Circuit protection can bite you if you do it too late in the project. You could put yourself in a situation where the space is not available for your ESD device. You end up settling for a non-optimal location, where the device won't function the way it's supposed to.
The best time to start thinking about such matters is after you've picked out the chip set and begun laying out the circuit board, Colby said. Doing it in that way, ESD ratings are available and designers know how robust or how sensitive the chips are. "Some of these chips are running at 1.5 volts and you don't have to do a lot to upset them," Capdevielle said. "The circuitry is more complicated and more sensitive than we sometimes realize."
Understand failure modes. "Everybody understands a fuse, but over-voltage may not be so obvious, and people might not realize the consequences," Capdevielle said. The consequences, however, do exist, even if they are not as catastrophic as those of over-current. Over-voltage has incapacitated the Hubble Telescope, shut down refineries, killed smartphones, and stopped roller coasters mid-ride. In some portable medical devices, over-voltages can even be life threatening, according to Capdevielle.
Sources of excessive current and voltage include lightning, ESD, motors, arc welders, and the aforementioned running shoes and hosiery, among others. "People understand lightning but they may not know it travels across the ground," Capdevielle said. "It can create huge glitches in power lines a mile away."
Define the threats. To accurately predict a product's circuit protection needs, the design engineer must first be able to imagine how it will be used. "You have to know where the product might end up," Capdevielle said. "You have to understand its environment and what might be adjacent to it. A device will be more susceptible to a factory setting than to an office."
Once the designer understands the environment, he or she can begin making accommodations. "You should start with the connection points," Capdevielle said. "You should have an over-voltage device, not three inches away, but as close as possible to the connector."
CAD and CAE tools often provide analysis of ESD and other effects. This is very much like security in software. Frankly, when design a program, at any level, the main concern is not security, or any number of other -ilities, but the funtionality of the program. Design tools have been used for a long time to ensure that the software meets these requirements. Hardware design has to take this into account as well.
It typically isn't too complicated to make a device immune static discharge and surges, but it should certainly not be an afterthought. Something as simple as adding a little series resistance to keyboard lines may be all it takes to keep from turning your silicon back into sand. It's worth investing in, or renting a static discharge (simulator) gun.
As a side note, I was performing static discharge tests on a device with a membrane switch panel. The device passed several tests, I turned off the gun and the reached for the device to turn it off. I was greeted by a very hearty jolt, as the front panel was holding a healthy charge and using the switch panel as a capacitor.
It's funny what people think about as an afterthought. Earlier in the mobile era, people said it was always the battery that was an afterthought. Hence, we had more OEM battery shapes and sizes than the battery OEMs would care to remember. Then it was the wireless technology, which designers often thought of (for good reason) as being a black box that they could simply drop into their design. Now it's the circuit protection. What's next?
In addition to circuit protection, power supplies are an afterthought as well. Being an engineer, I'm guilty of focusing on the key subcircuits of the products I've design and realized they needed clean efficient power to operate. To eliminate this afterthought mindset, I'll include in my system architecture design a sub-block for power supplies, interconnectors, and circuit protection devices.
Avnet Electronics is hosting its annual virtual Power Forum conference. Circuit Protection, power supplies, interconnectors, batteries, and electromechanical devices lectures will be presented in this virtual event. For additional information the weblink is shown below.
I remember hearing the same things about batteries, Rich. I suppose most suppliers would just like to see their technology get promoted to a higher spot on the designer's list.
Designs must also take into account suppression of electromagnetic and radio-frequency interference, and compatibility with other devices. Lack of attention to these effects can cause an expensive failure when a product undergoes compliance testing.
I recall an engineer telling me a device would not pass an emissions test because someone had cut ventillation slots in a metal case for the device. Thise openings served as perfect slot antennas for signals generated inside the box. If you don't think about this "stuff" you can get into trouble rather quickly.
Reading this article brought back a lot of memories. At one place I worked we had an automated ESD tester for our ram chips - you could seat the part in the fixture and program in the voltages and it would both zap and test the part. In product engineering at another company we would do ESD testing on our discrete hall effects with a handheld tester. You would adjust the voltage and then place the tip on the lead and give it a zap - then test it afterwards. While it is important to have protection circuitry - hopefully the problem is also being addressed at the discrete level like we were doing. We always cracked up laughing at our hand held ESD "Zapper" - the instructions specifically directed the user "Do Not Use As A Cattle Prod."
Charles, this is an extremely important article. When I was in the aerospace industry we would frequently visit Titan II silos to affect repairs. As a member of AFLC, I worked on the thrust chamber and the turbopumps for that defense system. Initiation of launch required the firing electrical "squibs". This was accomplished with a 28 volt DC source. With this low voltage, it would not be impossible to ignite the squibs with static discharges. This has happened. Due to this possibility, we were not allowed to bring or wear metal objects into certain levels of the silos. No belt buckles, no wedding rings, no watches, no jewelry of any kind. Even the eyelets in our shoes had to be taped or booties worn. For some areas, special clothing was also a necessity. I can certainly understand the need for proper circuit protection regarding electronic devices such as the ones you mentioned. This will be an ongoing problem unless engineers and engineering managers address the possibilities.
Static discharge has caused a few accidents with missiles. in `84 there was a fatal accident of a Pershing II that burned its 4 tons of propellant. The motor was being removed from the casing at the time of the accident and the discharge ignited the motor from the inside. The unfired igniter was recovered from the scene.
It truly is an important topic, bobjengr. I can't prove this, but I suspect circuit protection often gets overlooked because it never gets mentioned in school.
A fire control panel was not working and I was tasked with fixing it. I discovered that a large mechanical relay was throwing off some energy. In my experiments to discover how much, I ended up routinely disconnecting my mouse and resetting my PC with the relay's discharge.
I added the proper relay suppression system, and the panel work perfectly after.
Unfortunately, I was a victim of the afterthought.
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