes to allow a two-stage LC filter to be used. Why did I do this? The tool will output a design that meets the specs even without this, but I happen to know that achieving lowest cost is easier with the extra filtering than the alternative of really bulking up the output caps. While you may not have the benefit of my experience, you will find that this tool set is so fast and efficient that you can easily try several ideas with an investment of only a couple of minutes to optimize things to your liking. The rest of the settings can be altered as needed, but for this example, no further changes are required.

Now we submit the design by clicking “Submit & Goto Basic” and then “Submit/Run”. After a few seconds, the tool completes the design and displays the results. First, we get a schematic:

This schematic is from the eval board, and not every component might be needed for every design, but it is complete. Our tools target an eval board, so taking this approach makes sense and it is easy to tweak the schematic for a final end application. Next, we get the Bill of Materials:

Surprise! Upon doing the very detailed design calculations, the ADP5070 works for both rails. We can save some money by using a lower current part. Remember that I said the selector guide does a partial design and is conservative? Unlike so many of the other design tools out there, ADIsimPower almost never will suggest a part that won’t work, which saves a lot of time from going down dead-end streets.

Note that we have exact part numbers, cost and area here, too. The cost is for 1k or 1-reel pricing, and obviously each company negotiates their own prices for components, so it may not be exact for your organization, but it is a very good starting point.

The next section shows the computed design performance. With multi-rail tools, this section gets pretty long, so I’ll only show channel 1 for brevity:

Note that the tool predicts 2mV of ripple and a crossover frequency of 26.5kHz with 57° of phase margin.

I don’t want to toot my horn too much, but I want to point out that this is a SEPIC design. This topology seems to unnecessarily scare most design engineers (including myself many years ago) because it is a 4th order power stage which makes the control loop analytically intractable. Fortunately, we model the loop in its entirety and are able to numerically solve it so we can recommend a stable design with the most loop bandwidth possible. Even with a second LC filter stage in the loop.

Ok, we are rolling now. It took a lot longer to read this than to do it in practice. Let’s finish the design. Of course, you can print the results now, but perhaps you or your management have a few doubts still? Would a simulation help ease some concerns? We can help with that.

I want to make an important point here. Simulation models show you what will happen if you choose a set of components. Simulation is an analysis tool that doesn’t help choose the component set that meets any specifications. Simulation only helps verify a design. AdisimPower is a synthesis tool. It chooses all components to ensure the design specs are met. AdisimPower uses a very advanced and proprietary database of component specs along with significantly more accurate and detailed models of the power part than can ever be expressed in a datasheet to produce robust designs that meet the predictions when built.

May I make another point here? Our competitors seem to think that a nominal model of the external components is good enough, but I can state that it isn’t. I’ve tried designs in competitor’s tools, and frankly, I am disappointed that I can by default get unstable and totally unworkable designs from them and certainly ones that don’t meet the required specifications. ADIsimPower tells you what is really needed. If you are using a 22uF ceramic output capacitor, it won’t have 22uF if the output voltage is 5V. You may need most of that capacitance to meet ripple specifications and stability. Unlike all the other design and simulation tools out there ADIsimPower uses the effective value at the operating point for its computations as well as in the exported simulation model. Ok, I’m getting off the soapbox now.

Step 3: Design verification

To simulate the design, we just click the “Simulate with ADIsimPE/SIMPLIS” button. This brings up a dialog box asking what exactly should be simulated. You can choose what you need, but let’s focus on the most common runs. For our design, let’s check the loop stability and transient response. Here is the box:

The default is a very good choice for verification. We choose “Export & Run Simplis Model”. Now, you have to have downloaded and installed ADIsimPE for this to fully work. Assuming that you have done so, when you click the button in a few seconds you will load the simulation model loaded with your design specs.

When the simulation loads all that is needed is to run it. Let’s do that. First, here is what we get to simulate:

Isn’t it nice that we already have the effective component values pre-loaded for us? Take a look at 1Cout2. This is a 10uF cap, and you can see that at the operating point, it only has 5uF of effective capacitance. This is how we can match the tool output, the simulation results, and later lab measurements.

We are ready to run the simulation. Now, with most spice models, it’s time to go get a cup of coffee and wait for a few minutes. With ADIsimPE, the simulation completes in about 15 seconds, and includes a true AC analysis that other simulators cannot even perform. I’ll cover the details of why this is so in another post, but the reason for this is that ADI’s switching models are developed in SIMPLIS instead of spice.

Here are the results from the AC analysis:

Notice that the SEPIC gain and phase closely match the tool predictions: 26.4kHz and 64° of phase margin. That is about as good as it gets!

Here is the transient analysis:

Zooming in to look at the ripple on the SEPIC rail, we see 1.2mV of ripple. Pretty close to the 2mV the tool predicted and again a little conservative to account for tolerances:

There are many other things that can be analyzed, but at this point, the design is looking pretty solid. We have a complete paper design. With only seven minutes of time invested, we now have:

  • Selected the optimum IC
  • Generated a full schematic/BOM
  • Obtained all the operational parameters
  • Verified the performance meets spec

If you would like to test the design yourself, you can order an evaluation board. Every tool has an eval board, and you can order one at no cost by clicking the “Order Blank Evaluation Board” button in the spreadsheet and filling out the information.

Many customers would like an example layout as well. Each tool includes the layers:

So there you have it. Even if you have practically no knowledge of power, you can very rapidly get a complete design and layout using ADIsimPower and ADIsimPE. Give it a try!