Category Archives: Electronic Components

Boeing’s Fix for its Flaming Lithium Batteries: Is There A Fatal Flaw?

dubaiupsplanecrash“Boeing Co. is confident that proposed changes to the 787 Dreamliner will provide a permanent solution to battery problems that grounded its newest jet, a senior executive said Monday.” –Reuters, 11 March 2013

The reported changes include “adding ceramic insulation between the cells of the battery and a stronger stainless steel box with a venting tube to contain a fire and expel fumes from the aircraft.” –Reuters, Alwyn Scott and Tim Hepher and Peter Henderson, 5 Mar 2013

Why is Boeing confident? This is a mystery because, based on available published data, it does not appear that Boeing has positively determined the root cause of the battery fires. Furthermore, as for all safety-critical applications, the certainty of the cause should be determined beyond a reasonable doubt. This stringent requirement would be certified by a panel of independent experts of unquestioned expertise and integrity, who have no financial interest in the outcome of their review.

Without positive identification of the root cause, Boeing may be indulging in a logical fallacy that I have seen employed before, with very bad results. The fallacy is in trying to fix what is assumed to be the problem (e.g. inadequate thermal insulation between battery cells). But what if the assumption is wrong? If so, the “fix” could be ineffective, or even make things worse. For example, improving cell insulation will trap more heat within the cells, raising the cell temperature. If the true root cause is related to higher cell temperature, the added insulation could make cell failure more likely, not less.

There are many other troubling scenarios that can be hypothesized, and the only way to disprove them is to dig in and find the true root cause, beyond a reasonable doubt (including rigorous validation as discussed here: “Flying the Flaming Skies: Should You Trust the Boeing Dreamliner?“)

-Ed Walker

P.S. A good review of the genesis of the Boeing battery problem can be found here: “NTSB report shows Boeing’s battery analysis fell short,” Dominic Gates, Seattle Times

Boeing’s Flaming Lithium Batteries: Was This A Risk Worth Taking?

boeing_batteryIn DACI’s 1st Quarter 2012 newsletter I predicted that a catastrophic safety event would eventually occur due to lithium batteries (please see “Li-Ion Battery Pack Hazards and our Psychic Prediction“). The recent fires in the initial flights of the new Boeing Dreamliner have come close to fulfilling that prophecy.

From “Detecting Lithium-Ion Cell Internal Faults In Real Time” (Celina Mikolajczak, John Harmon, Kevin White, Quinn Horn, and Ming Wu, in the Mar 1, 2010 issue of Power Electronics Technology) it is known that internal cell faults in lithium batteries can lead to thermal runaway, subsequently resulting in fires and/or explosions. Therefore the question arises: do the Boeing lithium batteries have an advanced internal construction that prevents cell faults, or mitigates thermal runaway in the event of a fault? If not, the Boeing team or vendor responsible for the battery system design is in big, big, trouble.

Although deficiencies in basic battery chemistry and/or construction appear to offer the best root cause hypothesis for the fires, there are also other possible factors. For example, it has been reported that perhaps the charging system malfunctioned, causing the batteries to overheat. However, a properly designed charger for an aircraft application would have fail-safe protection, preventing an overcharge. Plus, it was also reported that charging sensors did not detect an overvoltage. Although these factors sound reassuring, they are not sufficient to eliminate the charger from consideration. For example, one can hypothesize a charging waveform that contains spurious high frequency oscillations that create high rms charging currents. This would not necessarily result in overvoltage, but could result in overheating.

It is also possible that battery “cell defects” are nothing more than cell imbalances that vary according to production tolerances. In other words, the lithium battery, by its very nature, tends towards thermal runaway unless the internal cells are very tightly matched. This sensitivity would become more pronounced with a higher number of cells and higher mass, which would explain why no explosions have occurred in  small button-style batteries, but do occur in the larger batteries.

There are other scenarios, including the thorny possibility that some combination of conditions conspired to create the failure. And, of course, the root cause may be highly intermittent, making detection extremely difficult. Such hypotheses are undoubtedly being examined by the Boing engineers. I wish them well, and hope that they are allowed to perform their work calmly, methodically, and thoroughly.

