Thursday, April 25, 2024

Circuit Protection: Too Important to be an Afterthought

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Additionally, in over-voltage protection, surge-handling capabilities of larger metal-oxide varistor products are now being achieved in smaller form factors, and even in surface-mount technologies.

A recent practice is to pack together sub-assemblies of a combination of two protection schemes. Natarajan M.M., vice president for South Asia of Arrow Asia-Pacific, explains, “To overcome electrical over-stress (EoS) and to protect equipment from consequent damage, an innovative solution is brought about by hybrid micro-assemblies, also called integrated product devices, in which two circuit protection devices, usually a combination of over voltage (OV) and over current (OC), are packaged together to provide efficient, cost-effective and space-saving circuit-protection solutions.”

Patil adds, “A digital isolator is a unique ADI product that helps to form an isolation barrier between two sections of the circuit, namely, the low voltage section (very sensitive) and the other side that is facing the field (prone to damage by unwanted signals). A digital isolator prevents unwanted high-potential voltage from affecting the sensitive portion of circuit by creating a barrier. Another impressive technology is our Latch-Up Proof multiplexers (MUXes).”

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He continues, “A latch-up is a condition where a low-impedance path is formed between supply and ground, causing excess current to flow, heating up the affected part and eventually burning it out. The MUX or the switch is typically the first element to get exposed in a signal/data-acquisition system, so one of the ways to protect the circuit is to make this MUX as robust as possible. We do this by using a circuit-design technique known as trench isolation.”

Listen up, you
It has been mentioned umpteen times that circuit protection is extremely critical. Let us make that umpteen plus one, with this take by Tim Patel, technical marketing manager, Electronics Business Unit, Littelfuse, “Circuit protection is a critical part of design; it should not be an afterthought. You need to anticipate what faults can occur, what can cause reliability issues, what faults can come into the system that can cause surge events, how a short-circuit event occurs, and what will happen if you do not protect against it. You may need to redo the board layout, costing money and lost development time. You may end up with a less-than-optimal protection device or location, which results in functional failures, poor reliability and safety issues such as shock or fire.”

He adds, “Let me tell you a story. Smartphone and tablet industries have evolved so much that chargers are now directly connected to the wall; the wire is just a USB cable. Any short-circuit condition in wall-mount chargers will come directly from the wall into that charger. It is a very dangerous application if you do not have adequate circuit protection. The charger can explode, catch fire or put the other chargers bundled next to it on fire. To solve this potentially dangerous issue, a very small form factor axial fuse is placed inside wall-mount chargers. This is how changing form factors of end devices force small form factors for protection devices, too.”

Some golden rules to live by
If you have read up to this point, then you need not be told that circuit protection is quite a can of worms. You essentially have to weigh up risk, cost and consequences. In the end, though, you should not mind spending an extra ` 5000 on a circuit that costs Rs. 100,000, as opposed to a circuit that costs Rs. 10,000.

I talked to a host of designers and they all had their own views on what the golden rules are, which must be followed. But within the chaos of opinion, some guidelines did appear a bit more inline than others. Read on to find out if you follow these favourite guidelines of your peers.

According to Moorthy, “Always understand the standards that the product needs to comply with. This is the most significant input for a successful design. Take special care to note the environmental specs, in particular, humidity and altitude. Chances of ESD strikes are higher at low humidity, which can result in complete circuit failure. Increasing chances of lightning at higher altitudes precipitates the need for additional lightning protection to be provided to mitigate the same. I would also suggest working closely with enclosure designers, especially when the enclosure is made of plastic.”


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