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Circuit protection as a forethought for design engineers

Smaller, faster, prettier – these are the main criteria for today's electronics. A profitable lead for manufacturers, but also quite challenging for design engineers, that have to fit everything together in a much smaller environment.

Put safety on top of these requirements and the story is getting even more complex. Complex for both designers and manufactures and ultimately relevant for the end customers, who expect their products to be not only fully functional, but also safe.

Circuit protection is daily business
Imagine the following scenario: morning, jogging, wet weather… running shoes, hosiery, and nylon shirt. Taken together, for a brief moment, they can generate an electrostatic discharge (ESD) of 30,000V - a potential cardiac arrest for the smartphone or any other handheld device that plays your favourite music.

Imagine you walk up to your computer and bam! Suddenly, everything resets. Your first thought: a malware or virus, computer is too old or hardware is failing. You wouldn't think that the simple contact between your fancy dress and the computer has actually triggered a brief, but powerful electric incident.

Circuit protection is daily business; still, problems associated with over-voltage and over-current still remain an afterthought for most engineers. Why? Because, with their duties expanding and with design cycles compressing, most engineers relegate circuit protection to the end of the to-do list. "Faster" is the word of the day! These days, engineers have to design the core functionality of their devices as quickly as possible. They have to get the form factor done, get the software done, get the prototype built, and prove out the concept. Only then, if at all, will they have time to think about circuit protection.

Saving valuable development time
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, but experience nowadays is showing that it can get really high, even up to 30,000 volt parts.

The unfortunate result of leaving such matters to the last minute is that design functionality suffers. With devices getting smaller and smaller, and so many parts having to fit together, engineers can't find room anymore for circuit protection devices on their printed circuit boards. The result: they end up re-spinning the boards and losing valuable development time – time that could have been saved, had they thought about circuit protection from the very beginning. To make things worse, many times engineers do think of circuit protection, however they hurry up choosing whatever solution is at hand and the wrong protection device, resulting in functional failures, poor reliability, safety issues, shock, or even fire.

In order to guide the design engineering community to prevent such gloomy scenarios, here are some expert recommendations of engineers whose professional lives revolve around the subjects of over-current protection and shock immunity:

Think of circuit protection from the very 'chip' phase
Considering circuit protection too late in the project can really have some serious repercussions for any design engineer. One such scenario would be to put yourself in a situation where the space is not available for your ESD device. Or, even worse perhaps, you end up settling for a non-optimal location, where the device won't function the way it's supposed to.

To avoid these, the best time to start thinking about such matters is after the very moment you've picked out the chip set and begun laying out the circuit board. 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. Experience tells us that the circuitry is more complicated and more sensitive than we sometimes realize.

Understand the threats
Fuses are simple, everybody understands them, but over-voltage may not be so obvious, and people might not realize the consequences. However, consequences do exist, even if they are not as catastrophic as those of over-current. Did you know, for instance that over-voltage has even 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.

There are many possible sources of excessive current and voltage, starting from the common lightning, ESD, motors, arc welders, as well as the aforementioned running shoes and hosiery, among others. Take lighting, for instance: people understand lightning but they may not know it travels across the ground and can create huge glitches in power lines a mile away. And this is just one example for why, knowing and understanding the possible threats is so important.

Define the needs
To accurately predict a product's circuit protection needs, the design engineer must first be able to imagine how it will be used. In other words, you have to know where the product might end up, understand its environment and what might be adjacent to it. For instance, 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. The point to start is the connection points. The over-voltage device should not be five centimetres away, but, but as close as possible to the connector. Understanding the target environment of the product and the possible threats that might come up with that location will be crucial in selecting the right circuit protection solution.

Know your standards
Standards determine the design of every product, all the way down to the circuit protection. Moreover, the list of standards that designers need to be aware of is seemingly endless. They also differ internationally, regionally or even locally. For circuit protection, standards include those from Underwriters Labs, Energy Star, NEMA, ATCA, CSA Group, IEEE, and standards bodies in Canada, South America, Japan, Korea, and Europe, among others.

Standards are a big part of every project, every project or industry. Design engineers have the duty to be aware of all relevant standards before even commencing the project. Just like the components themselves, knowing and understanding the regulations that govern certain products locally or internationally, has to be part of the very first design phase as well. In other words, one can not create the product and then go look for the standards that would enable that product to work or not in a particular market.

Keep yourself informed
Because circuit protection isn't taught in universities, most engineers are more well versed in the complexities of product design than in the issues of over-current and shock immunity. That problem is compounded by the fact that most design engineers are also juggling several projects and have too little time to research the topic. Industry whitepapers, product specs, and case studies are always helpful to as a source of information and education but also as examples of what worked in certain situations, possibly similar to yours.

Overall, although not a topic officially thought in schools, circuit protection is a topic relevant to the every day life of the end consumer, to the manufacturer whose brand reputation relies on the top functionality and safety of its products and ultimately to the design engineer whose success is measured by the his development time and quality. It's something that definitely needs to be learned through experience and share of know-how across the industry.

Bharat Shenoy is director of technical marketing for Littelfuse's Electronics business unit. Visit the company's speed2design site here.

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Bharat Shenoy

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