Note: Because it may take quite a long time to conclusively establish a root cause, I would suggest that Boeing immediately begin planning to retrofit the lithium system with one containing battery types that have not shown the proclivity to explode; e.g. nickel metal-hydride, or sealed lead acid gel. Heavier, yes, but in this case safety and the economic timeline indicate that it would be wise to be prepared with a retrofit design.

(For some brief guidelines on design failure crisis management, please see Scenario #6: “Coping with Design Panic,” in The Design Analysis Handbook, Appendix A, “How to Survive an Engineering Project.”

-Ed Walker

4th Qtr 2011

(c) 2011 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Rising Sun Gets Snagged On Mountain And Breaks Apart! / DESIGN MASTER TIP: Minimizing Calculation Time / ANALYSIS: Why Do A Worst Case Analysis? / TECH TIP: Nice Overview of Considerations for External Components for Switching Regulators / MORE UNINTENDED CONSEQUENCES: Wind Power Kills Endangered Species / ANALYSIS QUIZ: Adjustable 3-Terminal Regulator Output Tolerance

NEWS BITE: Rising Sun Gets Snagged On Mountain And Breaks Apart!
Motorists cautioned to avoid area due to high temperatures.

First planet with two suns reported found
15 Sep 2011, NASA and World Science

DESIGN MASTER™ TIP: Minimizing Calculation Time

Do an initial run and check sensitivities. Thereafter set the variables to their respective worst case values to reduce calc time until the design is finalized. Then set the variables back to their full range for a final calculation to obtain probabilities for risk assessment.

ANALYSIS: Why Do A Worst Case Analysis?

 
TECH TIP: Nice Overview of Considerations for External Components for Switching Regulators
See “Power System Performance Requires The Right Actives And Passives” by Tim Watkins, 8 Sep 2011 Electronic Design

MORE UNINTENDED CONSEQUENCES: Wind Power Kills Endangered Species

In the Bay Area, when activists in the 1980s demanded a cleaner planet, the state responded with the Altamont Pass Wind Resource Area. The state-approved wind farm, built with federal tax credits, kills 4,700 birds annually, including 1,300 raptors, among them 70 golden eagles…

“There’s a big, big hypocrisy here,” Sue Hammer of Tehachapi Wildlife Rehab in Kern County said. “If I shoot an eagle, it’s a $10,000 fine and/or a vacation of one to five years in a federal pen of my choice.”

From “Energy in America: Dead Birds Unintended Consequence of Wind Power Development” by William La Jeunesse, 16 Aug 2011, FoxNews.com

ANALYSIS QUIZ: Adjustable 3-Terminal Regulator Output Tolerance

An LM317T regulator with 36V input is set for 24V nominal output, using 1/8W 1% 100ppm thick film resistors (10K and 549 ohms). The regulator must deliver 1A and operate from 0 to 50 C for 10,000 hours.


Q: What will be the approximate worst case output tolerance? (Answer will be posted in the next newsletter.)

3rd Qtr 2011

(c) 2011 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Mutant Singing Cantaloup Wins Karaoke Contest! / MORE UNINTENDED CONSEQUENCES: Hands-Free Faucets / DESIGN MASTER TIP: AC Rectifier Worst Case Analysis Made Easy / ART MEETS ENGINEERING: The Invisible Man / STATISTICAL DESIGN PITFALLS: Monte Carlo Is Not Worst Case Analysis

NEWS BITE: Mutant Singing Cantaloup Wins Karaoke Contest!

Freaky Robot Mouth Learns to Sing,”
Evan Ackerman, 13 July 2011, IEEE Spectrum

MORE UNINTENDED CONSEQUENCES: Hands-Free Faucets Harbor More Germs Than Standard Faucets

Details here.

DESIGN MASTER TIP: AC Rectifier Circuit Worst Case Analysis Made Easy

In our previous Newsletter we provided a pretty good estimate for the ripple current for the bulk capacitor in an AC rectifier circuit. But what if you have a large volume product and you need a full worst case analysis to ensure high reliability, but one that is not overly pessimistic so that you can minimize cost? Design Master can help you achieve that optimum balance.

As readers are aware, we’ve started to release some DMeXpert “fill in the blank” WCA templates to make the design engineer’s life a bit easier. One of these is our AC Bridge Rectifier Analysis (ACBR1 $19) which allows the designer to determine all of the worst case component stresses within a minute or two. The analysis includes the effects of source impedance Rs (such as transformer secondary winding ohms), which if present can be used to reduce capacitor ripple current requirements, hence reduce capacitor cost.

