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What Is A Power Budget?

Posted by Chris_Gammell Oct 24, 2010

Boy is it hot in here or is it just me? Why’s that gizmo over there giving off so much heat?

Power budgets are a necessity these days. Due to increasing regulation, we’re seeing devices that must comply with efficiency limits in their power conversion (using a switching power supply or otherwise).

So what is a power budget? Much like a budget you might have for your personal finances, a power budget shows where all the possible power will be used by a device to by breaking it down into components and categories. In some situations, you might be told up front that you will have 3W available to run your design. However, sometimes as designers we start by calculating the total power a system needs and then taking actions such as replacing parts or redesigning circuits to cut back power to an acceptable level. So why might someone want to do a power budget from day one?

  1. Power availability — While you might have more power today, it doesn’t mean you’ll have it tomorrow. Designing a system for 3 W power consumption may be acceptable now, but designing a lower power system may meet future regulations. And the trends in the industry point in that direction.
  2. Battery Life — If your device is running off a battery, you likely do not have a choice whether you are doing a power budget. You want to maximize the life of your device on a single charge (assuming it is using rechargeable batteries) and your customers want the same. Just a few weeks ago I was complaining publicly on The Amp Hour about my new device with poor battery performance. Doing a power budget will point to the components consuming the most power so you can later optimize for longer battery life (hopefully this was a design constraint from the beginning).
  3. Heat generation — Heat is an unfortunate side effect of working with electronics. However,  it also has a 3 direct effects on your product and how it is used.
    1. User discomfort — No one likes having a hot laptop sitting in their lap. Nor a cell phone that is uncomfortable to hold.
    2. Circuit robustness — An often quoted specification of an op amp is the voltage offset drift. This sensitivity to temperature can have dire effects in systems that rely on analog accuracy. However temperature changes can create conditions that are unfavorable and could even cause device failure (such as thermal runaway). The heat of the whole system can end up affecting individual components as the nominal temperature inside your device rises.
    3. Product lifetime — The lifetime of a product can be drastically reduced by higher than normal temperatures inside the device. Extreme temperatures can begin to dry out capacitors and cause others to fail catastrophically. While it is possible for systems to fail in a drastic manner, the more likely outcome is a product that does not last for its specified lifetime. An example might be a TV that has a less vibrant LCD after 5 years due to excessive heat and component drift and fatigue. If the product was designed to have lower heat, the product would have lasted longer. For more on how to design and prevent early failures, check out Dave’s video blog about heatsink design.
  4. Cost (sizing) — More power means you need larger components. Other than the obvious requirement of needing more space (duh), it often correlates to higher cost components. Not only will you need larger packages for your components such as op amps and comparators in order to better dissipate heat. You’ll also need a larger power supply with more reliability. If a 5W and 20W power supply with 12V output are compared, the 5W supply has smaller magnetics and less wiring because there is less total current that needs to pass through.

So let’s look at an example power budget (click for a larger version):

As you can see, not much more is required than your datasheets and a spreadsheet type program. Even simpler is a piece of paper but I prefer the built in math functions of the spreadsheet program. The first two columns (A&B) are simply identifiers to allow you to recognize which components correspond to which set of data. The next two columns (C&D) determine the multiplicative factor. If you have 5 components that contain 4 op amps per, then that will consume 20x the power of a device that has the same supply current needs but only one op amp per and there is only one on the board.  The next two columns (E&F) show how much current each individual component contributes and then the sum of all the components of that type contribute. Note that this parameter on a data sheet would be listed as “supply current” or “active current”. The “quiescient” number is when the device is in a resting state and will likely be much less than the active number (and not relevant for this example). Finally, the supply voltage is listed (in column G) to calculate power (using the formula P=I*V) which is listed in column I per device. All of these contributors are summed, an efficiency is estimated (I assumed a poor efficiency linear type supply) and the total power required input to the device is given. Further calculations could result from much of this initial data.

I would be remiss without mentioning something about power budgets: you’re still going to guess about certain things. In fact it will be many different things. You might not have perfect data about your components. You might not completely trust the “typical spec” of one of your components. This is the point where you design in a margin of error. However, just like many other aspects of engineering, this is where tradeoffs come into play. You might want to design in 4 times more power capability than you calculate (to feel safe), but there are cost and spec requirements to consider. You will have to determine how confident you are in your design and how many resources you have available to your design. In the above example where the 5V parts require 408mA from the supply (~2W), I might over spec the part by designing in a part that is capable of supplying 600mA. The (50%) margin of error allows for future expansion (might need to solder in an extra part or two) and also gives a cushion if anything was miscalculated. In some situations this 50% might be too much (think a very low-cost, high volume design) or might be too little (think a military, high reliability design). It all depends on the situation and requirements.

Power budgets can be very powerful depending on the amount of time and effort you put into them. Otherwise they are educated guesses which may or may not be helpful to your project; how helpful they are might also depend on where you are in the design cycle. As stated before, these budgets are more and more of a necessity in a world more power conscious and with devices that continue to shrink. Your customers will expect longer battery life and your products to have yet more features. Teach yourself how to do power budgets now and it will pay dividends for you in the future.



Originally posted by Chris Gammell at http://chrisgammell.com/2010/10/25/what-is-a-power-budget/
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Homeschooling

Posted by Chris_Gammell Oct 19, 2010

I have lots of thoughts about education, especially higher education. The theme that keeps popping up in my head though is that school isn’t too far removed from teaching yourself. Honestly, let’s look at the learning process:

  1. Encounter a “problem” that needs to be solved.
  2. Do background research and look at past examples of how it was solved.
  3. Apply your newly gained knowledge to the problem at hand.
  4. If a new problem arises that is not encompassed by the recently acquired wisdom, go back to step 2.
  5. Report on your findings to others.

