What Is a Chip Fuse?
A Chip Fuse is a type of chip component mounted on a circuit board.
It has the role of preventing accidents caused by melting even if an overcurrent flows and generates heat due to a circuit malfunction. The structure is almost the same as that of a chip resistor, but instead of a resistive element, a metal with a low melting point is used as the fusing element.
Uses of Chip Fuses
Chip Fuses are mounted on a circuit board. Examples of specific applications include battery-powered communication devices such as smartphones and cellular phones, portable-type audio equipment, and electronic circuits in cameras and other photographic equipment.
The purpose is to prevent accidents in the event of an overcurrent due to a malfunction of an element on the circuit board. However, like ordinary fuses, they are not intended for use in power circuits to which commercial power is connected.
This is because chip fuses are small in size, do not provide sufficient insulation distance, and cannot carry a large current. Chip fuses are intended only for the protection of certain circuits and boards.
Principle of Chip Fuses
A Chip Fuse is a single-layer or multi-layer ceramic substrate with a fusing element that conducts electric current. The fusing element is based on a relatively high conductivity material such as copper, gold, or copper-tin or silver-palladium alloys, and therefore the fuse resistance is small.
The fusing element is also made of laser-trimmed thick film deposits or etched metal layers to achieve the required characteristics. In other words, the shape and thickness are set so that the fusing element melts after a certain period of time under overload, when the current flowing through the Chip Fuse exceeds its rated value.
In addition, the fusing element must be protected from various ambient conditions in order for the Chip Fuse to operate reliably.
Multilayer Chip Fuses are surrounded by a ceramic substrate layer, so the fusing element does not need a special protective coating.
On the other hand, in single-layer Chip Fuses, the elements are usually coated with lacquer or epoxy as a protective coating to prevent changes in their characteristics.
In addition, there are various types of fuses, such as fast-acting types and inrush-resistant types that do not easily fuse with rush currents, and each type has different fusing characteristics.
If the fusing characteristics are not taken into consideration, the fuse may blow even during normal operation, while at the same time, circuit protection may not function in the event of an emergency, etc. Therefore, it is important to select a fuse with optimal fusing characteristics for the circuit in use.
How to Select Chip Fuses
If the current flowing through the circuit is constant, fuse selection is easy. In the event of a short circuit or other abnormal condition, the current is much higher than normal, so there is no problem if you select a fuse that will blow immediately when the operating current of the circuit is exceeded. Therefore, for circuits where a nearly constant current flows, a type of fuse called a “quick-opening type” should be selected.
However, the actual current flowing in a circuit is not constant in most cases, but varies in a complex manner depending on the operating conditions. For example, if a capacitor with a large capacitance is connected to a power supply circuit, an inrush current many times greater than the steady-state current will flow when the power is turned on. It is inevitable that a large current flows when a motor starts up (from the time power is turned on until rotation stabilizes).
Such rush current is normal, but depending on the fuse selection, the rush current may cause the fuse to blow. On the other hand, it is undesirable to set a fuse with a large current rating with a large margin, since this will reduce safety. In such cases, one countermeasure is to use “inrush-resistant fuses,” which have a characteristic of not blowing at a short period of rush current.
1. Rush Current
It is difficult to determine the relationship between the rush current in a circuit and the fuse blowing, and the waveform and time of the current must be carefully examined in relation to the fuse’s blowing characteristics.
It is necessary to select a fuse with appropriate fusing characteristics based on the change over time of the heat generated by the fuse. Fuse manufacturers provide many materials for this purpose, and we recommend that you make use of these materials.
2. Rated Voltage
Fuses are treated the same as wires in circuit design, but in practice they have a small resistance value, so some voltage drop is inevitable. Especially since recent electronic circuits have lower operating voltages, it is necessary to check the effects of such voltage drops.
Fuses also have a rated voltage and should always be used at or below that voltage. If Chip Fuses are used above their rated voltage, there is a risk of arcing after fusing, causing them to conduct again.
3. Rated Current and Derating
Derating is a consideration in the rated current. In the case of fuses, there are two types of derating: the common derating, where the fuse is selected so that the steady-state current of the circuit is less than 70% of the fuse’s rated current (varies slightly from fuse manufacturer to fuse manufacturer).
The other is derating for temperature. The derating should correspond to the ambient temperature of the board on which the fuse is mounted. In general, the amount of derating should be checked in the fuse manufacturer’s catalog or data sheet.
4. Other
In the case of glass tube fuses and the like, even if they blow due to excessive current, continuity can be restored by replacing the fuse after repairing the faulty part. Chip Fuses, however, are not intended to be replaced. In other words, when a Chip Fuse blows, the entire circuit board must be repaired and replaced instead of replacing the fuse.
In addition, it is difficult to visually confirm whether a Chip Fuse has blown or not. Although it is possible to electrically confirm blown fuses by using a tester, it is not possible to judge the blown fuse from its external appearance, as is the case with glass tube fuses.