As those who have studied AC rectifier circuits are aware, this seemingly simple circuit has resisted for decades all of the attempts to generate a single-formula solution, until recently, which we’ve included in ACBR1. Based on Keng Wu’s article, “Analyzing Full-Wave Rectifiers With Capacitor Filters” (1 Jan 2010, Power Electronics Technology), Wu’s formula allows a straightforward circuit solution, greatly reducing computational time. So with ACBR1 you can just fill in the blanks, click Calculate, and let Design Master do the rest.

ART MEETS ENGINEERING: The Invisible Man

Engineers who work for the military are sometimes required to design clothing, equipment, or even entire shelters to be “invisible” to various detection means. Chinese artist Liu Bolin has a gift for applying such camouflage in a non-technological way, as seen below. Hint: If you can’t spot Liu, look for his shoes first.

From “The Invisible Man: Dragon Series,” Vurdlak, 28 June 2011, http://www.moillusions.com

Some more fascinating photos here and here.

STATISTICAL DESIGN PITFALLS: Monte Carlo Is Not Worst Case Analysis

A lot of folks like to let a simulator crank out “worst case” results, using Monte Carlo statistical methods. But as we’ve explained previously (“Design Master vs Extreme Value, RSS, Monte Carlo, & Simulation,” and “Design Master vs Monte Carlo“), this can be not only time consuming, but risky. For example, Monte Carlo can easily miss small but significant errors (see example below). In addition, if the Monte Carlo runs are improperly implemented (such as including temperature or other dynamic variables) you will likely obtain wildly inaccurate results.

The Design Master Advantage

Instead of statistical sampling, Design Master uses a top-down approach to achieve safer and more cost-effective results, by (a) detecting the extreme limits of performance, and then (b) using a proprietary probability algorithm to estimate how often those results will exceed the specification limits.

EXAMPLE

Design Master results at 2 samples/variable versus
Monte Carlo at 10,000 samples/variable, for the gain of an 8-variable filter

As can be seen, the Monte Carlo analysis detected a minimum of 8.42 versus the actual minimum of 7.86, a 7% error, and a maximum of 16.0 versus the actual maximum of 18.8, a 15% error.

Metal Oxide Varistor (MOV) DMX Analysis File Released

Metal Oxide Varistor (MOV) DMX Worst Case Analysis File
MOV1 $12.50

(DMX files are available free to Design Master™ Professional Edition users who purchased or upgraded DM not more than one year prior to the DMX file release date.)

The MOV analysis determines whether a Metal Oxide Varistor transient voltage suppressor will (a) survive a specified surge voltage or current, (b) clamp the surge below a specified voltage limit, (c) not clamp the normal operating voltage, and (d) survive a specified number of surges. MOVS are typically rated with 8x20us current waveforms, and (just to be confusing) 10x1000us energy waveforms. MOVs also have a lifetime (number of allowable surges) that depends on peak current, pulse width, and temperature. To complicate things further, MOV clamping voltages are a nonlinear function of surge current. To help make the design engineer’s job a little easier, this analysis contains adjustment formulas for all of these factors. Also provides standard surge waveform requirements and helpful hints.

DMeXpert™ (DMX) files guide the user with pop-up instructions, component selection lists, standard part values, important formulas, and a variety of other tips that are activated when entering a Formula cell. It’s like having a design/analysis expert at your side.

Transient Voltage Suppressor (TVS) DMX Analysis File Released

Transient Voltage Suppressor (TVS) with Optional Steering Diode DMX Worst Case Analysis File
TVS1 $12.50
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(DMX files are available free to Design Master™ Professional Edition users who purchased or upgraded DM not more than one year prior to the DMX file release date.)

The Transient Voltage Suppressor analysis determines whether a TVS avalanche diode and optional steering diode will (a) survive a specified surge voltage or current, (b) clamp the surge below a specified voltage limit, and (c) not clamp the normal operating voltage. Good for any TVS diode and steering diode; just fill in the blanks using data sheet values, and get an answer in a few seconds. TVS diodes are typically rated with 10x1000us current waveforms. Steering diodes are typically rated with line frequency half-sine current waveforms. When the applied surge has a different waveform, however, the TVS and steering diode ratings must be adjusted accordingly. In addition, the ratings must also be adjusted for pulse width and temperature. To help make the design engineer’s job a little easier, this analysis contains adjustment formulas for all of these factors. Also provides standard surge waveform requirements and helpful hints.