Doesn’t this sound like work? Or studying on your own? Or doing a hobby project? How is this any different?

Since I’ve had this debate with friends before I can tell you what others say. They say that the classroom environment and being shown some of the methods before doing the problem is helpful. That having the theory explained directly helps the brain to acquire the necessary knowledge. That being able to step into the professor or teacher’s office and ask a question is a nice luxury to have. But does this always happen? I know I’ve had teachers I don’t understand (or very much disliked), notes that didn’t make any sense upon second reading and semesters where I’ve taught myself completely out of a textbook (and of course it happened to be the worst textbook of all my classes that semester).

Furthermore, there are resources today that allow individuals to continue learning on their own. Video resources like MIT Open Course Ware (OCW) can replace or augment self learning on particular topics. Message boards can provide a forum to interface with experts and to keep up on recent developments in your industry. The prices of equipment have nosedived in the past 10-15 years, allowing many more people to have a “lab section” in their house. And things like hackerspaces allow for social interactions and places to flesh out more advanced ideas.

So what’s really left? Motivation. When you’re paying $30,000 a year or are spending every Tuesday and Thursday in a classroom somewhere, you’re going to make the most of your time there. You’re going to do the homework and go get the help you need to figure out the subject matter because you aren’t allowed to put it off a month or a year. You’re going to be motivated by the piece of paper you receive at the end of your degree program saying that you completed all of the necessary requirements and did so while meeting or exceeding the expectations of your institution. Or you might even want to just prove you can do it. All of them really are valid reasons, they just don’t exist when you’re teaching yourself at home. External motivation is needed for many people (myself included) to pick up a book on a subject. In fact “motivation, momentary lapse of” is how this post came about. I was reading about active filters for a side project (where the motivation is showing off the side project and becoming “internet famous”) and I started thinking about how similar my current situation is to my former schooling. And all of the self-teaching and gained experiences are occurring without paying the $31,000 a year (yes, you read that right, tuition went up just since the last time I listed the number).

Do I hate higher education? No, I think there are some factors that make it invaluable to those that pursue it (and many more that benefit from the output). I may still try to get back to grad school myself some day. I love that there are institutions dedicated to research that might never get done otherwise. I’m glad that there are institutions that stress the rigor of the scientific method. I love that there are places where learning and advancing knowledge is the main purpose and task of those that attend. But all I’m saying is sometimes this happens in basements and bedrooms too.

Is learning at home without the structure of schools possible, especially in higher education? Does anyone ever teach themselves at home and why do you do it? What problems do you have with it? People currently enrolled in a University, do you find any fault with this thought?



Originally posted by Chris Gammell at http://chrisgammell.com/2010/10/19/homeschooling/
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Being an analog engineer, I’m around “more experienced” engineers on a daily basis. However, a group of younger engineers often find ourselves acting much older than we are, shouting things like “Get off my lawn!” and “Back in my day…” (really, we had a whole list).

Anyway, another common one that comes up is “They don’t make ‘em like they used to!”.  Do we as engineers know WHY they don’t make them that way anymore? Of course we do. The lower reliability and requirements for many more people to assemble the devices honestly doesn’t make sense these days. With lower priced labor the world over and low tolerance for waste and inefficient processes, I know I wouldn’t make the proverbial “them” that way anymore. It just doesn’t make sense.

But why am I mentioning this? In episode 12 of The Amp Hour, Dave Jones and I were discussing the Tektronix scope that I currently have disassembled and am attempting to piece back together in working order.  It’s the 485M, the military version of the very popular scope.  Right now I’m looking at getting the power supply back on its feet, the voltages were woefully low. More on that in later posts hopefully. For now, let’s concentrate on looking at the awesome design tactics and fabrications inside an old scope.

Note: I am a pretty bad photographer, please excuse any non-professional looking images.

A view of the quite complex button schema of old Tek scopes. Each button controls an individual switch, pot or selector switch. And yet it has many of the features of modern scopes match these exactly.

I LOVE modular design and this is a great example. If a technician (a Tek Tech?) found that a module wasn’t performing correctly this entire module could be switched out to check to see if it is indeed this module.

A closer view of the module. Of note is the resistor jumpered directly across the signal lines of the end connector. Perhaps this is a later fix for a customer issue. It’s also a good view of a mechanical connector that reaches all the way back from the front of the module. It’s a compound switch, pulling on it activates the arm in one direction and pushing on it does some other completely different action.

A close up view of the modular connector. I also like seeing the layout patterns done by hand before CAD programs were prevalent. Interesting to see where they flooded the ground planes.

A closer view of the analog components on one of the modules. Notice this was mainly resistors and a smattering of socketed op amps.

Another view of a mechanical arm reaching all the way to the back of the chassis. Likely a custom part as discussed on The Amp Hour.

This selector switch was the main voltage range switching. It had a compound action as well (inside was a fine tuning I believe) whereas the outside switch was the larger 1-2-5 multiple decade switching.

And finally, a view from the top. Note the >7 kV warning on the CRT tube. No touch!

So there it is, as Dave calls it, “nerd porn”. Isn’t it interesting to see how instruments were constructed not too long ago? It sure was more labor intensive and likely much more expensive than you can pick one up today on ebay. The benefit is that the hand-made and through-hole nature of this board makes it ripe for fixing AND without straining my fragile old eyes. Dangnabbit!



Originally posted by Chris Gammell at http://chrisgammell.com/2010/10/19/they-dont-make-em-like-they-used-to/