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DadOf3TeenieBoppers
I have seen 74AC logic turn into miniature volcanoes with a hole in the middle ...
Sanjib.Acharya
In general, the most vulnerable parts of the handheld devices through which ESD ...
Circuit protection can't be an afterthought
Charles Murray
8/20/2012 11:55 AM EDT
Running shoes, hosiery, and nylon shirts can be death for electronic circuits. Taken together, they can generate an electrostatic discharge (ESD) of 30,000V for the briefest of moments, sending handheld devices or laptop computers into a response resembling cardiac arrest. "You walk up to a computer and bam!" Bob Capdevielle, senior applications engineer for Littelfuse Inc., a maker of circuit protection devices, told us. "Suddenly, everything resets."
Still, problems associated with over-voltage and over-current remain an afterthought for most engineers. With their duties expanding and with design cycles compressing, most engineers relegate circuit protection to the end of the to-do list. "These days, engineers have to design the core functionality of their devices as quickly as possible," Jim Colby, manager of technology and business development for Littelfuse, said in an interview. "They have to get the form factor done, get the software done, get the prototype built, and prove out the concept. Then they have time to think about circuit protection."
Polyfuse LoRho SMD Resettable PPTC Resistors provide over-current protection and automatic reset for handhelds.
Source: Littelfuse Inc.
More than ever, though, that approach is creating problems for product designers. Cellphones, computers, and music players are getting smaller. Moreover, they're running on tiny voltages that are more susceptible to ESD, distant lightning strikes, motor switching, and stray currents from process machinery. "It's usually 10,000 or 15,000 volts," Capdevielle said. "But it can get really high. We're getting calls from people asking about 30,000 volt parts."
The unfortunate result of leaving such matters to the last minute is that design functionality suffers. Engineers can't find room for circuit protection devices on their printed circuit boards. They end up re-spinning the boards and losing valuable development time. Worse, they hurriedly choose the wrong protection device, resulting in functional failures, poor reliability, safety issues, shock, or even fire.
For those who face the gloomy prospect of such problems, however, there's hope. Following are the expert recommendations of engineers whose professional lives revolve around the subjects of over-current protection and shock immunity.
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kinnar
8/20/2012 4:03 PM EDT
ESD is more problematic in the cold and dry areas, on equatorial plane it is not that much problem creating. But still protection is must, self resetting fuses are great designs. This too requires proper selection in a way the fuses it self does not get damaged that it could not reset.
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Michael J.
8/20/2012 4:31 PM EDT
Any good ESD protection device will offer more than just protection from super high voltage discharge.
A breakdown voltage slightly outside the signal voltage range also prevent from damage of transients.
A combination of ESD protection and e.g. EMI-filtering also reduces the cost adder for the ESD only function.
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ReneCardenas
8/21/2012 11:28 AM EDT
It may not be thaught in college but it is among the first lessons learned after a poorly under-rated fuse is spec'd, and the rush currents get closer scrutiny in all those odd cases that nobody thought were possible.
Yeap, most EE experience that in at least the first project, I agree since most testing is limited by time2market constraints.
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Brutus_II
8/21/2012 12:08 PM EDT
To take it a step further, ESD and EMI protection can be combined as a single step (EMI from EMP generated from such nasty things as weapon detonation very high in the atmosphere at say, the center of the U.S.). The fix is simple and relatively inexpensive. Yet in a such a competitive industry where every last mil (1/10 cent) counts, such things are not usually considered save for military and a few other critical applications. Our cars and PC's are not considered critical, unfortunately.
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DrQuine
8/22/2012 12:10 AM EDT
Regardless of the tremendous number of opportunities for static electricity to damage portable electronic devices, such events seem to be rare. We've all walked across a rug on a dry winter day and experienced a shock when we touched a conductive surface ... Is it because of exceptional circuit design or is it simply that the outer case of the devices serves as a Faraday cage and protects the contents that we rarely experience catastrophic events for our handheld devices under such circumstances?
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Sanjib.Acharya
8/26/2012 5:34 AM EDT
In general, the most vulnerable parts of the handheld devices through which ESD could cause damage are the ports (such as USB) exposed to the user. Proper protection is needed for the ports.
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DadOf3TeenieBoppers
8/27/2012 10:56 AM EDT
I have seen 74AC logic turn into miniature volcanoes with a hole in the middle of the package, bond wires strung out in the air, and all of the silicon vaporized when someone wearing a nylon shirt rubbed up against the board one day. The poor guy nearly jumped out of his skin when it happened.
Static happens. Plan for it. An ounce of prevention is worth a pound of cure.
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