DMeXpert™ (DMX) files guide the user with pop-up instructions, component selection lists, standard part values, important formulas, and a variety of other tips that are activated when entering a Formula cell. It’s like having a design/analysis expert at your side.

AC Full Wave Bridge Rectifier DMX Analysis File Released

AC Bridge Rectifier DMX Worst Case Analysis File
ACBR1 $19

(DMX files are available free to Design Master™ Professional Edition users who purchased or upgraded DM not more than one year prior to the DMX file release date.)

This updated and easy-to-use analysis provides all of the key waveforms, voltages, and currents for the AC full wave bridge rectifier circuit, including the effects of source ohms. Output includes average input amps, rms input amps, input watts, Rs watts, capacitor rms amps, average load volts, average load amps, and output watts.

Capacitor Current

DMeXpert™ (DMX) files guide the user with pop-up instructions, component selection lists, standard part values, important formulas, and a variety of other tips that are activated when entering a Formula cell. It’s like having a design/analysis expert at your side.

2nd Qtr 2011

(c) 2011 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Miraculous Emergency Landing on Railroad Track! / DESIGN MASTER TIP: AC Rectifier Bulk Capacitor Ripple Current / HUMANITARIANISM: Capitalism + Volunteer Engineering Helps Haitians / UNINTENDED CONSEQUENCES: Nanny Engineering / SHAMEFUL BEHAVIOR: Shanghai Euchips Industrial Co. Used Fake UL Label

NEWS BITE: Miraculous Emergency Landing on Railroad Track!

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Ground-Effect Robot Could Be Key To Future High-Speed Trains” by Evan Ackerman, 10 May 2011 IEEE Spectrum

DESIGN MASTER TIP: AC Rectifier Bulk Capacitor Ripple Current1 


Pretty good estimates of capacitor ripple amps for full wave rectifiers driven by low source impedance can be obtained by using the equations below. Note that an exact solution requires iteration, which can be done automatically by the Design MasterTM worst case analysis software. If you don’t have Design Master, you can get some quick results by first estimating the ripple voltage and solving for tC. Then calculate Vripple to see if your estimate was close. After a couple of iterations you will zero in on good values for tC and Vripple, and then you can solve for total capacitor ripple amps.

A PRETTY GOOD ESTIMATE for BULK CAPACITOR RIPPLE AMPS

1. tC = charge time, sec = ACOS(1-rRIP)/(2*Pi*f)
where
ACOS = inverse cosine function (COS-1)
rRIP = ripple ratio, Vripple/Vdc.
Vripple = ripple volts peak-peak = Idc*tD/C
Vdc = average DC output volts
Idc = average DC output amps
tD = discharge time = 0.5/f – tC, seconds
C = bulk capacitance, F
f = line frequency, Hz
2. Dc = charge duty cycle = 2*f*tC
3. Dd = discharge duty cycle = 1 – Dc
4. ICchg = ripple amps rms due to charge from full wave rectifier
= Idc*SQR(1/Dc-1)
5. ICdis = ripple amps rms due to discharge to load
= Idc*SQR(1/Dd-1)
6. ICload = rms content of pulsed load amps (e.g. input of switchmode regulator) if applicable. If load amps is purely DC, set ICload to 0.
7. ICtot = total capacitor ripple amps rms = SQR(ICchg^2+ICdis^2+ICload^2)

The great thing about analysis, as compared to simulators such as SPICE, is that you can learn a lot by reviewing analysis equations. For example, if you set the ripple voltage ratio to a desired amount (e.g. 15%), and rearrange the Vripple equation to solve for C, you can readily obtain a graph of the required ripple amps rating versus output current, regardless of input or output voltage. Now you’ve generated a general-purpose design guideline to use for numerous applications.

