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　　How to design chip fuses

　　In electronic products, chip fuses have two functions: to protect end-users from harm and to protect circuits from damage. These features benefit both device users and manufacturers. In the past decade, the market demand for electronic devices serving information technology, mobile, and consumer applications has been rapidly increasing. Along with this rapidly growing demand, the risk of unexpected situations in electronic devices has also increased, requiring the use of overcurrent protection devices such as chip fuses to avoid electrical overload and other risks.

　　Before analyzing the electrical characteristics of various chip fuses in the market, the most important thing is to first understand the basic design principles behind each technology.

　　Standard fuses may be based on metal wires placed inside sealed ceramic or glass tubes filled with air or sand, while chip fuses are based on completely different principles. Most chip fuses appear to be standard chip components and are made of single or multi-layer ceramic substrates. Some old designs in the past were based on epoxy fiberglass substrates similar to printed circuit boards (PCBs).

　　The fuse elements on single-layer or multi-layer substrates are based on highly conductive materials such as copper, gold, or similar copper tin (Cu Sn) or silver palladium alloys. These composite materials can enhance the ability of fuses to withstand surge currents. But their response to thermal stress is often unstable, which increases the possibility of incorrect melting after multiple surge cycles.

　　To achieve the desired characteristics, depending on the substrate type, the fuse element may be a thick film deposited by laser cutting or a metal layer chemically etched. It is also possible to use fusion welded gold wire. Due to the fixed shape and thickness, if the current reaches a certain level, the fuse element will melt after a certain period of time under overload conditions.

　　In order to fulfill its function as a functional layer of chip components, the fuse element must not be affected by environmental conditions. For single-layer chip fuses, the fuse element is usually coated with epoxy resin paint. The fuse element of a multi-layer chip fuse is naturally protected by each substrate layer. Due to the fact that chip fuses can operate at rated operating currents of up to 7-8A, they require surface mount device (SMD) connections to have low impedance characteristics.

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