We are surrounded by devices containing circuits all around us in our day-to-day life, and all these circuits require protection at various levels to avoid the device from getting damaged. Many non-protected circuits have had adverse effects on human lives, and so providing protection to a circuit has become an important requirement.
Bharat Shenoy, global technical director of LittelFuse, talks to Dilin Anand and Sneha Ambastha about circuit protection
Q. What are the key things to be considered while selecting a circuit protection component for a design?
A. Couple of key things that the designers should know include the basic electrical parameters where the device will operate (maximum system voltage, maximum running current going through the device, etc), the maximum operating temperature as most of our products are temperature sensitive, the environment in which the components are going to work and the environmental faults the products need protection from. In order to design the correct component, we need to know faults specifically. Sometimes customers do not know that and we may over-design, which can cause higher cost and more space, and also under-design, which can cause failure. Customers also need to know how sensitive the circuit is, which means that when would it get damaged and the factors that can damage it. Let us take the example of fuses, which are not the same always. Some fuses are such that if we pass 20A through them, they would trip in one second whereas others are such that if we pass 20A through them, they would trip in one microsecond. So if a product (used for circuit protection) gets damaged in a microsecond or millisecond and cannot withstand even a second fault, then it can lead to some damage to the circuit.
Q. What are the parameters based on which circuit protection components are selected for lightening protection?
A. There are fuses that protect against overcurrent and varistors that protect against overvoltage. Other than those electrical parameters, there are several environment-related parameters like lightening. Lightening has a very specific waveform, and it is very important to know its characteristics like the peak voltage, the maximum current, the time duration and the number of pulses. IEEE and UL have developed some standards that simulate lightening—not direct lightening but the effect of lightening storms either inside or outside the building.
Q. What are the technologies available to designers to select from, for lightening protection of electronic systems?
A. When you look at lightening protection or protection against overvoltage surge or overcurrent surge, there are several technologies that can be used. One of those is varistor—it is a ceramic-based technology, and we call them metal oxide varistors because they are made of zinc oxide. They are very strong and can handle a lot of surge but are very bulky. They are cost-effective (a very low-cost technology) and have been in use for many years in spite of being somewhat slow (do not react instantaneously). On the other hand, lightening takes a few microseconds, so till the time varistor starts working, some part of the lightening already passes through. A superfast technology that causes lightening protection is called TVS diode. As soon as lightening comes in, it starts working, so there is very little energy that is let through. However, it cannot handle higher surges. A varistor can handle ten times higher surges than the diodes. The diodes are very expensive, especially when they are big, because of our requirement that they should handle high surges. Then there is another technology called the gas tube. This has very high lightening protection capability but is the slowest of all. It is a technology where there is an arc within the gas tube.
Q. Charger being an important part of the mobile computing world, what should the designers keep in mind whilst designing a charger?
A. People have many devices with a big or small screen, lot of processing power and lot of memory. Some devices also have big batteries but people do not want to wait long to get them charged. They need to be charged quickly. So we need a very high-current charger but it cannot be very big because we need to carry it in our pocket or in our bag. It has to be smaller than a laptop charger but bigger than a small phone charger. So designing such a charger is not an easy task, it has to be custom designed. A custom-designed charger is a charger that can withstand electrical transients. The fuse inside the charger should open in a safe way and should not have any loud popping sound, any smoke or black soot coming up. At the same time, the fuse needs to open up very fast and also very safely so that it does not cause any issue. It should meet the requirements of the electrical transient tests and short-circuit tests or interrupt current test, and these tests should counterbalance each other. The charger needs to be fine tuned with the product it is made for, and also requires very close customer engagement and several iterations of testings in the lab to get it right.