See the example graph below for the capacitor ripple amps requirement versus DC load currents from 100ma to 5 amps, for a 15% ripple voltage and a 60Hz source. For example, for 2 amps of load current, the capacitor will require a ripple current rating of 4.4A

1. Excerpted and revised from DACI Application Note, ” Why Power Designers Need More than SPICE to Avoid Blow-Ups.”

HUMANITARIANISM: Capitalism + Volunteer Engineering Helps Haitians

Non-governmental organizations operating on free-market principles can offer the most effective assistance to those in need. For an example click here.

UNINTENDED CONSEQUENCES: Nanny Engineering

Yes, the government does provide some essential functions. Unfortunately, it doesn’t have the self-control to restrict itself to those functions. The result is that we are plagued by governmental busybodies who like to justify their salaries by telling the rest of us how to behave, in areas that are none of their concern.

For example, as pointed out in Engineering Thinking, there have been numerous regulations passed that restrict our right to choose the products we may want, such as incandescent light bulbs. In that case, the government has deemed such bulbs unacceptable due to low efficiency. But if incandescent light bulbs are inefficient, that fact becomes evident in our electric bill; why do we need the government to tell us how best to spend our money?

Furthermore, perhaps some of us would, regardless of efficiency, prefer to use the incandescent type. But no, the governmental busybodies have decreed that you don’t get to freely choose. Forget about all of the other parameters that might be of more importance to you: short-term cost, color rendering, lifetime, reliability, and environmental hazards. Also, some folks in chilly climates might even appreciate the extra heat that incandescent bulbs provide. But none of these considerations matter to the one-solution-fits-everybody government.

Now, as typically happens following such governmental decrees, we find that they are rife with unintended consequences; e.g. the compact fluorescent lamps (CFLs) that the government wishes us to use instead of  incandescent bulbs have significant disadvantages:  (a) substantially lower lifetimes than expected, (b) may emit hazardous fumes (click here), (c) emit electromagnetic interference (EMI), (d) emit a color that can disrupt melatonin production and thereby cause sleep disorders, (e) sometimes create an irritating buzzing nose, (f) contain hazardous materials that pose significant environmental disposal hazards, and (g) will kill the domestic incandescent bulb industry, and replace it with products that are primarily foreign-made.

Some years ago the government illustrated similar brilliance by outlawing magnetic ballasts, again simply on an efficiency basis. It should be no surprise that the electronic replacement ballasts were more expensive, had shorter lifetimes, were less reliable, contained hazardous materials, and emitted a lot of EMI (so much so that some hospitals refused to use them because of their tendency to interfere with medical instruments).

Recently some smart engineers from China, unencumbered by the U.S. regulatory dictatorship, have created a magnetic ballast whose efficiency is better than electronic ballasts, at lower cost, longer life, higher reliability, using non hazardous materials. [1] Congratulations!

Sad to say, this is the sort of advance that was typically accomplished by U.S. engineers, back before the government decided to play Nanny Engineer.

Note 1: “A ‘Class-A2’ Ultra-Low-Loss Magnetic Ballast for T5 Fluorescent Lamps — A New Trend for Sustainable Lighting Technology,” Hui, Lin, Ng, and Yan, Feb 2011 IEEE Transactions On Power Electronics.

SHAMEFUL BEHAVIOR: Shanghai Euchips Industrial Co. Used Fake UL Label

Details here.

1st Qtr 2011

(c) 2011 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Astronomer Discovers Giant Eye Staring At Earth! / DESIGN MASTER: New Pricing Structure / COMPONENTS: The Most Popular Op Amps? It’s A Secret! / CONCEPTUAL DESIGN: Look At The Big Picture / COOKING & MEASUREMENTS: Why Engineers Get It Right

NEWS BITE: Astronomer Discovers Giant Eye Staring At Earth!


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The Helix Nebula, “Hubble’s Best Photos” by Lauren Effron, Discovery News

DESIGN MASTER: New Introductory Pricing Structure

Effective for 2011, we’re changing our introductory price structure for DM to a flat fee of $85/copy, single user ($25 for upgrades), and providing modeling packages in DMX template format at an added charge. DMX is a DM feature that allows expertly-designed “fill in the blanks” templates to be used for thorough and efficient worst case analysis. Click here for details.

With the introductory pricing structure, DM will only be supported via email, and only with regard to DM operation, not circuit or part modeling.  For those who may need help with modeling, please see DM V8’s expanded help content, and refer to the available DMX templates. If more extensive assistance is required, please contact us with regard to a DM training seminar, or for the provision of custom circuit or part models.

