Category Archives: Ripple Current
In 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.”
(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.
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.
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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!
“Ground-Effect Robot Could Be Key To Future High-Speed Trains” by Evan Ackerman, 10 May 2011 IEEE Spectrum
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)
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
5. ICdis = ripple amps rms due to discharge to load
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
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.
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.  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.