This year, I had an opportunity to talk with members of the IEEE Student Chapter at Stanford University about some of the exciting things we've been doing at Littelfuse, particularly the IndyCar racing and NASA Speed2Design programs we ran over the last two years.
Racing and space exploration are just two examples of how pervasive the need is for circuit protection. Electronics are everywhere in racing, not only on the cars, but in the communications equipment in the pits and garages, as well. Much of that equipment is much more sensitive than in the past, because it involves digital electronics, which requires a higher level of circuit protection. And though static shocks might be little more than a wintertime annoyance to most humans on earth, these discharges can spell real trouble for critical electronics in orbit or on extraterrestrial missions.
Unfortunately for a lot of young engineers today, the intricacies of designing adequate circuit protection often aren't taught in universities. Many graduating engineers are more interested in pursuing careers in product design than in issues like overcurrent and shock immunity. If you have the same product design mindset but realize that your job demands a broader awareness of circuit protection, some highlights of my presentation to the Stanford students may serve as an introduction.
- Circuit protection components are vital elements in applications from consumer electronics and telecom circuits to automobiles and industrial equipment. They protect devices and business assets against harm caused by short circuits, power surges, electrostatic discharge, lightning, electrical load switching, and other electrically harmful occurrences. Common types of circuit protection devices for electronic products include thyristors, transient voltage suppression diodes and diode arrays, fuses, positive temperature coefficient devices (PTCs), varistors, gas discharge tubes (GDTs), and ESD suppressors.
- Designing the appropriate circuit protection is critical to preventing dangerous products from reaching the public. For example, lithium ion cells, like the ones in laptops, are subject to thermal runaway. Raising the temperature of a Li-ion battery above 140°F leads to a compromised cell, which can lead to an explosion and fire. Battery packs for laptops and other portable devices contain many levels of protection to assure safety under (almost) all circumstances when in the hands of the public. The safety begins with the battery cell, which includes a built-in temperature switch called a PTC that protects against high current surges, a circuit interrupt device (CID) that opens the electrical path if an overcharge raises the internal cell pressure to 1,000 kPa (145 psi), and a safety vent that releases gas in the event of a rapid increase in cell pressure. In addition to these internal safeguards, an external electronic protection circuit prevents the charge voltage of any cell from exceeding 4.3 V. Furthermore, a fuse cuts the current if the skin temperature of any cell approaches 90°C (194°F). To prevent the battery from overdischarging, a control circuit cuts off the current path at about 2.2 V/cell.
- All too often, circuit protection is the last thing some engineers think about, because they prioritize designing the core functionality of their devices as quickly as possible. However, treating circuit protection as an afterthought has the potential for serious consequences. Once the PCB is designed, it can be difficult to find room for circuit protection devices, making it necessary to re-layout the boards, which costs more money and wastes valuable development time. Even if they don't have to re-layout the board, time pressure may cause some engineers to choose a less-than-optimal protection device or put a device in a non-optimal location, resulting in functional failures, poor reliability, safety issues, shock, or even fire. The best time to start thinking about circuit protection is after the chip set has been selected and before beginning to lay out circuit boards. At this point, ESD ratings are available, and designers can know how robust or how sensitive the chips are.
- The beginning of every product should always be to understand the impact of relevant standards. Standards relevant to circuit protection include those from Underwriters Labs, Energy Star, NEMA, JEDEC, CSA Group, IEEE, ANSI/NEMA, and standards bodies in Canada, South America, Japan, Korea, and Europe, among others. Circuit protection manufacturers' websites are good places to look for guidance. For example, the Littlefuse Speed2Design site is geared toward helping time-challenged designers identify circuit protection solutions.
If you're interested in learning more about the next generation of circuit protection users, see the slideshow of my Stanford presentation on the Speed2Design website.