Offer expires 30 April 2011.

COMPONENTS: The Most Popular Op Amps? It’s A Secret!

We were looking for some guidance in building up op amp models for our DMX templates, so we asked several major semiconductor vendors this question: “What are your most popular op amps?” Surprisingly, although the technical contacts said they would like to provide the data, they couldn’t; their sales departments considered the data proprietary, due to competition concerns.

Therefore it seems our alternative is to create our own “favorite” lists. If you have some preferred op amp (or other analog IC) part numbers that you would like to share with the DM design community, please send them in. We’ll post them in future issues of this newsletter. The data will also provide guidance for our development of part models.

CONCEPTUAL DESIGN: Look At The Big Picture

In the 12 December 2010 issue of Electronic Design, Louis E. Frenzel discusses some modern versions of the ubiquitous 555 timer IC (“And You Thought The 555 Timer Was Dead?“).

In that article, Mr. Frenzel also refers back to an earlier article of his , in which he asks, in essence: why in the world does the 555 timer still exist? (“The 555: Best IC Ever Or Obsolete Anachronism?“, 12 Dec 2007, Electronic Design).

Might we suggest an answer? The 555 provides a simple common function in an elegant low-cost manner. This does not mean it is always the best solution (Mr. Frenzel mentions some alternatives), but it certainly is an attractive option, particularly for circuits that don’t contain a uP.

I suspect that Mr. Frenzel was deliberately being provocative to stimulate conversation, particularly with his implication that all modern products contain a uP. If that were true, then yes, why add a part when the uP can provide the timing capability for free? But a huge number of products do not contain uPs, because they simply don’t need uPs.  In fact, avoiding a uP has additional advantages in addition to cost avoidance, including better reliability (by keeping things simple), and by eliminating radiated emissions noise. We have saved some of our customers a lot of money by showing how a dirt-cheap non-uP circuit can achieve the functions they need, while also avoiding the noise issues that would require EMC compliance certification.

Mr. Frenzel quotes from a 1997 article, “…the 555 is dated mainly because it is no longer compatible with the mostly low-voltage (less than 5 V) circuits in use today. Furthermore, it consumes excessive power compared to today’s circuits.”  But this ignores the fact that the 555 has been continuously reincarnated over the years with better performance, operating at lower voltages and current, and available in various modern packages.

From a broader perspective, we think it’s important to stand back and look at the big picture during the concept design phase. We shouldn’t just indulge our prejudices, and grab the parts and designs we’re familiar with. Rather, we should ask: what’s the optimum design? Among other considerations, this means we need to challenge our assumption that newer means better. Maybe, maybe not. There are lots of situations where older-technology designs are much better for certain applications — all things considered — than their modern counterparts. A few examples:

  • 4000 series CMOS: these senior ICs are great solutions for low cost, low speed, noisy industrial applications.
  • Mechanical relays: ancient, but like the 555, continuously modernized, and still the best solutions for numerous higher power industrial designs.
  • Older IC technologies in general: lower cost, less EMC issues, and less susceptibility to electromigration, which is becoming a significant concern with modern minimal-geometry ICs.

COOKING & MEASUREMENTS: Why Engineers Get It Right

“Cooking is awesome because it’s applied science. Biology, chemistry, physics, they all come into play when you’re cooking. I don’t know that I necessarily do anything specialized. I know that some of my more particular habits come out when I’m cooking. I very rarely cook meat, for instance, without a thermometer. I know there are these old-school cooks who would turn their noses up at me because they can tell by giving their roast a touch or by poking it in the right spot and seeing what color the juices are, they can tell when it’s ready to come out of the oven. Well, I can tell when it’s ready to come out of the oven when it’s the right temperature and that is okay with me. You know, my bread—before I invested in a thermometer, sometimes it was too dry and sometimes it was gummy in the middle, but now it’s right every time. I use a kitchen scale for almost everything. If you think about a cup of flour: If I measure the cup of flour it may weigh 4 ounces; if you measure your cup of flour, it may weigh 5 ounces, because the flour is compressible. So you achieve a degree of precision when you measure things that way. Nothing drives me crazier than when you look at a recipe and they call for 1 large onion. I’m guessing that one large onion in Texas is massively different that one large onion in southeast Missouri. You know I can get an onion from the store here that weighs almost a pound. I have no idea if that’s what they meant. So my life got substantially more precise when I was able to start making notes, you know—this much onion worked, this much onion didn’t.”
April Woods, as quoted in “Geeks Cooking: April Woods, the Hungry Engineer,” an interview by Susan Hassler in IEEE Spectrum

 

2nd Qtr 2010

(C) 2010 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material

This Issue: NEWS BITE: Sneak Peek of “Smiley” Terminator 5! / NEWS BULLETS: Unintended Consequences Strike Again / DM V8 Scheduled for Release This Year / SPECIAL REPORT: Aging (Drift) Considerations for Electronic Components

NEWS BITE: Sneak Peek of “Smiley” Terminator 5!

Photo: Osaka University and Kokoro Company
(“Geminoid F: More Video and Photos of the Female Android” by Erico Guizzo, April 20, 2010 IEEE Spectrum)

NEWS BULLETS: Unintended Consequences Strike Again

From “The climate changers: How wind turbines make their own clouds” by Andrew Levy, April 25, 1010 Daily Mail online.

DM V8 Scheduled For Release This Year

Design MasterTM V8 (Major Upgrade) is planned for release later this year, with several new features. We’ll keep you posted.

Also, based on requests from users, we have shifted from a subscription basis to a flat purchase basis, with upgrade fees for new releases. (Those who purchase V7 within a year of the release of V8 will receive V8 at no added charge.)

Sample and Part files are being updated for V8 to include the effects of aging (see the Special Report in this newsletter).

For more details on the current version, please click here: Design Master V7

SPECIAL REPORT

Aging (Drift) Considerations for Electronic Components

A key goal for an electronics circuit is to ensure that the circuit will function properly throughout its desired lifetime. Achieving this goal requires an understanding of how electronic component parameters can shift, or drift, as they age.

DMeXpertTM Tip                        Accounting for Aging Is Essential

For even moderate desired lifetimes of a few thousand hours, aging may have more of an impact on total error than other factors such as initial tolerance and temperature.

End-Of-Life, Lifetime, MTBF, and Other Confusion Factors Designed to Torment the Design Engineer

Aging analysis should not be confused with “reliability prediction” efforts, which employ terms such as end-of-life, lifetime, and MTBF (mean time between failures). These efforts are intended to provide a statistical estimate of when a component will fail; i.e. suffer such a large parameter shift that it becomes inoperable. Although these predictions may be of interest to a reliability engineer, the design engineer is concerned with parameter shifts that occur as the part ages but is still operable.

For example, end-of-life generally refers to parameter drift after several years (usually not well defined, but the term typically implies a period of 7-15 years), whereas “lifetime” may refer to the time for actual part failure (wearout), which is generally exhibited as gross inoperability of one or more key functions. MTBF (mean time between failure) calculations are supposed to predict useful lifetime, but they can be highly inaccurate.

DMeXpertTM Tip                        MTBF and Lifetime Predictions

Don’t bother. As we’ve said before, it makes much more sense to spend resources on improving processes and designs than to spend those resources on making predictions that are based on inadequate samples and circular reasoning, predictions that often prove to be wildly inaccurate and misleading.1

Note 1: Our research has not uncovered a single example of a long-term study of failure rates or lifetimes that compares those actual values to a baseline prediction methodology. If you know of one, please let us share it with our readers.

The Design MasterTM Approach to Aging

When the desired operating hours are high, it’s important to use parts that define how their key attributes (resistance, capacitance, offset voltage, output current, etc.) shift with time, and also with applied stresses. Unfortunately, this information if often very hard or impossible to find, which leads to the following guidelines:

DMeXpertTM Tip                        Selecting Electronic Parts

For long-life designs, use parts that have aging effects clearly defined in the manufacturer’s data sheets or application notes.

DMeXpertTM Tip                        Don’t Predict, Confirm

Don’t try to predict the lifetime of a circuit.

Do specify the required lifetime of a circuit and confirm that, considering the effects of aging, the circuit will perform properly over that lifetime.

If you must use parts that have poorly-defined lifetimes (unfortunately this is often the case), then your product will need to have periodic maintenance (which can be manual, or automated via built-in self-test) that monitors performance degradation. In either case, the user will be know when the equipment should be recalibrated or receive preventive maintenance.

If you are building a product that cannot be manually maintained (e.g. a space probe) then the design will require not only the best (lowest aging) components, but will also likely need auto-calibration circuits to maintain performance.

CAUTION: Use “Useful Life” ONLY for Maximum Allowable Drift Points

The “useful life” figure provided by many part vendors is, well, not too useful, because it is not expressed in designer-friendly terms. Life testing and related data presentation seem to be controlled by statistically-oriented reliability engineers, but the dominate Big Gorilla factor in determining real-world useful life is the allowable parameter shift that a designer can tolerate, not an end-of-life guesstimate embedded in a generic test standard. For example, a failure definition for a capacitor life test, such as a capacitance change of -30%, or a change in ESR of +300%, will typically far exceed what is acceptable for a long-life design application.

In addition, “useful life,” for valid economic reasons, is based on very small samples, which means that a lot of assumptions and guesswork are involved.1 Also, the limits of the useful life value are not usually provided in the data sheet (no minimum values or standard deviations).

For these reasons, “useful life” or similar lifetime data provided by vendors should not be used for predicting life, but used only for defining maximum allowable parameter shifts.

Note 1: Part vendors make educated guesses as to the life of their product. They don’t say “educated guesses”; that would make some of their customers nervous. If anyone doubts the validity of this, here’s a challenge: scour the literature and send in a data sheet that provides a designer-friendly specification for a component’s useful life; e.g., “This component has a useful life, as defined by a tolerance shift of not more than +/- 5%, of 5000 hours minimum when operated within its maximum ratings.” In all cases we’ve seen, when you look at the fine print you find that useful life figures are based on small-sample statistical testing; i.e. the stated hours will typically be a mean value for a defined failure rate, as further defined by arbitrary large parameter shifts, with a 60% confidence level based upon an assumed exponential distribution — got a headache yet?

Our View                                         A New Paradigm for Part Vendors

Wouldn’t it be great if part vendors defined the min/max drift rates of all key part parameters, and dispensed with “useful life” mumbo-jumbo and related reliability “predictions”?

Modern ICs = Shorter Lifetimes

Due to decreasing geometries in ICs, wearout lifetimes (the time at which the part becomes inoperable) have been decreasing, and have become a significant reliability concern for electronics that are expected to operate for many years. Since shorter wearout lifetimes translate into a higher rate of aging versus time, aging analysis becomes even more critical when using modern ICs. The corollary is the following non-intuitive rule-of-thumb:

DMeXpertTM Tip                       Designing for Long Life

For designs with a desired life of many years, use older-technology ICs whenever possible.

With shorter wearout times and correspondingly quicker aging, it is becoming more and more necessary to employ auto-calibration so that circuits can periodically readjust themselves to compensate for drift errors.

DMeXpertTM Tip                       Ensuring Proper Auto-Cal

The required auto-cal dynamic range must be sufficient to compensate for the maximum accumulated drift over time.

Appendix A

Accounting for Aging Effects

Consider a simple resistor. A resistor’s deviation from its ideal nominal value is defined by its initial tolerance, and by reversible and irreversible variables. For a resistor,

Reversible: Temperature coefficient (static variable)

Irreversible: Aging (or drift), a time-dependent permanent deviation that is a function of exposure to elevated temperatures, voltages, humidity, etc. (dynamic variable)

Aging address irreversible deviations versus time, and should be factored with the initial tolerance and temperature coefficient to obtain the total tolerance:

TOTAL TOLERANCE =

(1 + TOLi) * (1 + TOLtc*(Tap-Tref)) * (1 + TOLage*Life)

where

TOLi = initial tolerance; e.g. -0.01/+0.01 for a 1% part

TOLtc = temperature coefficient, /C; e.g. -1.0E-4/+1.0E-4 for a +/-100ppm/C part

Tap = application temperature, C (typically ambient temperature Ta or case temperature Tc)

Tref = data sheet reference temperature, C (typically 25C)

TOLage = aging tolerance, /hour for a reference operating time; e.g. +/-20% for 2000 hours, or -1E-4/+1E-4

Life = desired operating time, hours

(For severe applications (e.g. circuits exposed to a radiation environment), additional tolerance terms need to be included in Total Tolerance.)